CECW-EI
Engineer Circular1110-2-6052
Department of the Army
U.S. Army Corps of EngineersWashington, DC 20314-1000
EC 1110-2-6052
1 January 2001
EXPIRES 31 DECEMBER 2002
Engineering and Design
STRUCTURAL DESIGN OF PRECAST AND PRESTRESSED CONCRETE FOR OFFSITE PREFABRICATED CONSTRUCTION OF HYDRAULIC STRUCTURES
Distribution Restriction StatementApproved for public release;
distribution is unlimited.
DEPARTMENT OF THE ARMY EC 1110-2-6052U.S. Army Corps of Engineers
CECW-EI Washington, DC 20314-1000
CircularNo. 1110-2-6052 1 January 2001
EXPIRES 31 December 2002Engineering and Design
STRUCTURAL DESIGN OF PRECAST AND PRESTRESSED CONCRETEFOR OFFSITE PREFABRICATED CONSTRUCTION OF HYDRAULIC STRUCTURES
1. Purpose. This EC provides interim guidance for structural engineers in the design and constructionof precast and prestressed hydraulic concrete structures. The primary emphasis is on float-in and lift-intype structures.
2. Applicability. This circular applies to USACE Commands having responsibility for Civil Worksprojects.
3. Distribution. This document is approved for public release, distribution is unlimited.
4. References. Required publications are listed in Appendix A.
5. Action Required. This EC should be used as interim guidance pending publication of the finalEM. Any comments regarding improvements or clarification should be submitted to HQUSACE(CECW-EI), Washington, DC 20314-1000, within one year of the publication of this EC.
FOR THE COMMANDER:
3 Appendices:Appendix A: References DWIGHT A. BERANEK, P.E.Appendix B: Specification Requirements Chief, Engineering and Construction DivisionAppendix C: Design Example Directorate of Civil Works
CECW-EI
Engineer Circular1110-2-6052
Department of the Army
U.S. Army Corps of EngineersWashington, DC 20314-1000
EC 1110-2-6052
1 January 2001
EXPIRES 31 DECEMBER 2002
Engineering and Design
STRUCTURAL DESIGN OF PRECAST AND PRESTRESSED CONCRETE FOR OFFSITE PREFABRICATED CONSTRUCTION OF HYDRAULIC STRUCTURES
Distribution Restriction StatementApproved for public release;
distribution is unlimited.
i
DEPARTMENT OF THE ARMY EC 1110-2-6052U.S. Army Corps of Engineers
CECW-EI Washington, DC 20314-1000
CircularNo. 1110-2-6052 1 January 2001
EXPIRES 31 December 2002Engineering and Design
STRUCTURAL DESIGN OF PRECAST AND PRESTRESSED CONCRETEFOR OFFSITE PREFABRICATED CONSTRUCTION OF HYDRAULIC STRUCTURES
Table of Contents
Subject Paragraph Page
Chapter 1IntroductionPurpose......................................................................................................................1-1 1-1Applicability..............................................................................................................1-2 1-1Distribution ...............................................................................................................1-3 1-1References .................................................................................................................1-4 1-1Background ...............................................................................................................1-5 1-1Scope .........................................................................................................................1-6 1-1Mandatory Requirements ..........................................................................................1-7 1-2
Chapter 2Precast Concrete Applications, Components, Manufacture,and Structure ErectionApplications ..............................................................................................................2-1 2-1P/C Components: Manufacture and Classification ..................................................2-2 2-2Permanent Structures.................................................................................................2-3 2-3Erection of Prescast Members...................................................................................2-4 2-3
Chapter 3Materials for Precast ConcreteConcrete for P/C Units ..............................................................................................3-1 3-1Concrete Materials and Mix Design for In-Fill Placements......................................3-2 3-1Reinforcement ...........................................................................................................3-3 3-2Embedded Metals......................................................................................................3-4 3-3Lifting Devices, Couplers, and Connection Devices ................................................3-5 3-4Grout .........................................................................................................................3-6 3-5
Chapter 4Materials for Prestressed ConcreteConcrete ....................................................................................................................4-1 4-1Components for Prestressed Concrete (Pretensioning) .............................................4-2 4-1Components for Prestressed Concrete (Posttensioning)............................................4-3 4-2Materials Selection....................................................................................................4-4 4-3
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Subject Paragraph Page
Chapter 5LoadsIntroduction ...............................................................................................................5-1 5-1Stripping Loads .........................................................................................................5-2 5-1Handling and Storage Loads .....................................................................................5-3 5-1Transportation Loads.................................................................................................5-4 5-3Erection Loads...........................................................................................................5-5 5-6Ballasting...................................................................................................................5-6 5-8Lifting Loads.............................................................................................................5-7 5-8In-Service Loads .......................................................................................................5-8 5-9Progressive Failures ..................................................................................................5-9 5-9Floating Stability .....................................................................................................5-10 5-10
Chapter 6ConnectionsStructural Functions ..................................................................................................6-1 6-1Monolithic Action .....................................................................................................6-2 6-1Water Stops ...............................................................................................................6-3 6-1Watertightness Requirements....................................................................................6-4 6-2Joint Preparation........................................................................................................6-5 6-2Underwater Connections ...........................................................................................6-6 6-2Match Casting ...........................................................................................................6-7 6-4Bracing ......................................................................................................................6-8 6-5
Chapter 7In-Fill Concrete PlacementTremie Placement......................................................................................................7-1 7-1Placement Equipment................................................................................................7-2 7-1
Chapter 8DetailsCover Requirements..................................................................................................8-1 8-1Minimum Reinforcement for Beam, Plate, and Shell Elements ...............................8-2 8-1Spacing Requirements for Reinforcement ................................................................8-3 8-2Detailing Requirements for Prestressing Reinforcement ..........................................8-4 8-2Surface Treatments for Composite Action................................................................8-5 8-3
Chapter 9Strength and Serviceability RequirementsRelation to EM 1110-2-2104.....................................................................................9-1 9-1Strength and Serviceability Requirements ................................................................9-2 9-1Load Criteria .............................................................................................................9-3 9-1Serviceability.............................................................................................................9-4 9-10Prestressed Concrete Design Criteria ........................................................................9-5 9-12Reinforcement Requirements ....................................................................................9-6 9-14Ultimate Strength Design ..........................................................................................9-7 9-14Strength Design of Composite Members ..................................................................9-8 9-15Fatigue Design...........................................................................................................9-9 9-16
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Subject Paragraph Page
Appendix AReferences
Appendix BSpecification Requirements
Appendix CDesign Example
EC 1110-2-60521 Jan 01
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Subject Paragraph Page
Appendix AReferences
Appendix BSpecification Requirements
Appendix CDesign Example
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Chapter 1Introduction
1-1. Purpose
This engineer circular (EC) provides guidance for structural engineers in the design and construction of precastand prestressed hydraulic concrete structures. The primary emphasis is on float-in and lift-in type structures.
1-2. Applicability
This circular applies to USACE Commands having responsibility for Civil Works projects.
1-3. Distribution
This publication is approved for public release; distribution is unlimited.
1-4. References
Required publications are listed in Appendix A.
1-5. Background
Traditional construction methods used in navigation projects include construction of cofferdams and placementof cast-in-place and mass concrete. These methods result in extended construction schedules and disruptionto navigation traffic with corresponding large costs. Other construction methods have been investigated toreduce costs and to alleviate impacts to navigation and the environment. Prestressed and precast float-inconstruction has been successfully used in the construction of offshore oil drilling platforms for several yearsand recently in the construction of bridge foundations. This type of construction has also seen limited use inUSACE navigation projects. However, USACE has not previously published specific criteria for designs ofthis type.
1-6. Scope
a. This circular references industry standards, primarily ASTM, AASHTO, and ACI, for the basic designrequirements for prestressed concrete. These standards are then modified herein to make them applicable tothe hydraulic structures typical for Civil Works projects including float-in and lift-in construction. Guidanceis provided for the selection of materials, determination of loads, design for strength and serviceability, andthe development of details.
b. Specification requirements and three design example calculations are presented in Appendixes Band C, respectively.
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1-7. Mandatory Requirements
This circular provides design guidance for the design of USACE structures. In certain cases guidancerequirements, because of their criticality to project safety and performance, are considered to be mandatoryas discussed in ER 1110-2-1150. In this circular, the following requirements are mandatory: the multipliersof Table 5.1, the coefficients of Tables 5.2 and 5.3, the amplification factors of Table 5.4, the minimumconcrete cover of Table 8.1, the allowable crack widths of Tables 9.3 and 9.4, the allowable stresses for crackcontrol of Table 9.5, the multipliers of Table 9.6, and the allowable stresses of Tables 9.7 and 9.8.
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Chapter 2Precast Concrete Applications, Components,Manufacture, and Structure Erection
2-1. Applications
Sophisticated design and innovative construction techniques are becoming common practice withinUSACE to minimize project costs while maintaining or improving project quality, durability, andoperability. Use of precast concrete (P/C) construction can result in lower costs by more effectivematerial usage and reduced onsite labor. New designs are likely to combine precasting with cast-in-placeconcrete to provide composite action or to develop continuity.
a. Definition. P/C is concrete that has been cast into the desired shape prior to placement in astructure. P/C components can be designed and used to serve as dual functions: forms for cast-in-placeconcrete and as a durable exterior finish. P/C construction involves concrete forming, placing, finishing,and curing operations away from the project site and then erecting the P/C components as part of acompleted structure.
b. Advantages. There are several advantages with P/C construction. Precasting operations generallyfollow an industrial production procedure that takes place at a central precast plant. Thus, high concretequality can be reliably obtained under the more controlled production environment. Since standardshapes are commonly produced in precasting concrete, the repetitive use of formwork permits speedyproduction of P/C components at a lower unit cost. These forms and plant finishing procedures providebetter surface quality than is usually obtained in field conditions. P/C components may be erected muchmore rapidly than conventionally cast-in-place components, thereby reducing onsite construction time.P/C components can be designed as in situ forms for underwater construction so that the use ofcofferdams may be eliminated or substantially limited. The precasting process is also sufficientlyadaptable so that special shapes can be produced economically.
c. Combination of methods. The combination of precasting conventionally reinforced flat panelmembers joined with second placement concrete and posttensioning has proved economicallyadvantageous for several USACE projects. For example, this type of precast construction has beensuccessfully used for re-facing lock walls, tainter gate pier construction, and guide wall construction.
d. Previous successes. The so-called “in-the-wet” or “offsite prefabrication” construction is anextension of marine construction methods that have been successfully used. This innovative methodutilizes P/C modules as the in situ form into which tremie concrete or other infill material is placeddirectly without use of a cofferdam. The precast elements may contain all or much of the primaryreinforcement. The tremie concrete is designed to work in composite action with the P/C modules.Numerous investigations have been conducted and designs considered by several USACE districts toevaluate the feasibility of the in-the-wet method at various potential sites of U.S. waterways. Thesestudies have shown that the “offsite prefabrication” method can provide substantial benefits in cost,construction schedule, risk reduction, facility utilization, river traffic alleviation, and environmentalimpact.
e. Special requirements. P/C construction has its special requirements. First of all, additionalengineering effort is generally required to detail P/C components, develop construction sequences, specialconstruction tolerances, and specifications, and to optimize the design. Secondly, special labor crews andequipment may be needed to erect the P/C units. Where underwater erection and joining of P/C
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assemblies are involved, special efforts and techniques are required for positioning, installation, tolerancecontrol, inspection, and quality control.
2-2. P/C Components: Manufacture and Classification
a. Components. All P/C components are fabricated offsite as modules and erected onsite to becomepart of a completed structure. Individual P/C members may be prestressed or conventionally reinforced.The manufacture of P/C primarily involves three steps: (1) the assembly and installation of reinforcingand prestressing steel, (2) the production and placement of concrete and the subsequent curing, and (3) thelifting, storage, and loading-out of the completed component. P/C production is basically an industrialmanufacture process. It is appropriate to require more rigorous control on cleanliness, temperature, andmoisture of the aggregates and on accuracy of batching and mixing time, and on tolerance.
b. Manufacture.
(1) P/C normally requires more thorough curing than conventional cast-in-place concrete, becauseP/C sections are generally thinner and more highly stressed during handling, transportation, and erection.Therefore, adequate means of curing must be specified and enforced. Steam curing at atmosphericpressure is widely employed in P/C manufacture to accelerate the early-strength gain and permit dailyturnover of forms. The adoptions of a proper cycle of steam curing and subsequent water curing isessential for good quality P/C. When P/C products are removed from steam curing, the moistureextraction from the concrete is accelerated due to the change in temperature and humidity, so the productsmust be covered for protection from wind and rapid changes in temperature and moisture.
(2) External vibration is useful in eliminating surface defects but generally cannot extend its effectsmore than 6 to 8 in. into concrete. For thicker concrete panels, external vibration is not effective toconsolidate the inner portion of the concrete. Internal vibration is required for adequate consolidation ofconcrete.
c. Classification. In terms of their functionality and structural characteristics, P/C components usedin hydraulic structures can generally be classified into two categories: (1) P/C panels and (2) P/Cassemblies.
(1) Panels. P/C panels are the basic elements that can be used either individually as resurfacingpanels or used as structural components in a large precast assembly. During the construction stage,individual precast panels are mostly subjected to loads from lifting hoists and erection bracing. Designconsiderations for precast panels should follow the relevant guidelines in PCI MNL-120 “PCI DesignHandbook” and ACI-ASCE Joint Committee Report 550.
(2) Assemblies. P/C assemblies refer to large prefabricated concrete modules for “in-the-wet”construction of hydraulic structures. These assemblies are typically boxlike structures ranging from adozen feet to several hundred feet in length and width. They are erected at project sites, often underwater,as in situ forms into which tremie concrete can be directly placed without use of a cofferdam. For float-inconstruction, the P/C assemblies must have bottom plates to allow flotation and contain severalcompartments to allow sequential ballasting. For lift-in construction, the P/C assemblies normally do notcontain any bottom plate but are fitted with a lifting frame to distribute lifting loads.
(3) Precasting similar components. In P/C construction, substantial economy can be achievedthrough repetition of precasting numerous precast components of the same or similar shape and size.Although prefabrication of P/C assemblies can be carried out in several ways, the most common
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prefabrication method is to assemble a large P/C module by erecting smaller pieces of P/C panels/shellsand making closure pours at the junctions of these panels/shells. Sometimes, the individual panels arepretensioned and the assemblies are posttensioned to meet the strength and serviceability requirements.
(4) Analysis of critical loads. Due to their special transportation and erection process, the P/Cassemblies used for “in-the-wet” construction usually experience much more complex loads than thosefor P/C panels for smaller projects. To design large P/C assemblies, thorough engineering analysis mustbe conducted for all the critical load cases during their fabrication, outfitting, transportation, erection, andunder the service condition.
2-3 Permanent Structures
a. Prestressed members. Completed structures built from P/C components may be conventionallyreinforced, prestressed, or both. Prestressed concrete may be constructed by either the pretensioningmethod or the posttensioning method. Pretensioning is the imposition of prestress by stressing thetendons against external reactions before placement of fresh concrete in the forms, then allowing theconcrete to set and gain a substantial portion of strength before releasing the tendons so that the stress istransferred into the concrete. Posttensioning is the imposition of prestress by stressing and anchoring thetendons against already hardened concrete.
b. Reinforced members. Conventionally reinforced P/C members and systems shall be designed inaccordance with EM 1110-2-2104 except as modified herein. Prestressed concrete members and systems,whether they are cast-in-place or precast, shall be designed in accordance with this EC.
2-4 Erection of Prescast Members
For the “in-the-wet” construction, erection of P/C modules can be carried out by either float-in method orlift-in method or a combination of both.
a. Float-in method. The float-in construction method entails transportation of prefabricated largeconcrete modules from their casting yard or outfitting site to the project site through floatation and/or bymeans of external buoyancy tanks. Once the float-in modules are precisely positioned over the site with asuitable mooring system, guide piles, or taut lines and winches, they are lowered down to the preparedfoundation by means of ballasting. Float-in P/C modules usually take the form of floating structures withmany compartments for sequential ballasting.
b. Lift-in method. Lift-in construction entails transportation of prefabricated P/C modules from theircasting or outfitting yard to project sites by towed barges or floating cranes. The lift-in concrete modulesthemselves do not float. Heavy lift equipment must be used to control position of the modules whilelowering them down to the river bottom to acceptable erection tolerances. Auxiliary guiding systems,such as mooring systems, tensioned guidelines, and guide horns, are often used to assist the positioning.Installation of lift-in P/C segments is largely independent of water level but is somewhat constrained byriver flow velocity, with a normal upper limit of 2 meters per second (6 ft/sec) on basis of priorexperience. However, units have been installed in current up to 3 meters per second (10 ft/sec) wherespecial procedures have been implemented.
c. Selection of method. Selection of the erection method for the P/C modules is an important designdecision. Each erection method has its special implications to project cost, construction schedules, rivertraffic, towing and mooring system, positioning accuracy, and level of risks during construction. In many
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ways, the erection method will at least in part determine the size and configuration of the P/C modules,the foundation treatment, construction sequence, and schedule. In general, a thorough evaluation shouldbe made in the early stage of design to determine the effects of the erection methods, because the erectionmethod and equipment to install precast modules will affect the structural concept and layout, fabricationof P/C components, and construction logistics.
d. Effects on cost and schedule. The number of P/C modules and underwater joints between thesecomponents has significant effects on the construction cost and schedule. Underwater joining of P/Ccomponents is costly and difficult to perform, and the installation of numerous small P/C modules alsohas adverse implications to the cost and quality control. In principle, use of large prefabricated modulesusually provides considerable benefits in cost saving, project schedule, and construction quality control.However, the size of the P/C modules is primarily limited by a number of factors such as the draftrequirements and the lift capacity of crane barges that are available or economically obtainable.
e. Positioning the module.
(1) For in-the-wet construction, accurately positioning the precast modules underwater is one of thecritical operations. Underwater positioning of the precast modules requires intensive onsite coordinationof several operations, including surveying, sequential ballasting and/or lift crane maneuvering, operationsof guiding devices and hydraulic rams, positioning control with flat jacks, taut lines, and winches, andinspection by divers.
(2) Surveys carried out during the erection shall require a high degree of accuracy for vertical andhorizontal alignments of the P/C modules. Engineering specifications require multiple survey systemssuch as DGPS, lasers, and underwater sonic sensors. It is more reliable and expedient for the operators torely on spotting and controlling targets above water than on underwater instrumentation. Therefore, theland-based survey method and GPS should be the primary systems for monitoring the placement, whileuse of underwater sonic devices and divers are supplementary methods that cross-check the other surveyreadings.
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Chapter 3Materials for Precast Concrete
3-1. Concrete for P/C Units
In general, the standard requirements, recommendations, and restrictions applicable for cast-in-placeconcrete should also apply to P/C. Therefore, materials selection, mixture proportioning, and batching andmixing of P/C should generally conform to relevant provisions of EM 1110-2-2000 and follow therecommendation in the ACI Committee 211 reports, except for the special recommendations discussedbelow.
a. Compressive strength. Precast and prestressed concrete often has 28-day compressive strengthsin the range of 28 to 55 MPa (4,000 to 8,000 psi). Such concrete can be produced with reasonableeconomy, provided proper care is taken in mixture proportioning and concreting operation. With properuse of water-reducing admixtures and pozzolanic materials, it is realistic and desirable to control thewater-to-cementitious material ratio within the range of 0.35 to 0.43.
b. Cements and admixtures. High early-strength cements and/or accelerating admixtures aresometimes employed in P/C production in order to achieve quick form turnover and early prestressing.Such applications may be permitted for production of thin-walled panels. In precasting thick walls orslabs of large concrete masses, high early-strength cements and accelerating admixtures should beprohibited due to the potential problem of thermal cracking and excessive shrinkage.
c. Aggregate size. P/C usually has a relatively high cementitious materials content. Fine aggregatesshould grade in coarser ranges. The nominal maximum aggregate size should be in the range of 13 to20 mm (1/2 to 3/4 in.). In highly reinforced components, steel congestion may exist in certain areas suchas at the end blocks of posttensioning anchorage. In such cases, use of 10-mm (3/8-in.) maximumaggregate size is recommended to facilitate proper placement and consolidation of the concrete.
d. Advantages of lightweight aggregates. P/C made with lightweight aggregates sometimesprovides substantial advantages in handling, transportation, and erection. The aggregates are expandedshale, clay, or slate. They are light in weight because of the porous, cellular structure of the individualaggregate particles, achieved by gas or stream formation during processing the aggregates in rotary kilnsat high temperature. Concrete made with these lightweight aggregates usually has a unit weight between14 and 19 kN/m3 (90 and 120 pcf) compared with about 23 kN/m3 (145 pcf) for normal weight concrete.The strength of lightweight aggregate concrete can be made comparable to that of stone aggregateconcrete through proper materials selection, mix proportioning, and control of the water-cement ratio.Abrasion and erosion properties of lightweight aggregate concrete are not fully understood for marineexposure conditions. Until sufficient experimental evidence supports adequacy of the abrasion resistanceof lightweight aggregate concrete, lightweight aggregate concrete should generally not be used inapplications where the concrete is exposed to rapidly flowing water.
3-2. Concrete Materials and Mix Design for In-Fill Placements
The precautions normally applied to concrete materials should be applied to underwater concrete placedby the tremie method. Cement and pozzolans must meet appropriate specifications. Aggregates must beclean, sound, and evaluated for potential harmful chemical reactions. Mixing water must be clean andfree of harmful materials. Admixtures must meet appropriate specifications. All materials should betested to ensure compatibility.
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a. Cement and pozzolans. Selection of the appropriate type of cement and pozzolans must be basedon evaluation of service conditions, available aggregates, heat generation, and availability of thematerials. The acceptability of a particular pozzolan (workability, water demand, and rate of strengthgain) should be verified before final selection. Cementitious materials content must be adequate toproduce a flowable and cohesive concrete mix.
b. Aggregates. Well-rounded natural aggregates are preferred over crushed angular aggregatesbecause round aggregates generally produce concrete with increased flowability. Aggregates should bewell graded. The maximum aggregate size for reinforced concrete is usually in the range of 20 to 25 mm(3/4 to 1 in.) and, for nonreinforced concrete, in the range of 20 to 38 mm (3/4 to 1-½ in.). Fine aggregatecontent should be 42 to 50% of the total aggregate weight.
c. Desired characteristics. Tremie concrete should not be proportioned on the basis of strengthalone. Concrete for tremie placements must flow readily and be cohesive to resist segregation andwashout. Standard tests for these characteristics include the slump test, slump flow test, washout test,bleeding test, and time-of-set test. Appropriate uses and limitations of these tests and acceptable testresults are discussed in detail in WES Technical Report INP-SL-1.
d. Testing. The proposed concrete mixtures should be tested using standard ASTM tests forbleeding, time of set, air content, unit weight, slump loss, compressive strength, and yield to ensurecompatibility of components and suitability of the concrete for its intended purpose.
e. Final selection. Final selection of a concrete mixture should be based on test placements madeunderwater. Test placements should be examined for concrete surface flatness, amount of laitancepresent, quality of concrete at the extreme flow distance of the test, and flow around embedded features.
f. Temperature considerations. The potential temperature increase should be evaluated using asimple iterative or finite-element technique. Anticipated thermal gradients should be considered. Basedupon the predicted concrete temperatures and gradients and the nature of the concrete placement, adetermination of the seriousness of the prediction may be made. Maximum temperatures and gradientsmay be reduced by using a lower cement content, replacing cement with a suitable pozzolan andlimestone powder, precooling of aggregates, or lowering the placement temperature.
3-3. Reinforcement
Reinforcing steel for precast components is often prefabricated and preassembled on a template or stand,where the location of every piece is marked and wire ties are used to hold the members in place. Exceptfor welded wire or bar mesh prefabricated in the steel fabricator’s plant, tying and clamping of reinforcingbars should generally be used instead of welding. Full penetration butt weld and manual welding inprestressed concrete production should be prohibited. Nonprestress reinforcement generally consists ofdeformed bars or welded wire reinforcement (previously referred to as welded wire fabric). Reinforcingbars should be deformed except that plain bars may be used for spirals or for dowels at expansion orcontraction joints. Reinforcing bars are generally specified to be Grade 60. In some situations, Grade 40or Grade 70 reinforcement may be specified.
a. Reinforcing bars. The most widely used type and grade of bars conform to ASTM A 615 Grade60 and include bars with sizes from No. 3 through No. 11, No. 14, and No. 18. When welding is requiredor when more bendability and controlled ductility are required, such as in seismic-resistant design, low-alloy reinforcing bars conforming to ASTM A 706 should be considered. It is recommended to use
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smaller bars closer together than larger bars further apart to reduce potential crack width. Reinforcingbars should conform to one of the following ASTM specifications:
(1) A 615 Specification for Deformed and Plain Billet-Steel Bars for Concrete Reinforcement.
(2) A 706 Specification for Low-Alloy Deformed Bars for Concrete Reinforcement.
(3) A 767 Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement.
(4) A 775 Specification for Epoxy-Coated Reinforcing Steel Bars.
b. Welded wire reinforcement. Welded wire reinforcement (WWR) is a prefabricated reinforcementconsisting of cold-drawn wires welded together in square or rectangular grids. Each wire intersection iselectrically resistance-welded by a continuous automatic welder. Pressure and heat fuse the intersectingwires into a homogeneous section and fix all wires in their proper position. WWR may consist of plainwires, deformed wires, or a combination of both. WWR can also be galvanized or epoxy coated. Weldedwire fabric (WWF) conforms to one of the following ASTM standard specifications:
(1) A 185 Specification for Steel Welded Wire Fabric, Plain, for Concrete Reinforcement.
(2) A 497 Specification for Steel Welded Wire Fabric, Deformed, for Concrete Reinforcement.
(3) A 884 Specification for Epoxy-Coated Steel Wire and Welded Wire Fabric for Reinforcement.
c. Wire sizes. Wire sizes are specified by a letter, W or D, followed by a number indicating thecross-sectional area of the wire in hundredths of a square inch. Plain wire sizes use the letter W; deformedwire sizes use the letter D. Wire sizes from W1.4 to W45 and D2 to D45 may be specified. Wire spacinggenerally varies from 50 to 300 mm (2 to 12 in).
d. Corrosion protection. When coated reinforcing bars are required as a corrosion protectionsystem, the bars may be either zinc-coated or epoxy-coated and shall conform to ASTM A 767 or ASTMD 3363 (AASHTO M284), respectively. Epoxy-coated reinforcing bars are generally used in structuresthat are exposed to a saltwater environment.
e. Splicing reinforcement. The most common method for splicing reinforcing bars is the lap splice.The effect of congestion at lap splice to concrete placement should be considered in design. When lapsplices are undesirable or impractical, mechanical connections may be used to splice reinforcing bars.
3-4. Embedded Metals
a. Embedded metals shall be electrochemically compatible with reinforcing and prestressing steel toavoid galvanic corrosion. Aluminum, copper, and stainless steel should not be used as embedments unlesspositive measures are taken to ensure absolutely no contact between the embedment and reinforcing steel.
b. Steel embedments with exposed surfaces, such as anchor bolts, have a tendency to corrode, sincethey become the anode and the reinforcing steel becomes a large cathode to fuel the corrosion potential.Exposed steel anchor or other embedments should be epoxy coated or separated from reinforcing steelwith plastic spacers.
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c. There are other important considerations for both precast and prestressed concrete membershaving embedded metals. During steam curing of concrete, metal embedments in precast members mayexpand more rapidly than the surrounding concrete. Any significant thermal change in configurationduring steam curing may lead to the local cracking of concrete, which has little tensile strength whensteam is first applied. Provision of sponge rubber gaskets can be effectively used to eliminate theproblem.
d. Prestressed concrete components usually experience substantial dimensional changes due toelastic shortening and plastic deformation of concrete. The deformation may lead to cracking at thecorners of the embedments. Design considerations should be given to these dimensional changes, andmeasures should be taken to prevent stress concentrations at the embedments.
3-5. Lifting Devices, Couplers, and Connection Devices
A lifting device consists of two parts: the anchorage element embedded in the P/C and the attachmentelement, which is attached to the anchorage to fasten the lifting line to the component. To provideadequate strength, the anchorage should bear against the reinforcement. A simple and common device isto embed several loops of prestressing strands in the concrete, leaving the loop exposed for attachment ofthe crane hook. Selection of the correct lifting devices depends on a number of factors concerned with thetype, weight, configuration, thickness, and strength of the precast component.
a. Location. The location of lifting devices in the components should be carefully considered,taking full account of the special loading that will be imposed on the concrete as a result of tilting, lifting,or moving the component, including an allowance for impact. For example, raising a horizontally cast P/Cpanel to a vertical position may induce stresses in the concrete that exceed any loading that may beimposed on the panel after it has been installed in a structure.
b. Selection. Selection of the lifting device and its location should be based on the manufacturer’srecommendation and an engineering analysis of the proposed installation. The locations and details oflifting and handling devices should be shown on the shop drawings. When requested, the engineeringanalysis should be reviewed and approved by licensed engineers with sufficient experience with P/Cconstruction.
c. Couplers. Lap splice can be used for splicing of reinforcing bars #11 or less. Mechanicalcouplers should be used to splice larger bars. The splicing couplers include threaded bars with couplers,hydraulically forged couplers, and swaged couplers. As in all mechanical items, their use requires propercare in storage, protection in transport and erection, cleanliness, and precision in installation.
d. Mechanical connections. Mechanical connections can be categorized as compression-only,tension-only, and tension-compression. Various types of mechanical connections are available that willhandle both tension and compression forces. These connectors use a variety of couplers that may be coldswaged, cold extruded, hot forged, grout filled, steel filled, or threaded. In most compression-onlymechanical connections, concentric bearing transfers the compressive stress from one bar to the other.The mechanical connection then serves to hold the bars in concentric contact. Tension-only mechanicalconnections generally use a steel coupling sleeve with a wedge. This is only effective when thereinforcing bar is pulled in tension. In general, a mechanical connection should develop, in tension orcompression, at least 125% of the specified yield strength of the bars being connected. This is to ensurethat yielding of the bars will occur before failure in the mechanical connection. Most mechanicalconnection devices are proprietary and further information is available from individual manufacturers.Some have been tested and approved for cyclic fully reversible action and endurance against fatigue.
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Descriptions of the physical features and installation procedures for selected mechanical splices aredescribed in ACI Report 439.3R.
3-6. Grout
When P/C components are placed adjacent to each other, load transfer between adjacent members is oftenachieved through a grouted keyway. The keyway may or may not extend for the full depth of the member.The keyway is grouted with one of several different grouting materials, which are described below.
a. Site mixed or prepackaged grout. Either site-mixed grout or prepackaged grout may be used tojoin precast members. Site-mixed grout shall generally conform to the performance requirementsspecified in ASTM C 1107 in terms of compressive strength, early expansion, and shrinkage. Site-mixedgrout shall also meet specific project requirements in consistency, unit weight, and air content through therelevant tests specified in ASTM C 1107.
b. Grades. ASTM Specification C 1107 covers three grades of packaged dry hydraulic-cementgrouts (nonshrink) intended for use under applied load. These grouts are composed of hydraulic cement,fine aggregate, and other ingredients and generally only require the addition of mixing water for use.Three grades of grout are classified according to the volume control mechanism exhibited by the groutafter being mixed with water:
• Grade A - prehardening volume adjusting in which expansion occurs before hardening
• Grade B - posthardening volume adjusting in which expansion occurs after the grout hardens
• Grade C - combination volume-adjusting which utilizes a combination of expansion before andafter hardening
c. Performance requirements. Performance requirements for compressive strengths and maximumand minimum expansion levels are given in ASTM C 1107. Although these grouts are termed nonshrink,the intent is to provide a final length that is not shorter than the original length at placement. This isachieved through an expansion mechanism prior to any shrinkage occurring.
d. Cementitious materials. Different cementitious materials may be used to produce grout. Theseinclude portland cement, shrinkage-compensating cement, expansive portland cement made with specialadditives, epoxy-cement resins, and magnesium ammonium phosphate cement.
e. Epoxy resin grouts. Epoxy-resin grouts can be used between P/C members where increasedbonding and tensile capacity are required. When these are used, consideration should be given to thehigher coefficient of thermal expansion and the larger creep properties of epoxy grouts.
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Chapter 4Materials for Prestressed Concrete
4-1. Concrete
Concrete used for prestressed construction is characterized by high strength, because prestressed concreteis generally subjected to higher forces and high strength concrete results in more economical designs. It iscommon practice that the 28-day compressive strength of prestressed concrete ranges between 35 and55 MPa (5,000 and 8,000 psi). Use of high strength P/C permits substantial reductions in member sizesand dead load, allowing lighter members to span longer distance. High strength concrete has a higherelastic modulus than low strength concrete, so that loss of prestress force resulting from elastic shorteningof the concrete is reduced. Creep losses, which are roughly proportional to elastic shortening, are alsoreduced.
4-2. Components for Prestressed Concrete (Pretensioning)
a. Casting bed. Pretensioned concrete members are commonly produced in plant conditions. Thetendons, usually multiwire-stranded cables, are stretched between abutments. With the forms in place,concrete is cast around the stressed tendons. High-early-strength concrete is often produced with steamcuring to accelerate the hardening of concrete. After sufficient concrete strength is attained, the jackingpressure is released. The strands tend to shorten but are prevented from doing so because they are bondedto the concrete. The prestress force is transferred to the concrete by bond and no special anchorage isneeded. Tendon eccentricity can be varied along the length of the member by holding down the strands atintermediate points and holding them up at the ends of the span. Pretensioning is generally suited to massproduction of standard shape members.
b. Jacking and load-measuring equipment. In addition to the casting bed, pretensioning operationsrequire a number of special devices. Formwork should be designed with consideration of thermalexpansion and compression during concreting and curing. Jacking abutments should be designed formaximum jacking loads with an appropriate safety factor of 4 or above. All jacking and load-measuringequipment shall be calibrated according to the standard industry practice in PCI MNL-116 and AASHTOLRFD Bridge Specifications.
c. Stress measurement devices. Stress measurement devices include pressure gauges on hydraulicjacks, dynamometers, and/or load cells connected into the stressing system. Under all circumstances,stress induced in the tendons shall be determined by two independent measurement methods: direct stressmeasurement by pressure gauges or dynamometers or load cells, and force computed from the actualelongation of the strand based upon its physical properties and compensation adjustment.
d. Strand and splice chucks. Strand chucks and splice chucks should be capable of anchoringjacking loads positively and with a minimum of differential slippage.
e. Hold-down devices. For pretensioned concrete with deflected strand profiles, a hold-down deviceshould be fastened in such a way as to permit the necessary longitudinal movement and angular rotationdue to shortening of the pretensioned concrete member under release of prestress.
f. Pretension in load. The pretensioning load shall be applied in two increments. An initial load isapplied to the individual strands to straighten them and provide a reference point for measuringelongation of the strands. The final load is then applied.
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g. Jacking force release. For all but lightly pretensioned concrete, jacking forces shall be graduallyreleased by hydraulic jacks after concrete reaches adequate strength. For lightly pretensioned concrete,release of jacking forces by burning the strands with low heat may be permitted.
4-3. Components for Prestressed Concrete (Posttensioning)
a. In order to prestress concrete by the posttensioning method, usually hollow conduits or ductcontaining the unstressed tendons are placed in the member forms, to the desired profile before pouringthe concrete. The tendons may be stranded wires or solid steel rods. The conduit is fastened to auxiliaryreinforcement to prevent accidental displacement. After the concrete has gained sufficient strength, thehardened concrete itself is used to provide the reaction for the stressing jack. The tendon is anchored byspecial fittings at the far end of the member, stretched, and then anchored at the jacking end by similarfittings. Tendons are normally grouted in their ducts after they are stressed. The grout bonds to thetendon and to the inner wall of the duct, permitting transfer of force. Grouting improves corrosionresistance and increases ultimate flexural strength. Use of unbonded tendons is permitted only for specialcases such as temporary structures or transport condition.
b. A complete posttensioned system includes tendons (bars or strands), anchorage devices orbearing plates, ducts, end caps, grout tubes, couplers, and a corrosion protection system. A briefdescription of these components is provided below. Additional valuable information can be found in Post-Tensioning Institute Post-Tensioning Manual and AASHTO LRFD Bridge Design Specifications.
(1) Tendons and anchorage. Tendons can be high-strength low-alloy steel bars or strands. Tendonspass through ducts placed within the concrete and sometimes terminate at embedded bearing plates oranchorage devices (dead end). The embedded ends of the prestressing steel are anchored by a positivemeans rather than by gripping devices, which are vulnerable to slippage resulting from penetration ofgrout into the anchorage device. Tendons are anchored at the dead end with embedded bearing plates thatrange in thickness from 25 to 63 mm (1 to 2-1/2 in.). Stress is introduced by stretching the tendons at thelive end by a hydraulic jack. Live-end anchorage devices may consist of a wedge, bell, or flat platesystem. The dead-end termination points of individual tendons can be staggered from one another todistribute the transfer of load from the tendons to the concrete. Where there is considerable curvature ofthe tendons, the tendons are often stressed at both ends. Strands may also be continuous extending fromthe live end to a fixed loop or 180-degree bend (that acts as the dead-end anchorage) back to the live end.
(2) Tendon ducts or sheathing. Ducts encase the tendons to separate them from the surroundingconcrete for tensioning. The ducts also function as protection for tendons during placement of concreteand act as a part of the corrosion-protection system. Ducts shall be rigid or semirigid, either galvanizedferrous metal or polyethylene. Polyethylene ducts are corrugated to increase crushing resistance and tointerlock with surrounding concrete. After the prestressing operation is completed, the ducts enclosingthe tendons are filled with a portland cement grout. The exposed ends of the prestressing tendons (liveend) are encapsulated in concrete for corrosion protection. Polyethylene-sheathed strands, prefilled withgrout, are increasingly being used in posttensioning operations to provide a second barrier againstcorrosion.
(3) Anchor accessories and corrosion-protection systems. The concrete surrounding the anchoragesystem provides a level of corrosion protection for the tendons but, because of its porous nature, allowssome penetration of moisture that eventually leads to corrosion. Corrosion of tendons must be avoidedbecause it can lead to pitting, stress corrosion, and potential failure of the tendons. Since replacement oftendons is not an economical solution to address corrosion, special measures such as proper selection ofduct material and associated hardware are required to prevent or deter initiation of corrosion. A proper
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duct system prevents moisture penetration to the tendons, and the proper grout system provides thenecessary environment that inhibits the corrosion process. The anchorage system shall be, at a minimum,doubly protected against corrosion with the duct system providing one level of protection and the groutproviding the other. The duct system, including sheathing and all connections, shall be watertight orgastight. The grout system shall ensure that tendons are completely encapsulated and no air voids arepresent. Where aggressive chemical environments are encountered or where corrosion is of greatconcern, anchor plates and anchorage ends can be encapsulated in plastic. Nonmetallic sheathing is lesssusceptible to corrosion than metallic sheathing and provides better corrosion protection for the tendonwhen properly installed. End caps are placed over the live end of the anchors and anchor nuts or wedgesafter stressing is complete and excess tendon removed. End caps shall be filled with grout or anticorrosioncompound and should be fitted with a sealing device. Grout tubes extend from the sheathing to allowaccess for grouting. Couplers are available to splice tendons; however, tendons are produced in sufficientlengths to make the use of couplers less frequent.
(4) Epoxy-coated strands.
(a) An organic epoxy-coating over seven-wire prestressing strands can vary in thickness from 0.64 to1.1 mm (25 to 45 mils). Two types of coatings are available. A smooth type has low bond characteristicsand is intended for use in unbonded posttensioned systems, external posttensioned systems, and staycables. An epoxy-coated strand with particles of grit embedded in the surface is used in bondedpretensioned and posttensioned systems. In addition to the strand having an external coating, it can alsobe manufactured with the interstices between the individual wires filled with epoxy. This prevents theentry of corrosive chemicals, either by capillary action or other hydrostatic forces. This type of strandshould be specified when there is risk of contaminants or moisture entering at the ends of tendons. Epoxy-coated strand should comply with ASTM A 882 and ASTM A 416.
(b) For pretensioned applications with epoxy-coated strands where accelerated curing techniques areemployed, the temperature of the concrete surrounding the strand at the time of prestress transfer shouldbe limited to a maximum of 66 !C (150 !F). The epoxy coating will not be damaged if the concretecuring temperature is limited to the recommended value. PCI Report JR-383 provides more specificinformation on the use of epoxy-coated strands.
4-4. Materials Selection
a. Concrete. The minimum compressive strength (28 day) of the concrete in the anchorage zoneshall be 35 MPa (5,000 psi.). Concrete strengths higher than 55 MPa (8,000 psi) shall not be used withoutapproval from USACE (CECW-E). Higher concrete strengths may be considered if required and whencontrols over materials and fabrication procedures can ensure the required strength. The maximumconcrete aggregate size should be selected based on reinforcing bars placement.
b. Tendons. Posttensioning bars shall be of high-tensile alloy steel, conforming to the requirementsof ASTM A 722. Strands shall be ASTM A 416 with a minimum strength of 18.62 MPa (270 ksi).
c. Steel for reinforcement. Reinforcing steel shall be deformed bars conforming to ASTM A615,Grade 60. Welded bars and bars subjected to dynamic impact loads shall conform to ASTM A 706, Grade60.
d. Ducts. Galvanized steel pipe ducts shall conform to ASTM A 53 and the requirements specifiedin AASHTO LRFD Bridge Design Specifications, Division II, Article 10.3.3. To provide corrosionprotection, ducts for tendons shall be mortar-tight and nonreactive with concrete, tendons, and filler
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material. Ducts shall have an inside diameter at least 3 mm (1/8 in.) larger than the tendon diameter.Ducts shall be stiff enough to retain their profile and shape during concreting. Ducts shall also be strongenough to avoid rupture or cracking by accidental contact of the vibrator. In aggressive environments, thewall thickness of the metal ducts should not be less than 2.0 mm. Elsewhere, the minimum thickness ofmetal ducts may be 0.8 mm (black) or 0.6 mm (galvanized). Anchorage devices shall be encapsulatedwithin second placement high quality concrete.
e. Grout. Grout for bonded posttensioned tendons shall be made of Type I or II portland cement,pozzolans, potable water, and chemical admixtures described in (f) below. Fine aggregates with amaximum size of less than 300 μm may be used in the grout. However, the common practice is to useneat grout without aggregates. The ratio of water to cementitious material (w/cm) specified in the groutshall be below 0.43 and less than the w/cm ratio used to make the P/C. Adding pozzolanic additives aspartial replacement of cement is generally beneficial in terms of reduction of permeability and bleeding.Fly ash used in the grout shall conform to ASTM C 618. The dosage of fly ash is generally controlled inthe range of 10 to 25 percent by weight of cement.
f. Admixtures.
(1) The types and performance requirements of chemical admixtures that may be used in grout forposttensioned members are as follows:
(a) Water-reducing and retarding admixtures shall conform to ASTM C 494 standards for Type B,Type D, Type F, and Type G admixture. In addition, compatibility of these admixtures with the cement,mineral additives, and other admixtures used shall be established during the grout trial tests.
(b) Air-entraining admixtures shall conform to the requirements of ASTM C 260. Air entrainmentnot only enhances durabilty in a freeze-thaw environment but also increases cohesion and reducesbleeding of the grout. Typically fine aggregates are needed to develop a stable air void system.
(c) Antibleeding admixtures shall be tested for effectiveness in accordance with ASTM C 940.Resistance to bleeding is a very important consideration for grout in long and curved posttensioning ductsand vertical ducts. Antibleeding admixtures have a proven record in eliminating detrimental bleeding andenhancing the thixotropic property of the grout.
(d) Corrosion-inhibiting admixtures may be considered if the structure will be exposed toaggressively corrosive environments. Corrosion-inhibiting admixtures are currently not covered inASTM specifications. Approval of the admixtures shall be based upon independent laboratory testing andpast project experience.
(2) In the past, certain gas-forming expansion admixtures, such as aluminum powder and cokebreeze, were used in grout in an attempt to reduce voids in the hardened grout in posttensioning ducts.However, there has been no evidence to date to show that the use of gas-forming expansion admixturescontribute to volume stability in the grout. In general, expansion admixtures shall not be used in any groutmaterial.
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Chapter 5Loads
5-1. Introduction
P/C components are fabricated offsite and then transported and erected onsite to make part of a completedstructure. Under various construction and service conditions, they are subjected to different loads thatvary in magnitude, direction, and duration. This chapter identifies various loading conditions that areunique to P/C designs. General structural design load criteria for precast/prestressed concrete structuresare described in Chapter 9.
5-2. Stripping Loads
Stripping loads are determined by self-weight of the P/C member, the orientation of the member, impact,and suction between the precast product and the form, the number and location of handling devices, andany additional items that must be lifted such as forms that remain with the member during shipping. Inchecking the strength of a P/C component against concrete cracking when it is lifted off the form, formsuction should be added to the self-weight of the member as an external load. Form suction can becalculated by using a multiplier on the member’s self-weight as defined in Table 5-1. A more accuratemethod is to establish a suction load that is independent of the member’s self-weight and apply this loadover the form’s contact area with the P/C component.
Table 5-1Equivalent Static Load Multipliers to Calculate Stripping LoadsProduct Type Smooth Mold Multiplier (form oil only)
Flat, with removable side forms, no false joints or reveals 1.3
Flat with false joints and/or reveals 1.4
Fluted, with proper draft 1.6
Sculptured 1.7
5-3. Handling and Storage Loads
Handling and storage loads are mainly influenced by the orientation of the member, locations oftemporary supports, and location with respect to other stored members.
a. Member orientation. The most critical time in handling a precast member is when it is initiallylifted from the form. The concrete strength is lower and, in pretensioned members, the prestressing forceis higher than at any other time in the life of the member. To minimize concrete stresses due to theeccentricity of prestress, pretensioned flexural members are handled with lifting devices as close aspractical to the location where the member will be supported in the structure. With the exception ofmembers with pretensioned cantilevers, lifting devices are located near the ends (Figure 5-1).
b. Handling points. Concentrically prestressed or conventionally reinforced members are handled attwo or more points in order to restrict the concrete tensile stresses below the cracking limit. Normally, afactor of safety of 1.5 is applied to the concrete modulus of rupture. In addition, an impact factor isapplied to the dead weight of the member. Optimum lifting locations equalize positive and negativemoments in members of constant cross-section where the section modulus is the same at the top and
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Figure 5-1. Lifting device for precast concrete members
bottom. For example, members lifted at two points will have equal positive and negative moments if thelifting points are located at the 1/5th point (0.2 times the member length) from the ends. The use ofoptimum lifting locations is not always necessary, as long as the concrete stresses are within allowablelimits. In many cases, available handling equipment determines the lifting locations.
c. Lateral stability. Long, slender sections can become unstable when handled with lifting deviceslocated near the ends. The most important parameter for lateral stability during handling is the lateralbending stiffness of the member. Inclined slings, such as shown in Figure 5.1(b), may introducesignificant compression in the P/C member that should be included in calculation of buckling loads. Thesimplest method to improve lateral stiffness is to move the lifting devices in from the ends. However,doing so normally increases the concrete stresses at lifting and, sometimes, the required concrete releasestrength.
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5-4. Transportation Loads
A float-in P/C module is subjected to several significant loads during its transportation from itsprefabrication yard to the project site. The module should therefore be analyzed for its global responses,local responses, and stability. The global responses address stresses and deflections of the entire modulein response to external forces as a beam. The local responses include stresses in concrete plates or shellsbetween bulkheads or stiffeners under hydrostatic and hydrodynamic pressures. The floating stability of afloating module is addressed in paragraph 5-6.
a. Global responses.
(1) The first type of global loads on a floating structure is the still-water bending moment and shear.The buoyancy force distribution usually varies very little along the length of the structure, but the weightdistribution is often much more uneven along its length with presence of concentrated weights, such asthose due to pier walls, bulkheads, baffle blocks, and heavy equipment. The disparity between the weightand buoyancy distributions causes the still-water moment and shear in the module. To calculate theseforces, the module may be divided into about twenty segments. The net load distribution is obtained bytaking the difference between the cumulative weight and the buoyancy forces from these segments. Theshear and moment distributions are then the first and second integrals of this net load.
(2) Besides the still-water condition, the sagging and hogging conditions caused by passage of wavescan also induce significant forces in float-in modules. When the weight distribution of a floating structureis closely matched by the buoyancy force distribution, dynamic wave and wind loads may constitute 80%or more of the design loads. Figure 5.2 is a sketch of a floating module in the still-water condition, asagging wave condition, and a hogging wave condition. The lower part of the figure shows the weightdistribution and buoyancy distribution correspondence to the three cases. The sagging condition usuallyexacerbates the load effects of the still-water condition, while the hogging condition reverses the loadeffects of the still-water condition.
(3) To assess the sagging and hogging wave loads, design criteria should specify height and length ofthe design wave induced by river current, wind, and passing vessels. If the float-in module will remainafloat during its construction and outfitting for a substantial period, the design should include the StormWave condition pertinent to the construction site. Structural calculations must be performed to determinethe sagging and hogging components. These additional forces are then added to the still-water momentsand shear. The wave action must be checked in both the longitudinal and transverse directions of thefloat-in module, since they depend on the orientation of the vessel as well as the wave length and height.Any significant torsional moment induced by the design waves should also be included in structuralcalculations. These wave-induced forces are, by the rules of naval architecture, applied to develop themaximum bending moments and shears in the float-in modules.
b. Local responses.
(1) All the environmental forces should be determined and included in structural calculation. Thedesign loads may be based upon actual measurements at the site. In absence of field-measured data, wind,current, wave drift, and wave force shall be calculated using the following formulae:
shCCV.P 261050:Wind = (5-1)
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Figure 5-2. Shear force and moment distributions under still-water condition and sagging and hoggingconditions
where
P = wind pressure, Pa
V = wind velocity, m/s
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Ch = height coefficient as defined in Table 5-2
Cs = shape coefficient as defined in Table 5-3
Table 5-2Values of Height Coefficient Ch
Height from Water Surface to Center ofDesign Surface Area
0-15 m(0-50 ft)
15-50 m(50-100 ft)
30-46 m(100-150 ft)
46-61 m(150-200 ft)
Height coefficient Ch 1.00 1.10 1.20 1.30
Table 5-3Values of Shape Coefficient Cs
Shape of Components Shape Coefficient Cs
Hull 1.0
Deck house 1.0
Isolated structural shape (cranes, angles, channels, beams, etc.) 1.5
Cylindrical shape (all sizes) 0.5
Rig derrick 1.25
g
WVC
2:Current
2
= (5-2)
where
C = current pressure, Pa
W = water density, N/m3
V = current velocity, m/s
g = gravitational acceleration, m/s2
Resultant environment load: F = PAwind + CAcurrent (5-3)
where
Awind = area of segment above water
Acurrent = area of segment below water
(2) The towboat will be required to overcome bow waves, effect of static drift forces, wind, andcurrent. The influence of the pull force and wind loading on the draft is considered negligible.
c. Mooring. Contingency mooring should be provided along the route if the transportation of theprecast module takes more than 24 hours. Contingency mooring should also be provided at the outfittingsite if the site will be exposed to flood. The contingency moorings are designed for at least 100-yearflood. Single point moorings are typically used for the contingency mooring line, along with an anchor to
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prevent yaw. The magnitude and heading of wind, currents, waves, and dynamic excursions make itdifficult and risky to rely on two mooring lines acting efficiently at the same time. Transportation loadsdue to waves shall be determined in close coordination with hydraulic engineers.
5-5. Erection Loads
For in-the-wet construction, typical erection loads on a P/C module include mooring line forces,ballasting loads and/or lifting forces, in-fill concrete pressure, thermal pressure of in-fill concrete,concentrated loads from support points, environmental loads such as current and wave drift, and wavesurge forces.
a. Mooring line force.
(1) During erection of a precast module, a mooring system is usually used to position and hold thesegments while they are being lowered by a crane or ballasted down to the riverbed. The mooring forceshould be a sum of steady (static) loads and transient (dynamic) loads. The steady loads are the wind,current, and wave drift forces discussed in paragraph 5-4. Furthermore, there is a significant surfacewave surge when a large segment sets on the riverbed and extends over the water surface. The wave surgeis due to obstruction of wave flow by the segment. The wave surge force and the resulting overturningmoment should be included in structural calculations by using proper theories such as Molitor’s empiricalmethod (Quinn 1971). The resultant steady loads on mooring lines are as follows:
Fsteady = PAwind + CAcurrent + Fsurge (5-4)
where
Fsurge = wave surge force due to obstruction of wave flow.
(2) Transient loads include vertical wave load causing a vertical excursion of the modules that willstretch the mooring lines. In general, the transient load is considered only for unusual conditions, such asmajor flooding. The resultant mooring line loads are calculated without any load factors. The safetyfactors are built into the mooring line and anchor pile capacities. A safety factor of 5 to 6 is usually usedon the breaking strength of mooring lines. This is assumed to compensate for wear, impact, combinedstresses at bends over sheaves, and material uncertainties.
b. In-fill concrete pressure. In-the-wet construction typically utilizes P/C modules as in situ formsfor in-fill tremie concrete. As the in-fill concrete is being placed into the forms, the hydrostatic concretepressures on the forms increase proportionally. The form pressures in combination with the thermalexpansion of the in-fill concrete may dictate the design of the precast modules. Tremie concrete pressureon the precast modules should be adequately designed in accordance with EM 1110-2-2104 and TechnicalReport INP-SL-1. In principle, the magnitude of the form pressures primarily depends on the rate of theconcrete placement and the rate of the tremie concrete slump loss. If placement of concrete is slowenough to allow the concrete at the bottom to stiffen, the form pressure will correspondingly decrease. Asimple design method is to assume liquid pressure of the concrete without considering the time-dependentreduction of the form pressure. However, in some cases, neglecting the reduction of the form pressurescan result in uneconomical design of the precast module. As an alternative, a bilinear pressure diagram asshown in Figure 5-3 may be used as the design form pressure. In other words, the time period requiredfor the concrete to reach zero slump is recorded as t0. The form pressure is calculated as follows:
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0t t whentRWp <∗∗= (5-5)
0max t t whentRWpp >∗∗== 0 (5-6)
where
pmax = maximum form pressure
W = buoyant weight of concrete
R = placement rate
t = time lapse from initiation of the placement
t0 = time for concrete to reach zero slump
Figure 5-3. Design diagram of underwater concrete form pressure
c. Thermal loads. The thermal expansion of in-fill concrete due to heat of hydration should bechecked for “in-the-wet” construction of navigation structures. The thermal expansion of mass concreteand steep temperature gradient in the P/C form can lead to unacceptable cracks in the P/C. Thermal loadsmay be estimated with an approximate method such as the Schmidt method as described in ACICommittee 207.1 report. For major projects, it is recommended that the adiabatic temperature test of in-fill concrete and nonlinear incremental stress analysis (NISA) be conducted to determine the thermal loadand its effects.
d. Superimposed additional loads. Once a P/C module is set down on its supports or foundation,additional loads, such as lifting frame, equipment, and additional P/C components, are likely to beimposed upon the module. Unless the in-fill concrete reaches the design strength and acts compositelywith the P/C module, the module should be designed to carry all the imposed loads.
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5-6. Ballasting
a. Ballasting of a float-in P/C module may be performed at different construction stages for variouspurposes. Ballasting during launch or during outfitting is usually performed to trim or to lower the centerof gravity for required floating stability. Ballasting is also performed at the project site for the purpose ofsetting down the segment onto a prepared foundation.
b. Ballasting of a floating module is performed by filling its interior compartments with either solidor liquid ballast. Concrete may be used for permanent solid ballast. During all phases of transport andoutfitting, water ballast is usually used for minor trimming of the floating modules. Water is frequentlythe primary ballast material during the set-down operation. Gravel or sand may also be used for ballast.Iron ore has been used where high density ballast is required.
c. The staging of the ballast sequence and the amount of ballast at each stage shall be specified inthe design. The layout of compartments and sequence of ballasting shall be such as to reduce the overallstresses in the structure by filling the compartments in a pattern that results in a more uniform distributionof the dead loads. All compartment bulkheads and keel plates as well as the entire floating module shallbe designed to withstand the ballast loads at different stages of ballasting.
5-7. Lifting Loads
a. The lift-in P/C modules cannot float by themselves. Towed barges or floating cranes are usuallyused to transport and place the modules. The lifting operations of a crane do not represent one welldefined load case, but a sequence of different load cases. Uncertainties with respect to internal force dis-tribution and possible accident loads require an adequate safety margin. The designer should in principleconsider the entire lifting sequence step-by-step and identify the most critical dynamic load case for eachstructural member. The dynamic effects of lift operations can be influenced by a number of factors, suchas (1) environmental conditions, (2) motions of crane barges, (3) stiffness of lifting gear/ equipment,(4) lift weight, and (5) whether lift is in air or water. The dynamic effects are mainly due to accelerationof lifted objects. The dynamic effects may be significantly increased when lifting a heavy load off a bargein a wave environment, since the barge may drop suddenly before the crane has lifted the load.
b. In lieu of refined analyses, the dynamic effects may be included in calculations by means ofdynamic amplification factors, as defined in Table 5-4.
Table 5-4Dynamic Amplification Factor of Calculating Lifting LoadsLift Weight < 100 tons 100 – 1,000 tons 1,000-2,000 tons >2,000 tonsDynamic aplification factor 1.15 1.10 1.05 1.05
c. The basic design load case is the dynamic hook load. The design lift hook load shall becalculated as follows:
SL)WW(DF ra ++= (5-7)
where
F = design hook load
Da = dynamic amplification factor
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W = lift weight
Wr = weight of rigging
SL = special loads such as tugger line forces, guide forces, wind forces on the lifted object
d. To design the precast modules for the lift loads, a load factor of 1.7 shall be applied as amultiplier to the design load in order to account for uncertainties of load inaccuracy, local dynamiceffects, and consequences of failure.
e. One of the critical components is the connection of the lifting gear to the lift-in precast segments.The gravity loads from the segment have to be picked up by the lifting gear. Similarly, the lifting forcesneed to be distributed to the precast segment. Lifting points and their attachments to the precast modulesare to be designed for the maximum lifting load plus any lateral load component. The general rule is thatthe attachment points of the lifting gear to the precast segments should have an elastic capacity equal tothe breaking strength of the wire lines or rods. This is to ensure that any failure will occur only in theslings, not the structure. Typically the maximum negative moments occur at the attachment points. Toprevent excessive cracking in the segment, additional strengthening and reinforcement confinementaround the embedment of lift device (padeyes) are often provided. For odd-shaped precast segments,torsion must also be considered. Padeye plates should be oriented in such a direction that the possibilityfor out-of-plane loading of the padeye plate and on the shackle is minimized.
f. P/C modules can be subjected to high bending stresses near the picking points during liftingoperations. Therefore, the flat plate modules can be fitted with a structural steel frame to facilitate thelift-in operation. The steel frame is usually secured to the top of the module before launching. The framecan serve multiple purposes. First, it distributes the lift force from the crane hoist to the concrete segmentthrough many picking points, thereby reducing bending moments in the segment. Secondly, the framemay serve as a spotting tower for accurate positioning of the segment underwater. And thirdly, it mayserve as a guide frame for lowering tremie pipes into the segment at specific locations so that underwaterconcrete can be placed.
5-8. In-Service Loads
The loads that are expected to be imparted to navigation dams, locks, and their appurtenant structuresinclude hydrostatic pressure due to differential elevations, uplift, lateral earth pressure, tow impact,hawser loads, ice and debris, wave and wind loads, bulkhead and gate loads, seismic loads, thermal loads,superstructure and equipment loads, sheetpile or cofferdam tie-in loads, and monolith joint loads. Designloads on navigation dam structures should be specified in accordance with EM 1110-2-2607. Designloads on navigation lock structures should be specified in accordance with EM 1110-2-2602.
5-9. Progressive Failures
In addition to checking stability of a floating structure, structural engineers should also check its strengthagainst additional forces and pressure when the structure experiences significant trim/heel or accidentallocal damage. For permanently floating structures, the Progressive Collapse Limit State (PLS) should bechecked against catastrophic failure. PLS corresponds to the condition that failure of one member due toaccidental overloading leads to progressive failure of adjoining members. The PLS design is usuallyachieved by a combination of the structural redundancy and ductility. At the element level, individualmembers should be designed for adequate ductility. At the level of the structural system, compartments
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provide contingency against accidental flooding during float-out and transport. All designated wallsbehind the external walls should be watertight. A float-in structure is commonly designed for flooding ofone perimeter compartment. A permanently floating structure is commonly designed for simultaneousflooding of two compartments without significant impact on its stability and strength, that is, multipleload paths are provided in all critical regions.
5-10. Floating Stability
a. Floating structures and float-in modules must meet the design requirements for floating stability.In essence, the structures should be able to remain floating upright for all afloat conditions, includinglaunching and ballasting down, and under all the possible environmental conditions pertaining to the siteand the period. They should also have adequate reserves of stability when certain accidental damageoccurs.
b. A floating structure may lose its stability due to several destabilizing effects, such as flooding ofits compartments or uncontrolled lifts from a mounted crane. A stability check according to navalarchitecture principles should be made against all potential destabilizing effects.
Figure 5-4(a). Center of buoyancy, center of gravity, and metacenter
Figure 5-4(b). Effect of metacentric height
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c. There are three important parameters controlling the stability of a floating structure: the center ofgravity (G), the center of buoyancy (B), and the water plane moment of inertia (I), as shown in Fig-ure 5-4(a). A reference point is often established at midship on the keel (K). When a floating structureheels or trims, the buoyancy force acts vertically upwards through B to intersect the axis of the structureat the “metacentric point” (M), as shown in Figure 5-4(b). The buoyancy force also imposes a rightingmoment on the structure. The righting moment is the product of the displacement and righting arm (GM)(sin θ) (GM, θ, Figure 5-4(a)). sin θ may be replaced by θ for small angles of list. Stability of thestructure requires that the righting moment restore the structure to the upright floating position onceexternal forces causing the heel or trim are removed. In naval architecture, this stability requirement
implies that the metacentric height should always be positive. In practice, the metacentric height (GM ) isusually kept above 1 meter (3.3 feet) for all directions of inclination. This stability requirement can betranslated into the following mathematical equation:
meter01.BMKGKBGM ≥+−= (5-8)
where KB and KG are the distance from the keel to the center of buoyancy and the center of gravity,respectively. BM is the distance from the center of buoyancy to the metacentric point. BM can becalculated as follows:
structuretheofntdisplacemethe
planewatertheofinertiaofmomentthe
V
IBM == (5-9)
For any rectangular structure, 12
3lbI = and .bldV = b, l, and d are the beam, length, and draft of the
structure, respectively.
d. Floating stability shall be checked for all the possible cases of flooding. There are a number ofreasons for flooding a floating structure, e.g., boat impact, incorrect valve operation, or ballasting down afloat-in module. One principal method of controlling stability is to subdivide a floating structure into asufficient number of small compartments so that accidental flooding is limited to a small part of thestructure.
e. If one or more compartments are partially filled with water or other liquid, the internal waterplanes will cause a shift of the center of gravity further away from the center of buoyancy upon heel ortrim of the structure. The net effect, often referred to as “free surface effect,” is a reduction in stability andthe metacentric height. The free surface effect can be approximately accounted for by subtracting itscontribution from BM as follows:
∑−=i
ii rAV
IBM 2 (5-10)
where Ai is the free surface area in a partially filled compartment and ri is the distance from the freesurface to the axis of the water plane of the entire structure in the direction of rotation.
f. Some float-in structures have partially or fully open tops during transport. Adequateconsideration must be given to the potential for overtopping, such as by waves or due to unintended list,and even to rainwater, because even a small amount of floodwater can lead to significant free-surfaceeffects.
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g. To provide protection against accidental flooding, the damage control measures require that allmanholes, hatches, and bulkheads in the compartments be sealed watertight and designed to withstand themaximum pressure head of the accidental flooding. The design also requires that all pipes and ducts beclosed off during nonusage periods so that flooding does not spread through those systems.
h. In ballasting a float-in segment down to riverbed, it is critical to check the stability when thesegment commences immersion into water. Since the waterline plane diminishes at that instance, themoment of inertia diminishes rapidly. Special caution must be taken to ensure adequate metacentricheight (at least 1 meter) for the stability of the float-in segment in all directions of inclination. Duringsetdown for a float-in segment with a top deck, if the segment is ballasted down uniformly so that theentire top deck becomes awash at approximately the same time, the segment will lose a significant portionof the water plane and may behave unpredictably. It is, therefore, good practice to purposely tilt thesemgent in one direction (usually in the longitudinal direction) so that the water plane is reducedgradually while the mooring lines and guide device hold the segment in place.
i. To check the stability of a float-in segment during submergence, further calculations should bemade to take into account (1) dynamic stability of the segments during final immersion, (2) effects ofsloshing of the free surface water on the overall stability, (3) effects of mooring lines, and (4) effects ofpressure reduction due to increased velocity under segments during final stages of setdown.
j. Lifting loads from a floating structure can substantially change its stability condition. In stabilitycalculations, the weight of the loads should be assumed to act at the height of the upper support point(e.g., the sheave blocks at the tip of the crane boom). In such cases, the center of gravity can be very high.The stability becomes excessively sensitive to the instantaneous transverse moment of inertia.
k. The above formulae are useful tools for quick assessment of the hydrostatic stability of a floatingstructure. They are valid for small heel/trim rotational angles. If a floating structure may experiencesubstantial trim/heel rotation, stability calculation should be based upon the righting moment stabilitycriterion, as specified by American Bureau of Shipping (1980, 1985).
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Chapter 6Connections.
6-1. Structural Functions
In terms of their structural functions, joints in hydraulic structures may be divided into momentconnections, hinged connections, expansion/contraction joints, and isolation joints. Offsite prefabricationand underwater installation of P/C modules implies that loads are generally transferred through a discretenumber of connections which may be more critical than those in the conventional cast-in-placeconstruction. Connections to tie superstructures into pile foundations and to join precast segements into amonolith are of paramount importance. Both temporary and permanent connections must be designedwith careful attention to details and construction procedure to ensure the critical load paths and durabilityperformance.
6-2. Monolithic Action
Each structural unit that incorporates P/C components shall be designed to act as a monolithic structuralsystem. Specific joint details shall be developed to accomplish monolithic action. Connection details formoment connections shall assure full transfer of moment, shear, and axial forces across the joints betweenadjacent components. Connection details for hinged connections shall assure full transfer of shear andaxial forces. Connection details for expansion/contraction joints shall assure full transfer of shear.Connection details for isolation joints shall assure no transfer of force and moments across the joints andallow free movements of adjacent components. This design approach requires the structural engineer tocoordinate the joint design with cost estimators and construction field personnel to develop cost-optimized P/C systems. Energy-absorbing designs for P/C structural systems (non-monolithic jointdetails) may have economical applications in seismically active regions. Designs based on thisphilosophy require approval from HQUSACE.
6-3. Water Stops
a. Materials. Water stops can be classified as rigid or flexible. Suitable rigid water stops are madeof copper or stainless steel. These water stops are costly and require special handling care to avoiddamage. For these reasons, flexible water stops are generally preferred. Flexible water stops can be madeof polyvinyl chloride (PVC), butyl, neoprene, and natural rubber. PVC water stops are thermoplastic andcan be easily spliced on the jobsite.
b. Reasons for failures. Most joint deterioration problems can be traced to failure of water stops.Improperly installed or damaged water stops permit seepage through monolith joints which in turnaccelerates penetration of deleterious agents into concrete around the joints. Water leakage also causes theconcrete to become critically saturated and thus susceptible to freeze-thaw damage. Past experienceshows that failures of water stops are primarily due to the following reasons: (1) excessive movement ofthe joint which ruptures the water stop, (2) honeycomb in concrete adjacent to the water stops due to lackof consolidation, (3) contamination of the water-stop surface which prevents bond to the concrete,(4) puncture of the water stop or complete omission during construction, and (5) breaks in the water stopdue to poor or no splice.
c. Criticality of selection and installation. Proper selection and installation of water stops arecritical to durability of hydraulic structures. In general, water stops must be capable of accommodating
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the anticipated movements of the adjacent concrete as the joints go through thermal movements.Adequate attention shall be paid in design and construction to allow sufficient consolidation of concretearound water stops. Engineering specifications shall require rigorous quality control and inspection forinstallation of water stops. A high degree of workmanship and special attention to splicing, intersection,and supports of water stops are essential. Conventional water stops shall not be used for underwaterconnection as discussed in paragraph 6-6.
6-4. Watertightness Requirements
a. Watertightness requirements shall be based on project-specific requirements. Joints for permanentfloating structures such as floating guard walls shall be watertight. Water-tightness of float-in structuresis less critical due to their temporary float condition.
b. Leakage through concrete walls and slabs is an important design consideration if there is asubstantial differential hydrostatic pressure on two sides of a concrete wall. In addition, water flowthrough the joints and cracks could cause long-term durability deterioration, such as leaching andreduction in corrosion resistance and freeze-thaw resistance.
c. In general, a partially cracked concrete section with a compression zone of no less than 30 mm(1.2 in.) is essentially free of leakage. Any section of a watertight component should be designed to havea compression zone no less than 30 mm (1.2 in.) and 0.25h in thickness (h is the overall thickness of thecomponents), whichever is less. For permanently floating structures, the watertight components aregenerally required to maintain a minimum compressive stress of 0.5 MPa (70 psi) across the section underservice loading conditions. The compression should take thermal strains into account. For temporaryloading conditions during construction and installation, tensile stresses up to 2 MPa (300 psi) areacceptable. Crack widths are generally checked at critical locations against static loads plus 40 percentdynamic loads.
6-5. Joint Preparation
a. Precast panel joints generally receive a light sandblast prior to assembly. Other methods such asa moderate pressure wash have been used successfully.
b. For underwater joints, exposed P/C mating surfaces could be severely contaminated withalluvium settlement that prevents bonding of underwater concrete or grout with the precast surface. Thus,wherever is possible, the joint surface should be immediately sealed from flowing water once thesegments are lowered to their final positions. Any exposed joint surface shall be cleaned with underwaterjetting prior to grouting or concreting.
6-6. Underwater Connections
a. Underwater construction of structural connections usually entails joining two or more P/Csegments and filling the voids between the segments with concrete, cement grout, or polymer grout. Forisolation joints, the filling materials may be rock, gravel, or polymer foam covered with concrete pavingblocks. In essence, construction of underwater connections entails three essential steps: (1) positioningtwo or more P/C segments to prescribed tolerances, (2) sealing the joint space between the segments, and(3) grouting or concreting the sealed joint space to make it a monolithic connection.
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b. The key to construction of underwater connections is the joining of two P/C sections throughunderwater grouting or concreting. Since the underwater operations have to be carried out in adverseconditions, with poor visibility and difficult accessibility, construction operations should be carried outabove water as much as possible. It is important that connection detail design facilitates simple andreliable construction. The design of underwater connections shall include or take into account theattainable tolerances, position-guiding system for mating the adjacent precast segments, sealing of thejoints, performance requirements of grout or concrete mix, grouting procedure, grouting system, and ventsystem.
c. Conventional water stops are generally not suitable for construction of underwater connections.To seal a joint, primary considerations should be given to preformed compression seal, Omega seals, Ginaseal, or J-seal so as to confine a space in the joint prior to filling the void with suitable materials.Selection of proper material and type of seals depends on the joint design details, installation procedure,and environmental exposure condition.
d. Strict onsite enforcement of engineering requirements and quality control is paramount for goodquality underwater connection. Under most circumstances, the effectiveness of divers’ inspection is verylimited due to the poor visibility. Divers’ inspection should be limited to such activities as checking thejoint seal and outflow from vents. In principle, onsite monitoring and quality control should be mainlycarried out above water. The critical items that need careful monitoring include (1) positioning of adjacentP/C segments, (2) sealing of the joints, (3) rate of grouting or concreting placement, (4) grout or concretedelivery system (leakage, plug, or spill-over), (5) venting system, and (6) complete grouting of the joints.Field trial testing and post-construction coring shall be considered as a part of the QA/QC program
e. In essence, design of underwater connections shall meet the following requirements:
(1) Durability requirements. Past experience shows that joint deterioration is the most commondurability problem in hydraulic structures. Common causes of joint failure include leakage-inducedfreeze-thaw damage and corrosion, spalling of monolith joints due to impact, and abrasion. Underwaterjoints are especially vulnerable to physical and chemical attacks. In principle, underwater connectionsshall be designed for durability against freeze-thaw deterioration, chemical attack, abrasion, erosion,cavitation, spalling due to reactive aggregates, and corrosion of reinforcing steel. Careful designconsiderations should be given to accommodate the special conditions for underwater construction.
(2) Strength reduction factors. Because of the poor visibility and difficult accessibility of underwaterwork, there is a higher degree of uncertainty associated with the quality and integrity of underwaterconnections than those of above-water connections. To take full account of the increased uncertainty inunderwater works, structural design of underwater connections shall use a strength-reduction factor of 0.6for flexure and axial tension and 0.57 for shear and torsion. There is no change in the load factors forunderwater connections.
(3) Required characteristics.
(a) Construction of underwater connections entails joining P/C modules, sealing the joints fromexternal water, and grouting or concreting the voids with suitable materials. The first requirement of theconnection design is to ensure simple and reliable execution of these three steps underwater. To this end,the selection of the seal, the grout mix, the sequence and rate of grouting, and the vent system shall be anessential part of the design.
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(b) All the design forces and moments shall be adequately transferred through the connection bymeans of shear keyways, reinforcement splicing, and composite action of P/C and underwater grout orconcrete.
(c) Underwater connections shall have durability characteristics at least equivalent to those of above-water connection.
(4) Common details. At this writing, most existing connection details were developed for thetraditional, in-the-dry method. Precaution shall be taken in using these details for the in-the-wetconstruction, because these connection details may not be suitable or constructable underwater or may nothave the required characteristics for underwater connections.
(5) Current research. Reference to ongoing research at the U.S. Army Engineer Research andDevelopment Center is recommended.
(6) Monitoring. Instrumentation for monitoring construction and performance of underwaterconnections is recommended.
f. In general, underwater connections are more costly and more complex to construct than above-water connections. It is often beneficial to minimize the number of underwater connections without anincrease in overall project costs.
6-7. Match Casting
a. Match-cast precast products are typically used in segmental construction to ensure the proper fit-up of mating surfaces between precast segments while providing for the profile grade and horizontalalignment required by design. For hydraulic structures, match casting applies to horizontal joints (e.g.,stacking one element on top of another) and to vertical joints where one element fits tightly against anadjacent element.
b. Two basic techniques are used to match-cast P/C segments, one employing a stationary form, theother involving a form that is moved for every casting. With the stationary form, the first segment is castwith endplates at both ends of the form. After this segment has been cured to a concrete strength adequatefor stripping, it is lifted out of the form and positioned adjacent to the form so that one of its ends servesas the endplate for the match-cast end of the second segment. The other end of the second segment isformed with one of the original endplates.
c. The positioning of the first segment relative to the form is critical, since it dictates the alignmentof the two segments in the completed structure. Sophisticated surveying techniques, together withadjustable screw jacks and stops, are normally used to accurately position the segment. Prior to casting,the match-cast end of this segment is coated with a debonding agent to allow separation of the segmentsafter casting. After the second segment achieves stripping strength, both segments can be stripped fromthe form. The first segment is moved to storage, while the conventionally formed end of the secondsegment assumes the role of the endplate for the third segment to be cast.
d. The “moving form” technique begins in a similar manner. However, after the first segment is castand cured, it is left stationary on the form pallet. The form is stripped, moved longitudinally, andpositioned adjacent to the first segment. The second segment is then match-cast against the first in thesame manner as described above. This approach has the advantage of decreasing segment handling butrequires multiple form pallets and significantly more space.
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e. A common method of joining match-cast segments is by “cementing” them together with a thinlayer of epoxy bonding agent, approximately 0.5 to 1.0 mm (0.02 to 0.04 in.) in thickness. Because theepoxy coat is thin, it is essential that the member ends be properly matched. The normal constructionsequence begins with the application of a slow-setting epoxy to the mating ends. The epoxy should beapplied in accordance with the manufacturer's recommendations. The ends are then assembled, and aninitial posttensioning force is applied across the interface. The best bonds across joints are obtained whenthe epoxy cures under a compression stress of about 0.27 MPa (40 psi). This is done progressively foreach pair of match-cast segments. Once a predetermined number of segments have been joined, and theepoxy in all joints has cured, a final posttensioning force is applied to the superstructure (or a portion ofthe superstructure).
6-8. Bracing
Bracing and guying methods shall be designed to support all construction loads including wind.Selection of connections shall give due consideration to the size, location. and capacity of the precastpanel and dead man.
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Chapter 7In-Fill Concrete Placement
7-1. Tremie Placement
a. Basic assumptions. These recommendations are based on the procedure by which concrete isplaced underwater using gravity flow through a tremie pipe. The concrete that is first placed into thetremie pipe is protected from water by a mechanical device (“pig” or “plug”) until a mound of concretehas developed at the mouth of the tremie and a seal is developed. The mouth of the tremie pipe isthereafter maintained within the mass of fresh concrete. The tremie pipe is normally not movedhorizontally while concrete is flowing. Horizontal distribution of concrete is accomplished by the flow ofconcrete from the tremie mouth or by stopping placement and repositioning the tremie and resumingplacement. Multiple tremies may be used to place larger areas.
b. Placement considerations. Several basic concepts are important when planning the layout andconfiguration of precast components that are filled with either structural or nonstructural tremie concrete.First, the chambers or voids to receive tremie concrete should not contain abrupt changes in section,openings, or pockets where laitance could accumulate. Second, reinforcement or embedded metalsshould be detailed to minimize restrictions to the flow of concrete. Lastly, the placement size (chamberdimensions) must be compatible with concrete production rate, equipment placement capacity, and thedepth of the concrete placement. Tremie spacing is based on the flow distance of the concrete. Flowdistances ranging from 5 to 20 m (16 to 66 feet) have been used with success.
7-2. Placement Equipment
a. Tremie pipe. The tremie pipe should be fabricated with structural steel pipe. A jointed tremiepipe configuration is recommended where deep placements are made to allow removal of upper sectionsof pipe as the placement depth increases. These joints shall be watertight, either bolted flange plates orthreaded screw are commonly used. The pipe and appurtenant features are designed for anticipatedhandling forces. The diameter shall be sufficient to prevent aggregate blockages; typical diameters varybetween 200 and 250 mm (8 to 10 in.) for the aggregate sizes given in paragraph 3.2.b. A suitable endclosure device or go-devil is provided to seal the tremie at the beginning of placement. A funnel orhopper is provided to direct concrete into the tremie. The tremie pipe should be marked to allow thedepth from the water surface to the concrete to be determined quickly.
b. Placement platform and other equipment. A stable platform must be maintained to provideadequate controlled support to the tremie pipes, since any movement of a supporting platform will causeexcessive disturbance to concrete, resulting segregation, and laitance. Thus, floating supports and cranebooms should not be used to hold the tremie pipe. A separate hoist system is required for controlledvertical tremie movements; a crane is usually not suitable for this purpose. A crane is needed to move thetremie pipe or to lift the tremie pipe out of the water. Airlifts or pumps are required onsite to removeunsuitable materials from low areas during placement.
c. Restrictions. Tremie concrete should not be vibrated. Divers should never be allowed to walk ona tremie concrete surface until it has achieved final set. After the concrete has reached a compressivestrength of 7 Mpa (1,000 psi) or higher, water jetting may be used to remove laitance left over theconcrete surface.
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Chapter 8Details
8-1. Cover Requirements
Minimum concrete cover requirements vary with the application of P/C components, as specified inTable 8-1.
Table 8-1Concrete Cover RequirementsApplication Minimum Concrete CoverSurfaces exposed to cavitation or severe abrasion and erosion. 100 mm (4 in.)
Surface exposed to moderate abrasion and erosion or where watervelocity exceeds 8 ft/sec
75 mm (3 in.)
Surface exposed to earth, weather, or where water velocity is lessthan or equal to 8 ft/sec
50 mm (2 in.)
Surface adjacent to structural in-fill placements One bar diameter for No. 14 or larger1 in. for No. 11 bars and smaller
8-2. Minimum Reinforcement for Beam, Plate, and Shell Elements
For P/C members, the minimum reinforcing steel area shall be the largest value of the followingrequirements:
a. For rectangular flexural elements, the minimum reinforcement area shall not be less than As givenby
bdf
'fA
y
cs 4
= (8-1)
and not less than 1.4bd/fy.
where
cf ′ and yf = specified compressive strength of concrete (in MPa) and specified yield strength of
reinforcement (in MPa), respectively
b and d = width and effective depth of the rectangular elements
b. For slab/wall panel elements, the minimum reinforcement ratio 0040.min =ρ with one-half oneach face in each direction. In addition, if a P/C panel will bond to cast-in-place concrete as a compositemember in the final structure, the minimum reinforcement steel area shall be 0.0028 times the gross cross-sectional area of the composite member, but not exceeding a steel area equivalent to No. 9 bars at 0.3 m(12 inches) in each face.
c. For flat slab in which computed tensile stress in concrete at service load exceeds (1/6) 'fc ,
reinforcing steel shall not be less than As calculated as follows:
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y
cs f.
NA
50= (8-2)
where Nc is tension force due to unfactored dead plus live load and fy shall not exceed 420 MPa.
d. If tensile stress occurs on one face of a P/C member during construction, transportation, or in ser-vice, the minimum steel area on the tensile face of the member shall not be less than As in Equation 8-3.
ey
ts bd
f
fA = (8-3)
where
ft = tensile strength of concrete
de = effective tension zone, to be taken as 1.5c+10db
c = concrete cover over reinforcement
db = diameter of reinforcement
8-3. Spacing Requirements for Reinforcement
Spacing of reinforcement at critical section shall not exceed two times the member thickness. In general,reinforcement in P/C slabs and walls shall not be spaced further apart than three times the memberthickness, nor 450 mm (18 in.).
8-4. Detailing Requirements for Prestressing Reinforcement
a. Prestressing tendons shall be confined within the reinforcing steel stirrups in webs and betweenlayers of transverse reinforcing steel in slabs, walls, and flanges.
b. Curved prestressing tendons shall be adequately confined by lateral reinforcement. Spacing of theconfinement reinforcement shall not exceed either three times the outside diameter of the duct or 600 mm(24 in.).
c. When the tendons curve away from the longitudinal member, such as at a typical intermediateanchorage as shown in Figure 8-1, prestressing force will produce high transverse force that is radial tothe tendons. The transverse in-plane force tends to shear off the concrete cover and split the concrete atthe junction. The transverse deviation force must be calculated, and fully anchored tieback stirrups mustbe provided accordingly to resist the transverse force.
d. When posttensioning ducts are spaced closer than 300 mm (12 in.) in slabs, the top and bottomreinforcement mats should be tied together with No. 10 hairpin bars. The spacing between the hairpin barsshall not exceed 450 mm (18 in.) or 1.5 times the slab thickness.
e. The clear spacing between straight posttension ducts shall not be less than 38 mm (1.5 in.) or1.5 times the maximum size of the coarse aggregates. The clear horizontal spacing between post-tensioning duct bundles shall not be less than 100 mm (4 in.). The clear vertical spacing between the ductbundles shall not be less than 38 mm (1.5 in.) or 1.5 times the maximum size of the coarse aggregates.
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Figure 8-1. Design requirements for curved prestressing tendons
8-5. Surface Treatments for Composite Action
a. When P/C shell structures are designed to behave compositely with cast-in-place or tremieconcrete, shear must be transferred across the interface between the two concrete layers. The twoseparate concrete placements are intended to act as a unit when resisting externally applied loads. Thesurface treatment of the precast component influences the mechanism of shear transfer across theinterface with tremie concrete. Surfaces can be smooth formed, sandblasted, intentionally roughened,corrugated, or patterned with shear blocks or holes. Corrugated surfaces with sufficient amplitudedevelop the shear capacity of concrete.
b. Typical designs use the "shear friction" concept at the interface. Design advantages are realizedwhen the surface of the precast member, which will interface with cast-in-place concrete, is intentionallyroughened to full amplitude of approximately 1/4 in., although the shear friction concept does not requireroughening. Roughening of surfaces is very common in the precast industry. Methods used depend uponwhether the surface to be roughened is exposed or formed.
c. A requirement common to both exposed and formed roughened surfaces is that they must beclean and free of laitance prior to placing the cast-in-place concrete. It is also generally desirable tomoisten the precast surface prior to the second placement.
d. The standard method of roughening exposed surfaces is to “rake” or “broom” the concrete whileit is still in its plastic state. After the concrete has been struck level, a workman rakes the surface with atool that creates grooves at a specified spacing and depth. These grooves normally run transverse to the
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direction of the anticipated shear force, and must be deep enough to produce the desired roughness, butnot so deep so as to dislodge individual aggregate particles near the surface.
e. Formed surfaces cannot be roughened in the same manner as exposed surfaces. Several methodsused to roughen formed surfaces include chemical surface set retarders, deep sandblasting or shotblasting, castellations, shear keys, and corrugated surface.
f. Surface set retarders, which locally retard the setting of cement, are painted onto the form in thedesired location prior to casting the concrete. After form removal, the retarder is pressure washed fromthe concrete surface, resulting in a roughened, exposed-aggregate finish. Set retarders are formulatedwith different strengths to result in varying depths of retardation. Normally, the strongest formulation isrequired to achieve the roughness desired for composite action. Sandblasting and shot blasting are donemanually after the product is stripped. They are labor-intensive. Shear keys and castellations are formedinto the concrete surface. Roughened formed surfaces are normally used at the interface with cast-in-placeconcrete joints.
g. Bush hammering is not recommended since it may actually degrade the bond strength at thecomposite interface.
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Chapter 9Strength and Serviceability Requirements
9-1. Relation to EM 1110-2-2104
This chapter on strength and serviceability requirements of precast/prestressed concrete expands theguidance in EM 1110-2-2104. In general, design of nonprestressed P/C members should follow theprocedures outlined in EM 1110-2-2104 except as modified herein. Design of prestressed concrete shouldfollow specific design procedures provided in this chapter.
9-2. Strength and Serviceability Requirements
All precast/prestressed concrete members of hydraulic structures must satisfy both strength andserviceability requirements. In general, special consideration shall be given to effects of time-dependentconcrete strength, support conditions, and dynamic loads on structural strength and serviceability of P/Cmodules during various stages of construction, service condition, and extreme environmental condition.
9-3. Load Criteria
Because all precast/prestressed concrete components of hydraulic structures experience loading stagesvery different than those of conventional cast-in-situ concrete hydraulic structures, the loading cases, loadcombinations, and load factors as stipulated in EM 1110-2-2104 are modified to account for the specialrequirements of the precast/prestressed concrete. Thorough engineering analysis must be conducted for allthe critical load cases and combinations during the fabrication, transportation, and construction and underthe service condition. Chapter 5 describes various load conditions unique to P/C designs. This chapterprovides general load criteria for basic load cases and factored load combinations.
a. Basic load cases.
(1) Dead loads (D). For long-term operating conditions, the weight of saturated solid ballast materialshall be used. Water weight may be considered as ballast for temporary conditions such as transport,setting, and maintenance dewatering. For preliminary design, a unit weight of 2240 kg/m3 (140 pcf) plusthe tendon/rebar weight shall be used for both prestressed and reinforced concrete. When usinglightweight concrete, the reinforced concrete is approximately 1840 kg/m3 (115 pcf) plus the weight ofreinforcement. Final designs shall be based on specific design mix weights and the design volume ofreinforcement. For draft and buoyancy analysis, an additional 3% increase in the concrete unit weightshall be included to account for swelling, water absorption, and construction tolerances. Concrete unitweights shall be field-verified prior to construction.
(2) Hydrostatic load (H). Vertical and horizontal loads induced by a static water head, excludinguplift pressures, shall be considered. Buoyant pressures present during transport and setting of a float-insegment shall also be included.
(3) Uplift (U). Vertical water pressure imposed on the base of the completed dam followingdevelopment of differential water heads upstream and downstream of the dam and/or vertical pressure dueto the underbase grouting operation shall be considered. In lieu of more accurate flow-net analysis todetermine seepage rates, a limit approach may be used. A typical structure shall be designed for the two
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uplift conditions listed below for in-service conditions as well as uplift during construction associatedwith the underbase grouting operation. The two in-service uplift conditions are:
(a) Uplift condition A: Assume the upstream sheet-pile cutoff is fully effective. The uplift pressureis constant and equal to the tailwater pressure head downstream of the cutoff.
(b) Uplift condition B: Assume the upstream sheet-pile cutoff is ineffective. Uplift pressuredistribution is determined by seepage analysis or other appropriate means. The base uplift pressures maybe reduced with drains. Guidance for drains specified in paragraph 3-3.d.(1).(c) of EM 1110-2-2200 isapplicable except that the drain effectiveness shall not exceed 33% of design. Uplift pressures shall beapplied at an assumed crack between the structure and underbase fill.
(4) Soil load (S). Lateral pressures shall be determined using the at-rest coefficient Ko.
(5) Thermal load (T).
(a) This load set includes self-straining forces and effects arising from contraction and expansionresulting from temperature change, shrinkage, moisture change, creep in component materials, andmovement due to differential settlement or combinations thereof.
(b) If P/C modules are composed of an assembly of thin P/C slabs and walls, an in-depth analysis ofthermal stresses for shell prefabrication will not be required. However, the differential thermal load onthe shell, between cooler portions of the shell surrounded by river water and areas of the shell heated byhydration of the cementitious in-fill material, may become significant enough to crack the shell concrete.Performance of the shell under this loading shall be addressed in the design. The need for detailed thermalanalysis, such as a nonlinear incremental stress analysis (NISA), shall be evaluated on a case-by-casebasis. NISA shall be performed following the guidelines in ETL 1110-2-365.
(6) Settlement (S). Load effects analogous to differential settlement are expected during launchingfrom a land-based construction site to the river. The magnitude of uneven support shall be developed bythe designer, included in the shell design, and documented in the design documentation report andcontract drawings or specifications.
(7) Live loads (LL). Transport loads shall include towing forces. Live loads (including pressure dueto in-fill concrete placements, mooring forces, and operating equipment) shall be included in the design ofindividual members. The placement of cementitious in-fill materials will produce a fluid pressure on theP/C shell. Loads from personnel and light equipment are generally negligible and may be neglected fromthe floating stability calculations.
(8) Wave load (WB). These are loads induced from a design wave when floating structures arebuoyant. Two wave sizes should be considered – a significant wave and a storm wave. The significantwave is anticipated within a one-year construction period. The storm wave is a 50-year event and isconsidered to be an extreme event. The minimum design wave height for inland waterways is typically0.6 m (2 ft) for the significant wave and 2 m (6 ft) for the storm wave. Final design wave load analysisshall be checked on the basis of local meteorological and hydrological conditions onsite.
(9) Dynamic wave load (WD). This load represents the lateral force of wave action that acts abovethe hydrostatic pressure. The wave force can be equated to an additional static head that acts in thefreeboard area.
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(10) Current load (C). This load consists of the forces applied to structures due to river current andflow patterns. Scaled modeling may be used to estimate the magnitude of force based on expected flowvelocity and direction. In lieu of scale model test results, the current force may be estimated usingEquation 5-2. This force can be neglected when the velocity is less than 0.6 meter per second (2 feet persecond), except when determining the towboat capacity to transport and mooring force to position the P/Cmodules.
(11) Wind loads (W). Wind loads shall be considered when determining the force of the towboat,mooring line, and, under extreme conditions, floating stability of P/C modules. Wind load shall be basedon American Bureau of Shipping’s Rules, as defined in Equation 5-1 and Tables 5-2 and 5-3. In any case,wind load shall not be less than 1.4 KN/m2 (30 psf).
(12) Ice loads. Ice loads shall be based on EM 1110-2-1612 requirements.
(13) Impact loads. Barge impact loads shall be determined by sound engineering principles andempirical information.
(14) Seismic loads (EQ). Seismic loads shall be determined in accordance with ER 1110-2-1806.
b. Load factors. The load factors applied to the ultimate strength design of precast/prestressedconcrete components shall follow the guidance in EM 1110-2-2104 with the following modifications:
(1) The required strength against loads or load combinations during stages of fabrication,transportation, and erection of these members may be verified by the load factor methods without use ofthe hydraulic factor Hf. However, an impact load factor Fi = 1.15 should be applied to proper loads toinclude the dynamic load amplification effect during transportation and erection of these members.
(2) To check the strength of P/C members during underwater construction of hydraulic structures, thelateral pressure of underwater concrete against P/C members may be taken as the hydrostatic fluidpressure with a load factor of 1.4. Alternatively, time-dependent effects of the underwater concrete maybe taken into account in calculation of the lateral pressure, and a load factor of 1.7 shall be used.
(3) For nonprestressed P/C members, the hydraulic factor Hf = 1.3 shall be applied only to themembers that will be permanently exposed to abrasion/erosion action during the service life of thestructure.
(4) For prestressed concrete members, the hydraulic factor Hf = 1.15 shall be applied only to themembers that will be permanently exposed to abrasion/erosion actions.
(5) For nonprestressed precast members that are not exposed to abrasion/erosion action, the ultimatestrength design need not include the hydraulic factor.
Table 9-1 illustrates an example of appropriate load factors specified for design of lift-in P/C modulesduring construction stages and during service.
c. Load combinations.
(1) The critical load combinations should be determined on a project basis through careful evaluationof the project requirements, P/C modules (size, shape, weight, etc.), prefabrication site and method,transportation and erection methods, river conditions, construction logistics, and schedule. In general,precast/prestressed concrete modules shall be designed for the following general construction stages:
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Table 9-1Load Factors for Ultimate Strength Design of P/C
Load DescriptionBasic LoadFactor
HydraulicFactor
Minimum DynamicImpact Factor Notes
Precast Yard LoadsDead load (on yard skids with dynamic factors)Posttensioning jacking force for anchorage design.Form suction.
1.41.21.4
1.01.01.0
1.15--
1
Transport LoadsDead load (rigged in water with dynamic factor)Hydrostatic pressureDynamic inertial/drag (tow / current )
1.41.41.3
1.01.01.0
1.15-1.15
1
Positioning LoadsDead load (rigged in water on landing pads with dynamic factor)Ballasting loadsHydrostatic pressureDynamic inertial/drag ( current )
1.4
1.71.41.3
1.0
1.01.01.0
1.15
--1.15
1
Construction LoadsDead loadHydrostatic pressureHydrostatic pressure of underwater concreteThermal load due to hydration of cement
1.41.41.71.4
1.01.01.01.0
1.15---
1
2
Operation Loading Condition
Permanent LoadsDead loadsEarth pressure loads
Hydraulic LoadsHydrostatic pressureDynamic inertial added massFlood stageBase uplift pressure
Impact LoadsBarge impact loadIce and debris impact load
Environmental LoadsWindFlow ice
1.41.4
1.71.31.31.7
1.31.3
1.31.3
1.31.3
1.31.31.31.3
1.31.3
1.31.3
--
----
1.01.0
-1.0
3,43,4
3,43,433,4
33
33
Notes:1. Dead loads include any construction framing and equipment attached to the members.2. Use load factor 1.7 when pressure accounts for slump, temperature, pour rate, mix design, and consolidation procedures. Use load factor 1.4 when pressure is taken to be full equivalent hydrostatic head.3. The hydraulic factor applies only to structural components that will be permanently exposed to abrasion/erosion action. For prestressed members, the hydraulic load factor of 1.15 may be adopted. For nonprestressed members subjected to direct tension, the hydraulic load factor is 1.65.4. Basic load factors are reduced by 0.75 if combined with short-term dynamic loads.
• Prefabrication and handling.
• Outfitting (afloat or onshore).
• Load-out (i.e., the operation required to transfer a P/C module from land onto a vessel fortransportation) or float-out (i.e., the operation required to transfer a P/C module from a dryconstruction site to a self-floating mode prior to towing).
• Lifting (onshore and offshore).
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• Transportation.
• Positioning.
• Submergence (lift-in or ballasting).
• Setting.
• Tremie placement or grouting.
(2) For both construction conditions and service conditions, distinction shall be made for usualcondition, unusual condition, and extreme condition as defined below:
(a) A usual condition is one that affects the structure for an extended period on a reoccurring basis.This condition includes normal operation and maximum operation. Maximum operation considers thedesign headwater and corresponding minimum tailwater without occasional or rare events added.
(b) An unusual condition is a loading condition that occurs for a brief period of time or will beexperienced infrequently. This condition includes the prefabrication work in a precast yard, setting downloads, maintenance dewatering, hurricane, and normal operation stage with operating basis earthquake.
(c) An extreme condition is a loading condition that is highly improbable and may happen only oncein the life of the structure.
d. Example – Load cases and load combinations for design of a float-in navigation dam.
(1) A 183-m (600-ft) long gated navigation dam structure with four tainter gate bays will beconstructed with the float-in method. The “in-the-wet” construction plan calls for breaking the dam intotwo segments of 102 m (333 ft) and 81 m (265 ft) in length. The segments will be constructed as closedbottom P/C box structures. The bottom of the boxes will be recessed to fit the pre-installed foundationcaissons onsite. As each segment module is fabricated in a casting yard, it will be launched by floodingand towed to the site for final outfitting. It will then be positioned over the foundation caissons with amooring system mounted on the segment. Each module will be ballasted down onto six landing caissonsand leveled with flat jacks. The pile tops and underbase will be grouted, and 3.4-m (11-ft) thick tremieconcrete will be placed in the segment module. Each module will then be dewatered and the remainder ofthe dam including tainter gates will be completed in the dry. The step-by-step construction sequence andeach critical operation are illustrated in Figures 9-1 through 9-6. For the analysis and design of the float-indam segments, the load cases and load combinations (LC) are presented below.
LC1 - PREFABRICATION CONDITION. Handling, launching (into the river), construction live loads,settlement, wind, and dead loads are the loads normally considered. Thermal loads will be considered tothe extent that deems to be necessary for the specific method of prefabricating the P/C modules.
LC2 – TRANSPORTATION LOAD CONDITION. The loads occur during their transportation from theprefabrication site to the project site. Both nonuniform weight distribution along the segments and waveforce induce significant stresses in the P/C module. Maximum wave-induced stresses are typically definedby the hogging and sagging conditions. The wave action must be checked about both the longitudinal andtransverse axes of the float-in segment. Wave action may also induce torsion in the module. Inlandwaterways transport must consider the height and period of a design wave caused by river current, wind,and/or passing vessels. Due to the short transport duration, only the “significant” wave load is considered
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Figure 9.1 Float-in Construction Stage 1
Figure 9.2 Float-in Construction Stage 2
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Figure 9.3 Float-in Construction Stage 3
Figure 9.4 Float-in Construction Stage 4
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Figure 9.5 Float-in Construction Stage 5
Figure 9.6 Float-in Construction Stage 6
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in this project. Inland transport must consider the height and period of a design wave caused by rivercurrents, wind, and/or passing vessels.
LC3 – SETTING LOAD CONDITION. This load condition addresses the submergence of the moduleonto the prepared foundation. Each dam segment may be partially flooded during submergence. Watershall be used to ballast the module down, so that the segment may be refloated if proper positioning is notobtained on the first attempt. The structural strength and floating stability of the segment shall be checkedfor a set of ballasting sequence. The ballasting sequence developed for design of the segment shall beincluded in the construction documents. The total weight of the set structure shall have a minimum 10%negative buoyancy during foundation grouting and curing. The weight is calculated with the poolelevation assumed at mean high water level.
LC4 – IN-FILL PLACEMENT. The set P/C module shall be evaluated under the pressure generated byunderbase grouting and in-fill concrete during placement operation. The magnitude of loads shall beconsistent with anticipated concrete/grout placement rate, temperature, slump, mixes, and placement andconsolidation procedures. The concrete shell shall have adequate strength to resist these loads. Theconcrete/grout placement procedure developed for the design of the shell shall be included in theconstruction documents. The sequence plan for in-fill placements shall reflect design assumptions.
LC5 – MAINTENANCE DEWATERING. One gate adjacent to a pier is assumed to be closed and seatedon the sill, while other adjacent gates are fully raised with maintenance bulkheads in the upstream anddownstream bulkhead slots. The two limiting uplift conditions (A & B) previously indicated shall beapplied. The gate bay analyzed shall be assumed dry between the upstream and downstream maintenancebulkheads. Other loads include water pressures from the upstream and downstream pool, lateral soilpressure from upstream, wind on bridge, piers, and control building.
LC6 – NORMAL OPERATION. The normal operating load condition is defined as follows: all taintergates are assumed to be closed and seated on the sill. The two limiting uplift conditions (A & B)previously indicated shall be applied. Other loads include water pressure from upstream and downstream,wind load, lateral soil pressure, and ice load.
LC7 – EXTREME OPERATION. The extreme operation load condition is defined as follows: one gateadjacent to a pier supported just off the sill by one hydraulic cylinder while the other adjacent gates fullyraised with maintenance bulkheads in place upstream and downstream slots. The two limiting upliftconditions (A & B) previously indicated shall be applied. The gate bay analyzed is assumed dry betweenthe upstream and downstream maintenance bulkheads. Other loads include water pressure from upstreamand downstream, lateral soil pressure, and wind load.
LC8 – EXTREME OPERATION. This extreme operating load condition is defined as follows: all gatesare closed and seated on the sill. The two limiting uplift conditions (A & B) previously indicated shall beapplied. Other loads include water pressure from upstream and downstream, wind load, lateral soilpressure, and barge impact load.
LC9 – EXTREME OPERATION. This extreme operating load condition is defined as follows: all gatesare closed and seated on the sill. Failure at downstream project results in a further reduction in the lowerpool elevation on the downstream side. The two limiting uplift conditions (A & B) previously indicatedshall be applied.
(2) For load cases LC1 through LC4, the P/C segments should be designed as stand-alonecomponents. For load cases LC5 through LC9, the composite action of P/C and concrete in-fill should beincluded in the design.
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(3) Table 9-2 shows appropriate load combinations and load factors used in design of the navigationdam in the above nine load conditions.
Table 9-2Factored Load Combinations for Design of a Navigation Dam by the Float-in Construction Method
LOAD FACTORSGROUP D H U S LL WB WD C W T MISC. Hr Hf
LC1 1.4 0 0 0 0 0 0 0 0 1.7 1 1LC2 1.4 1.7 0 0 1.7 1.7(1) 1.7 0 1.7 0 1 1LC3A 1.4 1.7 0 0 1.7 1.7(1) 1.7 0 1.7 0 1 1LC3B 1.4 1.7 0 0 1.7 1.7(1) 1.7 1 1.7 0 1 1LC4 1.4 0 1.4(3) 0 1.4 0 0 0 0 1.7 1 1LC5 1.4 1.7 1.7(2) 1.7 1.7 0 0 0 1.7 0 1 1.3LC6 1.4 1.7 1.7(2) 1.7 1.7 0 0 0 1.7 0 1.7*Ice 1 1.3LC7 1.4 1.7 1.7(2) 1.7 1.7 0 0 0 1.7 0 0.75 1.3LC8 1.4 1.7 1.7(2) 1.7 1.7 0 0 0 0 0 1.7*Imp 0.75 1.3LC9 1.4 1.7 1.7(2) 1.7 1.7 0 0 0 0 0 1.7*Imp 0.75 1.3NOTES: (1). SIGNIFICANT (TYPICAL) WAVE(2). UPLIFT CONDITION A or B(3). UNDERBASE GROUT PRESSUREHr = REDUCTION FACTORHf = HYDRAULIC FACTOR
9-4. Serviceability
The serviceability requires satisfactory performance and durability of the structural members during thedesign life of the hydraulic structures. Requirements commonly include abrasion/erosion resistance, cor-rosion resistance, crack control, deflection control, and general durability. Crack width and deflectionrequirements shall be checked by calculations with the Working Stress Design method (WSD). Otherdurability requirements shall be met by means of proper structural detailing and construction qualitycontrol.
a. Classification. All precast/prestressed concrete components of hydraulic structures shall beclassified into three categories: (1) no cracking, (2) controlled cracking, and (3) no requirement on crackwidth. The classification shall be specified in the project design criteria in accordance with theserviceability requirements of each precast component and in consultation with CECW-E.
b. Crack control. Crack control shall be based upon either crack width calculations or allowablestress criteria.
(1) Crack width criteria.
(a) For hydraulic surfaces that are exposed to flowing water, crack width at the concrete surface shallbe imposed to enhance the abrasion/erosion resistance and leakage control. The crack width criteria arebased on the allowable values in Tables 9-3 and 9-4 for sustained loading and temporary loading,respectively.
(b) Calculation of the crack width shall follow Provision 10.6 of ACI 318 as follows:
30760 Adf.W csβ= (9-1)
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Table 9-3Allowable Crack Width Under Sustained Loading
Crack Width for Sustained and Operational Loading ConditionsFor Nonprestressed Concrete ComponentsSurfaces subjected to abrasion/erosion action of flowing water )in0060(mm150 ...W ≤All other exterior surfaces )in0130(mm330 ...W ≤For Prestressed Concrete ComponentsSurface subjected to abrasion/erosion action 0=WAll other exterior surfaces )in0080(mm200 ...W ≤
Table 9-4Allowable Crack Width Under Temporary LoadingCrack Width for Temporary Loads (Construction, Transportation, and Inspection)For through-thickness cracks in.)01180(mm30 ..W ≤For surface cracks )in01970(mm50 ...W ≤
where β is defined as the ratio of distance to the neutral axis for the extreme tension fiber to distance fromthe centroid of the main reinforcement and prestressing steel. β may be approximately taken as 1.2 forbeams and 1.35 for plates and slabs. fs is working stress in reinforcement in MPa. W is in units of0.03 mm. A is effective tension area of concrete surrounding the flexural tension reinforcement andhaving the same centroid as that reinforcement, divided by the number of bars or wires, in mm2. dc isthickness of concrete cover measured from extreme tension fiber to center of steel bar or wire locatedclosest thereto, in mm. fs is stress in reinforcement at service loads, in MPa. The definitions of dc and Aare illustrated in an example in Figure 9-7.
Figure 9-7. Definitions of dc and A in crack width calculation
(2) Allowable stress criteria: In lieu of the crack width calculation, the allowable stress design criteriamay be used to control cracking. If the maximum stresses in reinforcement fs and the maximum stressincrease in prestressing steel ∆fps meet the limitations in Table 9-5, the crack width of concrete need notto be checked:
Table 9-5Allowable Stress Design Limitation for Crack ControlLoading Condition Change in Steel StressAt the stages of construction, transportation, installation,and inspection
ksi)518(MPa6127 ..f ps ≤∆
ksi)23(MPa6158.f s ≤Under normal service condition ksi)11(MPa875.f ps ≤∆
ksi)17(MPa2117.f s ≤Under extreme loading at service
ys f.f 60≤
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c. Deflection. Both reinforced concrete components and prestressed concrete components shall bedesigned to have adequate stiffness to meet the short-term and long-term deflection requirements.Evaluation of deflection of P/C components shall take into account effects of cracking, reinforcement, andtime-dependent factors.
(1) Calculation of both short-term deflection and long-term deflection of nonprestressed P/Ccomponents shall follow Provision 9.5 of ACI 318.
(2) For prestressed concrete members, short-term deflection or camber shall be computed by usualelastic mechanics formula using the effective moment of inertia Ig. The long-term deflection or cambershall be computed by multiplying the short-term deflection by the factor C2 as follows:
p
s
p
s
A
A
A
AC
C+
+=
1
1
2 (9-2)
where As is cross-sectional area of reinforcing steel and Ap is cross-sectional area of prestressing steel.C1 shall be taken from Table 9-6.
Table 9-6Multiplier C1 to be Used for Estimating Long-Term Deflection and Cambers in Prestressed Concrete Members (from PCI MNL-120)
Multipliers C1
Deflection and Camber Without Composite Topping With Composite ToppingAt erection:
(1) Deflection (downward) component – apply to the elastic deflectiondue to the member weight at release of prestress
(2) Camber (upward) component – apply to the elastic camber due toPrestress at the time of release of prestress
Final:
(3) Deflection (downward) component – apply to the elastic deflectiondue to the member weight at release of prestress
(4) Camber (upward) component – apply to the elastic camberdue to prestress at the time of release of prestress
(5) Deflection (downward) – apply to elastic deflection due tosuperimposed dead load only
(6) Deflection (downward) – apply to elastic deflection caused bythe composite topping
1.85
1.80
2.70
2.45
3.00
--
1.85
1.80
2.40
2.20
3.00
2.30
9-5. Prestressed Concrete Design Criteria
a. Allowable stresses.
(1) Stresses in prestressed concrete shall be computed by the Working Stress Method on the basis ofthe elastic, uncracked concrete properties and strain compatibility. The calculated stress in concrete shallbe limited by the allowable values in Table 9-7.
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Table 9-7Allowable Stress in Prestressed Concrete
Allowable Compressive Stress Allowable Tensile StressAt transfer (before losses)PretensioningPosttensioning
0.60 'fc
0.55 'fc
00
At service(After prestress loss)
0.45 'fc 'f c41
'fc21( under unusual conditions)
Anchorage bearing 21 MPa (3,000 psi) -
(2) The maximum tensile stress in prestressing steel at jacking and after transfer should be limited inorder to provide a margin of safety against tendon rupture, excessive inelastic deformation and prestressloss in the steel. The maximum stresses in prestressing steel shall be limited by the allowable values inTable 9-8.
Table 9-8Allowable Stress in Prestressing Steel
Stress Relieved Strands andPlain High Strength Bars
Low-RelaxationStrands
Deformed HighStrength Bars
At jacking 0.72 fpu 0.78 fpu 0.72 fpu
At prestress transfer 0.70 fpu 0.74 fpu 0.70 fpu
Effective stress afterprestress loss
0.68 fpu 0.72 fpu 0.64 fpu
b. Loss of Prestress.
(1) Loss of prestress is the reduction of tensile stress in prestressing steel mainly due to prestressanchorage seating loss, friction loss, elastic shortening of concrete member, concrete shrinkage and creep,and relaxation of prestressing steel over time. The anchorage seating loss, friction loss, and elasticshortening are instantaneous prestress loss prior to or at prestress transfer. Creep and shrinkage ofconcrete and relaxation of prestressing steel result in time-dependent loss of prestress force.
(2) For bonded prestressing, prestress losses generally have no effect on the ultimate flexural strengthof a member. But the losses affect service conditions such as cracking and deflection.
(3) In general, a high degree of refinement of prestress loss estimate is not warranted in preliminarydesign. For routine initial design, an approximate lump-sum estimate of prestress losses is practical.Lump sum estimates of time-dependent prestress losses shall be calculated in accordance with Section5.9.5.3 of AASHTO LRFD Bridge Design Specifications. These lump-sum loss estimates do not includeanchorage seating loss and friction loss. Anchorage seating loss is usually compensated by overjacking.The magnitude of overjacking shall be in accordance with the manufacturer’s recommendations andverified in the field during the early stage of prestressing construction, but not to exceed the allowablestresses in Table 9-8. Prestress loss due to friction and elastic shortening shall be calculated separately inaccordance with Sections 5.9.5.2.2 and 5.9.5.2.3 of AASHTO LRFD Bridge Design Specifications,respectively.
(4) When more accurate estimates of prestress losses are required, the total prestress loss should bethe sum of each individual loss calculated in accordance with Sections 5.9.5.2 and 5.9.5.4 of AASHTOLRFD Bridge Design Specifications.
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9-6. Reinforcement Requirements
a. The type and grade of mild steel reinforcement shall be limited to ASTM A 615 Grade 60 steel.Reinforcement of other grades and types may be permitted for special application subject to consultationand approval of USACE CECW-E.
b. High strength tendons and bars shall conform to ASTM A 722 with deformation conforming toASTM A 615. Prestressing strands shall conform to ASTM A 416. Prestressing wires shall conform toASTM A 421.
c. Unbonded posttensioning and external posttensioning should not be permitted as permanentreinforcement in hydraulic structures without approval of CECW-E. They may be used for temporaryprestressing to assist in transport and installation of large P/C segments.
d. For prestressed concrete flexural members, the maximum amount of prestressed andnonprestressed reinforcement shall comply with Section 5.7.3.3.1 of AASHTO LRFD Bridge DesignSpecifications.
e. For prestressed concrete flexural members, the minimum amount of prestressed andnonprestressed reinforcement shall comply with Section 5.7.3.3.2 of AASHTO LRFD Bridge DesignSpecifications.
f. For nonprestressed P/C members, the maximum reinforcement ratio shall comply with EM 1110-2-2104.
g. For nonprestressed P/C flexural members, the minimum reinforcement ratio shall comply withparagraph 8-2 of this circular.
h. For compression members, the maximum and minimum amount of prestressed andnonprestressed reinforcement shall comply with Section 5.7.4.2 of AASHTO LRFD Bridge DesignSpecifications.
i. For prestressed concrete members subjected to membrane tension (i.e., full section tension), theminimum amount of prestressing shall be such as to provide an effective prestress not less than 6 MPa(860 psi). In addition, the minimum steel area (prestressing steel and reinforceing steel) shall be checkedas follows:
y
c'c
pss f.
AfAA
90≤+ (9-3)
j. Transverse shear reinforcement shall comply with Section 5.8.2 of AASHTO LRFD BridgeDesign Specifications.
k. Membrane shear reinforcement may be required in the wall of P/C segments and should bechecked against the critical in-plane shear force incurred under the construction and service conditions.
9-7. Ultimate Strength Design
a. The strength requirements include the adequate load resistance of the structural members againstany potential loads and load combinations by means of the Ultimate Strength Design method (USM).
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b. The strength reduction factor shall comply with EM 1110-2-2104.
c. Flexural and axial strength design of precast and/or prestressed components shall comply withSection 5.7 of AASHTO LRFD Bridge Design Specifications.
d. Shear and torsion strength design of precast and/or prestressed components shall comply withSection 5.8 of AASHTO LRFD Bridge Design Specifications.
e. Design of posttensioned and pretensioned anchorage zones shall comply with Section 5.10.9 andSection 5.10.10, respectively, of AASHTO LRFD Bridge Design Specifications.
f. Development and splices of reinforcement shall comply with Section 5.11 of AASHTO LRFDBridge Design Specifications.
9-8. Strength Design of Composite Members
a. Provisions of this paragraph for composite structural members shall apply to P/C and cast-in-place concrete elements that are constructed in separate placements, but are designed to respond to loadsas a unit. Therefore, the composite members shall be designed for resisting shears and moments that willbe encountered during the service conditions.
b. In addition, P/C elements shall be designed for all the loads that will be encountered duringconstruction, including transportation and installation loads, thermal stresses, and hydrostatic formpressure from in-fill concrete.
c. Composite members shall be designed for serviceability and ultimate strengths. The ultimateflexural and shear strengths of composite members should account for different strengths of the P/C andthe in-fill concrete. In calculation of the ultimate strength of the final composite section, the straindiscontinuity at the interface of both concretes and the loading sequences can be ignored to simplify thedesign. Studies show that this simplification produces satisfying design that is generally consistent withrelevant test results.
d. Serviceability requirements include crack width calculations for nonprestressed compositemembers and concrete stress check for prestressed concrete composite members. The crack width may becalculated for the service conditions only. However, the residual stresses that are locked in the P/C duringplacement of the in-fill concrete shall be considered.
e. For prestressed composite concrete members, the need to control the tensile stresses on theexternal concrete surface often governs the choice of the prestressing force. The Working Stress Methodshall check the stresses on the external surfaces of composite members against the permissible stresscriteria. The stress calculation must take into account the strain compatibility between both concretes andall critical loads during the construction process, including hydrostatic form pressure and thermal stressesof the in-fill concrete.
f. If the in-fill concrete is of different strength than that of the P/C, a composite transformed cross-section shall be used to account for the difference in the elastic moduli of the two concretes in order toensure that the strains in both materials at the interface are compatible. The composite transformedsection is obtained by modifying the width of the in-fill concrete as follows:
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nbbE
Eb
cp
cim == (9-4)
where ciE and cpE are the elastic modulus of the in-fill concrete and P/C, respectively. The modified
width bm shall be used to calculate composite transformed section properties.
g. Flexural strength of the composite members shall be designed using the transformed sectionproperties.
h. Shear strength of composite members shall be calculated in accordance Section 5.8 of AASHTOLRFD Bridge Design Specifications. The entire cross section of the composite members shall be used inthe calculation. If the in-fill concrete is of different strength from that of the P/C, the lower concretestrength shall be assumed in the design.
i. If shear reinforcement is required to resist critical combinations of shear and torsion, thereinforcement shall extend across the entire composite section and be fully anchored in the P/C.
j. Shear strength on the interface of the P/C and the in-fill concrete shall be checked to ensure fullshear transfer between segments of composite members. Calculations of checking horizontal shearstrength on the interface shall comply with Provision 17.5 of ACI 318. Minimum reinforcement #3 at1.2 m (4 ft) on center shall be provided across the interface to ensure adequate shear transfer.
k. The deflection of composite members shall be estimated using the multiplier method defined inparagraph 9-4 of this circular, except that the moment of inertia shall be calculated on the basis of thecomposite transformed section.
9-9. Fatigue Design
a. When fatigue resistance is likely to be a serious problem in critical precast/prestressed com-ponents under cyclic loads or repeated impact loads, such as on hulls of a floating guard wall, the fatiguestrength requirements shall be met by limiting the allowable stresses in steel and concrete as follows:
(1) Maximum stress range in reinforcing and prestressing steel is less than 20,000 psi (140 MPa). Ifreinforcement is bent or weld, the maximum allowable stress range is 10,000 psi. (70 MPa).
(2) No membrane tensile stress in concrete.
(3) Maximum flexural tensile stress in concrete is less than 1.4 MPa (200 psi).
(4) Maximum compressive stress in concrete is less than 0.5 'fc .
(5) If maximum shear exceeds the allowable shear stress in concrete, or if the cyclic excursionsexceed 50% allowable shear in concrete, then stirrups should be designed without consideration ofstrength contribution from concrete. Calculation of the allowable shear in concrete may account for thefavorable effect of prestressing.
b. If any one of the above stress values in steel or concrete is exceeded, a more detailed fatigueanalysis based on the principle of cumulative damage should then be carried out to verify the fatiguestrengths of concrete, reinforcement, prestressing steel, and bond between concrete and steel.
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Appendix AReferences
ER 1110-2-1150Engineering and Design for Civil Works Projects
ER 1110-2-1806Earthquake Design and Evaluation for Civil Works Project
EM 1110-2-1612Ice Engineering
EM 1110-2-2000Standard Practice for Concrete for Civil Works Structures
EM 1110-2-2104Strength Design for Reinforced Concrete Hydraulic Structures
EM 1110-2-2200Gravity Dam Design
EM 1110-2-2602Planning and Design of Navigation Locks
EM 1110-2-2607Planning and Design of Navigation Dams
EC 1110-2-312Engineering for Prefabricated Construction of Navigation Projects
ETL 1110-2-365Nonlinear Incremental Structural Analysis of Massive Concrete Structures
American Association of State Highway and Transportation Officials 1998American Association of State Highway and Transportation Officials. 1998. “AASHTO LRFD BridgeDesign Specifications,” 2nd ed., Washington, DC.
American Bureau of Shipping 1980American Bureau of Shipping. 1980. “Rules for Building and Classing Steel Vessels for Service onRivers and Intracoastal Waterways,” 65 Broadway, New York.
American Bureau of Shipping 1985American Bureau of Shipping. 1985. “Rules for Building and Classing Mobile Offshore Drilling Units,”65 Broadway, New York.
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American Concrete InstituteAmerican Concrete Institute, Farmington Hills, MI
Committee Report 207.1-R-96, “Mass Concrete”
Committee 211, “Proportioning Concrete Mixtures”Committee Report 357.R-84, “Guide for the Design and Construction of FixedOffshore Concrete Structures”
Committee Report 357.1R-91, “State-of-the-Art Report on Offshore Concrete Structuresfor the Arctic”
Committee Report 439.3R-91, “Mechanical Connections of Reinforcing Bars”
American Concrete Institute-American Society of Civil Engineers Joint Committee 550 1996American Concrete Institute-American Society of Civil Engineers Joint Committee 550. 1996. “Designrecommendations for precast concrete structures,” ACI 550R-96.
American Society for Testing and MaterialsAmerican Society for Testing and Materials. Annual Book of ASTM Standards, Philadelphia, PA.
Designation A53/A53M-99b, Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless
A185-97, Standard Specification for Steel Welded Wire Fabric, Plain, for ConcreteReinforcement
A416/A416M-99, Standard Specification for Steel Strand, Uncoated Seven-Wire forPrestressed Concrete
A421/A421M-98a, Standard Specification for Uncoated Stress-Relieved Steel Wire forPrestressed Cocnrete
A497-99, Standard Specification for Steel Welded Wire Fabric, Deformed, for ConcreteReinforcement
A615/A615M-00, Standard Specification for Deformed and Plain Billet-Steel Bars forConcrete Reinforcement
A706/A706M-00, Standard Specification for Low-Alloy Steel Deformed and Plain Bars forConcrete Reinforcement
A722/A722M-98, Standard Specification for Uncoated High-Strength Steel Bar forPrestressing Concrete
A767/A767M-00, Standard Specification for Zinc-Coated (Galvanized) Steel Bars forConcrete Reinforcement
A775/A755M-00, Standard Specification for Epoxy-Coated Reinforcing Steel Bars
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A882/A882M-96e1, Standard Specification for Epoxy-Coated Seven-Wire PrestressingSteel Strand
A884/A884M-99, Standard Specification for Epoxy-Coated Steel Wire and Welded WireFabric for Reinforcement
C260-00, Standard Specification for Air-Entraining Admixtures for Concrete
C494/C494M-99a, Standard Specification for Chemical Admixtures for Concrete
C618-00, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolanfor Use as a Mineral Admixture in Concrete
C940-98a, Standard Test Method for Expansion and Bleeding of Freshly Mixed Grouts forPreplaced-Aggregate Concrete in the LaboratoryC942-99, Standard Test Method for Compressive Strength of Grouts for Preplaced-AggregateConcrete in the Laboratory
C953-87 (1997), Standard Test Method for Time of Setting of Grouts for Preplaced-Aggregate Concrete in the Laboratory
C1090-96, Standard Test Method for Measuring Changes in Height of Cylindrical Specimensfor Hydraulic-Cement Grout
C1107-99, Standard Specification for Packaged Dry, Hydraulic-Cement Grout (Nonshrink)
C1202-97, Standard Test Method for Electrical Indication of Concrete’s Ability to ResistChloride Ion Penetration
D3363-00, Standard Test Method for Film Hardness by Pencil Test
Gerwick 1993Gerwick, Ben C., Jr. (1993). “Construction of prestressed concrete structures,” 2nd ed., Wiley, New York.
Ghorbanpoor 1993Ghorbanpoor, A., and Madathanazalli, S. C. (1993). “Performance of grouts for post-tensioned bridgestructures,” FHWA-RD-92-095, Federal Highway Administration, McLean, VA.
Post-Tensioning Institute 1990Post-Tensioning Institute. 1990. “Post-Tensioning Manual,” 5th ed., Post-Tensioning Institute, 1717 W.Northern Ave., Suite 114, Phoenix, AZ 85021.
Post-Tensioning Institute 1998Post-Tensioning Institute. 1998. “Acceptance Standard for Post-Tensioning Systems,” Post-TensioningInstitute, 1717 W. Northern Ave., Suite 114, Phoenix, AZ 85021.
Precast/Prestressed Concrete Institute 1993Precast/Prestressed Concrete Institute. 1993. “Guidelines for the use of epoxy-coated strand,” ReportJR-383, PCI Journal, Vol 38, No. 4, pp 26-32.
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Precast/Prestressed Concrete Institute 1999Precast/Prestressed Concrete Institute. 1999. “Design Handbook,” MNL-120, Precast/PrestressedConcrete Institute, 209 W. Jackson Blvd., Suite 500, Chicago, IL 60606.
Precast/Prestressed Concrete Institute 1999Precast/Prestressed Concrete Institute. 1999. “Manual for quality control for plants and production ofprecast and prestressed concrete products,” MNL-116, Precast/Prestressed Concrete Institute, 209 W.Jackson Blvd., Suite 500, Chicago, IL 60606.
Quinn 1971Quinn, Alonzo D. (1971). “Design and construction of ports and marine structures,” 2nd ed., McGrawHill, New York.
U.S. Army Corps of Engineers 1999U.S. Army Corps of Engineers. 1999. “Assessment of Underwater Concrete Technologies for In-the WetConstruction of Navigation Structures, WES Technical Report INP-SL-1, Vicksburg, MS.
U.S. Army Engineer Waterways Experiment Station 1949U.S. Army Engineer Waterways Experiment Station. 1949. Handbook for Concrete and Cement (withquarterly supplements), Designation CRD-C 61-89A, Test Method for determining the resistance of freshlymixed concrete to washing out in water,” Vicksburg, MS.
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Appendix BSpecification Requirements
B-1. Qualification of the Contractors
a. Precast concrete (P/C) fabrication shall be performed by the contractor or subcontractor who hasa demonstrated capability of producing quality precast and prestressed concrete products in a reliable andconsistent way. Contractors in charge of P/C production must (1) show that they are capable ofestablishing the required plant and equipment onsite, (2) have experienced personnel, and (3) establish thequality control procedures to fabricate the P/C products at the required rate of production. Contractorsshall show that they have successfully completed at least three projects involving P/C works for stateand/or federal agencies in the last five years. The supervisor responsible for the precast/pretensionedconcrete work shall obtain a level III P/C Plant Quality Personnel Certification from the Precast/Prestressed Concrete Institute.
b. The contractor and each applicable subcontractor should have a minimum level of financialstrength. This may be demonstrated by sufficient bonding capacity, but this alone is no guarantee ofadequate financial strength. In addition to bonding capacity, the contractor should provide with his Q&ESubmittal an audited financial statement including balance sheet and income statement for the last threeyears. If the bidder is a joint venture, then each partner should submit a financial statement. A copy ofthe joint venture agreement should be provided.
B-2. Tolerances
a. Tolerances for surface leveling, horizontal alignments, and vertical alignments of the finalposition of precast components shall be clearly specified in the plans and specifications. The specifiedtolerances for the as-built dimensions of the structure should be established on the basis of therequirements for operations, strength, and durability of the structure.
b. The specified tolerances of the as-built structure are dependent on dimensional tolerances ofindividual precast products, erection tolerance of the precast components, and interfacial tolerancebetween adjacent components.
c. In general, the dimensional tolerances should be used as guidelines for acceptance and not limitsfor rejection. The engineer shall decide whether a deviation from the allowable tolerance affects safety,service performance, and durability of the structure.
d. The dimensional tolerance of P/C components shall generally comply with the consensusstandards of the industry, as described in PCI MNL-116. For example, dimensional tolerances of a P/Cpanel include length and width dimensions, thickness variation, and straightness (warping, bowing, andtapered edges). Different temperature effects and differential moisture absorption between the inside andoutside faces of a panel should be considered in design and construction to minimize warping andbowing.
e. The erection tolerance shall be determined in accordance with the current construction practiceand the project site conditions. The erection tolerance must be economically attainable in construction,especially for underwater positioning of large P/C segments. It may depend on the site conditions, theerection method, and equipment. Final erection tolerances should be reviewed and agreed to prior to the
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beginning of erection. Any change in tolerances made from the original plans and specifications shall beapproved in writing and noted in the contract documents.
f. In determination of erection tolerances, attention should be given to possible deflection and/orrotation of the P/C component and the foundation members supporting the P/C component duringerection.
g. The dimensional tolerance and the erection tolerance are interrelated and may be additive. Forexample, warping, bowing, and edge straightness of a P/C component have an important effect on theedge match-up and joint dimension during the erection. If the accumulated tolerance exceeds the totaltolerance specified for the structure, adjustment shall be made with the interface clearance betweenadjacent precast segments.
h. Interfacing tolerances and clearances are those required for joining different prefabricatedelements. The fabrication dimensional tolerances and erection tolerances must be considered in specifyinginterfacing tolerances. The clearances between adjacent prefabricated elements are important details,because they provide accommodation for adjustment of possible size variation and misalignment.
i. Typical dimensional tolerances for construction of large P/C assemblies are as follows:
(1) Fabrication of individual precast panels
• Overall dimensions of members: !1 mm per 1 m of length (1/8 in. per 10 ft), maximum 6 mm(1/4 in.).
• Cross-section dimensions: !6 mm (1/4 in.).
• Deviation from straight line: !1 mm per 1 m of length (1/8 in. per 10 ft), maximum 6 mm(1/4 in.).
• Bowing: L/240.
• Warpage: one corner out of the plane of the other three shall be less than 5 mm per 1 m (1/16 in.per 1 ft) distance from the nearest adjacent corner.
• Variation in slab thickness: - 13 mm (1/2 in.), + 19 mm (3/4 in.)
(2) Erection of precast panels to construct large P/C assemblies:
• Horizontal tolerance at bottom of erected vertical panels/walls: !13 mm (1/2 in.).
• Horizontal tolerance at top of erected vertical panels/walls: !13 mm (1/2 in. ).
(3) Casting bed.
• Level tolerance of casting bed: !13 mm (1/2 in.) over entire area
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B-3. Stressing Operation
a. The contractor in charge of P/C production shall include a schedule and sequence of stressing inthe production plan. The contractor shall maintain daily records containing information regardingmaterials testing, tensioning, detensioning, concrete proportioning, placing, and curing.
b. For all stressing operations, either pretensioning or posttensioning, the stressing load shall beapplied in two increments. An initial load is applied to the individual strands to straighten them andprovide a reference point for measuring elongation of the strands. The minimum initial jack load shall be5% for pretensioning and 10% for posttensioning. The initial jacking force shall be measured within anaccuracy of +445 N (+100 lb.). The final load is then applied such that the stress conditions in all strandsreach a uniform final stress.
c. Under all circumstance, stress induced in the tendons shall be determined by two independentmeasurement methods: (1) direct stress measurement by pressure gauges or dynamometers or load cells,and (2) force computed from the actual elongation of the strands based upon their physical properties andcompensation adjustment.
d. For double-curved tendons, or all single curved tendons over 30 m (100 ft) in length, or whereversharp bends of tendons exist, stressing from both ends shall be specified. Where stressing from both endsis impracticable, calculations and tests shall be made to determine friction losses and adequatecompensation shall be provided accordingly.
e. In posttensioning all structural components with multiple tendons, the stressing shall be carriedout in a carefully predetermined sequence and increments so that prestressing eccentricity can beprevented.
f. For all but lightly pretensioned concrete, jacking forces shall be gradually released by hydraulicjacks after concrete reaches adequate strength. For lightly pretensioned concrete, release of jacking forcesby burning the strands may be permitted, but is not recommended.
g. Stressed and ungrouted posttensioning tendons are very susceptible to corrosion. All post-tensioning ducts shall be grouted within 24 hours after final stressing.
B-4. Grouting
a. P/S ducts. Past experience shows that blockage of posttensioning ducts is a common problem inthe field. The causes of blockage are usually either entry of foreign objects (pieces of gravel or short bars)or in leakage of cement paste at splices. Ducts and their couplers shall be watertight to prevent leakage ofcement paste. Specifications shall require that the open end of the ducts be capped with red plastic coverand heat-shrinkage tape be used at splice. Care shall be taken to ensure that all ducts, anchorage block-outs, openings, and vents are kept clean prior to and after installing the tendons. Minor damage to ductsshall be satisfactorily repaired and sealed, or by removing the damaged segment and splicing a segment ofnew duct. Major damage shall require replacement of the duct. Prior to grouting, the ducts and tendonsshall be flushed with water to remove any contaminants and protective grease. The ducts shall be checkedfor blockage and leakage. The flush water shall then be removed by blowing compressed air through theducts.
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b. Grouting. Grouting of the ducts shall proceed as soon as practical once the tendons are stressed.Written grouting instructions shall be prepared to dictate the direction of grouting and sequence of closurefor vents, inlets, and outlets. The essential requirements are as follows:
(1) Grouting of each duct shall be one continuous operation. All vents shall be open when groutingstarts. The ducts shall be grouted from the lowest inlet in an upward direction. Grout shall be placed bypositive pressure in a continuous, one-way flow. The pumping pressure shall not exceed 1 MPa (150 psi)for oval/flat ducts nor 1.7 MPa (250 psi) for circular ducts without approval of the engineer. If thegrouting pressure exceeds the maximum allowable, the injection vent shall be closed and grout shall beinjected into the next vent in the prescribed direction. Grouting shall continue until all the water andentrapped air are removed from the duct and the grout emerging from the vents is of the same color andconsistency as the grout being pumped. Then, a pressure of 0.7 MPa (100 psi) shall be maintained for oneminute and the injection valve shall be sealed off under pressure. If the bleeding test show that bleedingwater of grout amounts to over 1% in volume, a standpipe shall be provided at the high-point outlets andanchorage to let out bleeding water. When one-way flow of grout cannot be maintained or when groutingis interrupted for more than 30 minutes, the grout shall be immediately flushed out of the duct.
(2) Vertical ducts with high rise can develop substantial bleeding due to strands acting as wicksunder the high pressure. Thus, standpipes and special thixotropic admixtures are employed to ensurecomplete filling of the ducts.
(3) Within 48 hours after grouting, the level of grout in the injection ports and vents shall be checkedfor complete filling of the ducts. In case voids are found, grout should be topped off through the outlets.Valves, caps, and vent pipes shall not be removed until the grout has set. The filled ducts shall not besubjected to vibration or shock within 24 hours of grouting. All steel vents shall be removed at least25 mm (1 in.) below the concrete surface and patched with epoxy mortar. All plastic vents shall beremoved to the concrete surface.
c. Pressure. The pumping pressure shall not exceed 1 MPa (150 psi) for oval/flat ducts nor 1.7 MPa(250 psi) for circular ducts without approval of the engineer.
d. Vents.
(1) The grouting procedure and arrangement of inlets and outlets should be designed to ensurecomplete filling of ducts, including forcing grout through wedges at anchorage. Inlets and outlets shall beplaced at the following areas: (a) the anchorage of the tendon, (b) the high points of the duct profile whenthe vertical distance between the high and low points is more than 0.5 m (20 in.), (c) the lowest point of atendon profile, (d) major changes of the cross section of the duct, such as trumpets of couplers, and(e) any location where air and water are likely to collect. All the inlets and outlets shall be detailed onworking (shop) drawings. Grout injection and outlet vents shall be equipped with positive shutoffs.
(2) The specification shall require that the contractor submit written procedures of groutingoperations for approval. As a minimum, the grouting plan shall include the following items:
(a) Type, quantity, and brand of materials used in the grout with all the certification required.
(b) Type of equipment to be used including their capacities in relation with demand and workingconditions.
(c) Types and sizes of hoses and connections.
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(d) Grout mixing and pumping procedure.
(e) Duct cleaning methods prior to grouting.
(f) Direction and sequence of grouting, rate of grouting.
(g) Contingency procedure for removing blockage.
(h) Contingency procedure for possible local regrouting.
(i) Contingency plan of equipment breakdown, including provisions for backup equipment and spareparts.
(j) QA/QC plan, the types and frequency of onsite tests of the grout mix
(k) Names of the persons in charge and their experience.
(3) If the grout mix design is not provided in the engineering specification, the specification shallprovide specific performance requirements in the following aspects:
(a) Degree of fluidity and pumpability (test in accordance with recommendation in FHWA-RD-92-095).
(b) Amount of bleeding water (test in compliance with ASTM C 940).
(c) Volume stability (test in compliance with ASTM C 1090).
(d) Strength (test in compliance with ASTM C 942).
(e) Permeability (test in compliance with ASTM C 1202).
(f) Set time (test in compliance with ASTM C 953).
B-5. Tremie Concrete In-fill Placements
a. Concrete specifications should cover at least eight key elements that contribute toward thesuccess of underwater concreting:
• Performance requirements of the concrete mixture.
• Quality of the concrete mixture components.
• Handling, storage, and testing of concrete mixture components.
• Batching, mixing, and transportation of the concrete.
• Equipment.
• Concrete placement procedures.
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• Curing and protection of fresh concrete.
• Quality control.
b. In general, specifications for underwater concrete should conform to the requirements ofEM 1110-2-2000. Only special items pertaining to large-scale underwater concrete construction arediscussed in this appendix.
c. Typical performance requirements of concrete mixtures (a range of suggested target values isincluded in parentheses):
• Slump (180-280 mm or 7-11 in.).
• Slump flow (310-760 mm or 12-30 in.).
• Slump retention (20% slump loss in 60 minutes).
• Bleeding (< 0.1% to 2%).
• Antiwashout property (<6-12% per CRD-C 61-89A).
• Time of the set (> 4 hours, <18 hours).
• Adiabatic temperature rise (<21 "C or 70 "F).
• Strength (> the design strength of concrete).
• Bond strength to the P/C form.
• Creep.
• Abrasion resistance, if concrete will be permanently exposed to river currents.
d. Prior to concrete construction, a comprehensive plan of concrete production, placement, andquality control shall be developed by the contractor and approved by the engineer. The plan shall includedescriptions and supporting calculations pertaining to the following items:
• Concrete mixture proportions (if requested), and the test data showing their compliance with thespecification.
• Concrete delivery system and placement plan with detailed placement schedules and the systemcapacities. The validity of the systems shall be supported by the calculation of concreteplacement rate and concrete delivery capacity. The systems shall be shown to meet the specifiedplacement rate.
• Equipment for production, transportation, and placement of concrete.
• Plan and details of placing, positioning, and supporting reinforcement, if any.
• The methods and equipment for sounding underwater concrete.
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• A contingency plan to deal with foreseeable incidents and breakdown, such as accidentaldischarge of concrete in water and blockage of the tremie pipe.
e. For placing concrete containing aggregates greater than 10 mm (3/8 in.), only the tremie methodshall be used. The tremie method is defined as placement of underwater concrete through a tremie pipeby means of gravity flow.
f. The tremie shall be of heavy gauge steel pipe with an inside diameter equal to or greater thaneight times the nominal maximum size of aggregates. The tremie pipe shall be marked to allowdetermination of depth to the tremie mouth.
g. The tremie shall be a straight pipe of uniform diameter and smooth internal wall surface. Underno circumstance shall the tremie be sharply bent to accommodate concrete placement. All splices shall beflush on the inside.
h. Joints between tremie pipe sections shall be gasketed and bolted so as to be watertight through thetremie placement.
i. A hopper or funnel with a size of at least 2 cubic meters (3 cubic yards) capacity shall beprovided on top of the tremie to facilitate transfer of concrete.
j. An adequate supply of extra end plates and gaskets shall be provided to allow resealing of thetremie, if necessary.
k. The tremie and hopper shall be supported on a stable frame or platform to keep its verticalposition and to prevent horizontal movement. A power hoist shall be provided to raise the tremie pipe in acontrolled manner.
l. A crane or other lift equipment shall be available at the site for complete removal of tremie forthe purpose of resealing or relocation.
m. The method selected for transporting concrete shall ensure delivery without segregation,excessive delay, and excessive temperature change. Pump line for horizontal delivery should be insulatedand in hot climate painted with reflective paint. Similarly, conveyer belts should be covered and insulatedand/or painted.
n. After the introduction of mixing water to concrete, no retempering of the concrete mixture shallbe permitted without the engineer’s approval.
o. Tremie placement shall be a continuous operation, uninterrupted until completion, if possible.
p. The placement rate shall be controlled by the rate of concrete delivered to the tremie hopper.Vertical movement of the tremie shall be carefully controlled to prevent loss of seal.
q. Throughout tremie placement, the tip of the tremie shall remain embedded in the fresh concrete atleast 0.6 m (2 ft) at all times. At no time shall concrete be allowed to fall through the water or slurry.
r. The spacing of tremie pipes and sequence of the placement shall meet the specificationrequirement of the maximum allowable horizontal distance of concrete flow.
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s. The starting point of the tremie placement shall be at the lowest points in elevation within theconfine of the placement.
t. During the placement, the tremie shall be relocated in accordance with the placement plan and onthe basis of the concrete flow as indicated by soundings.
u. For massive concrete placement, the tremie shall not be moved horizontally. To relocate, thetremie shall be lifted from the water, resealed, relocated, and restarted.
v. If the tremie loses the seal due to any reason, placement shall be halted immediately, the tremiebe removed, resealed, and restarted.
w. The wet method, i.e., the “pig” or “go-devils” method, shall not be used to restart a concreteplacement. The tremie or pumpline shall be resealed in the dry, reinserted into the fresh concrete torestart the placement. Restarting the tremie placement shall follow the standard procedure as specifiedbelow:
• Lift the tremie pipe out of water, and tie on an end plate with gasket.
• Position the pipe at the placement location. Then, lower the pipe down to seat the tip through thefresh concrete to rest on bottom.
• Start placement with 0.4 m3 (0.5 yd3) grout and fill the tremie up to 40% of water depth.
• Lift the tremie off the bottom by 15 cm (6 in.) and continue the placement.
x. At completion of the concrete placement, exposed laitance shall be green-cut after concrete hasset.
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Appendix CDesign Example
C-1. Introduction
Three example calculations have been selected from load case 1 (LC1), the transport stage of precastconcrete (P/C) segments from a casting yard to the outfitting site (Figure C-1). These sample calculationsare performed using AASHTO LRFD Bridge Design Specifications, SI Units, 2nd Edition, 1998. Allequation numbers refer to this guide. Note: All SI units are in N and mm unless otherwise noted.
The examples are as follows:
• Flexural check of a reinforced concrete section.
• Shear check of a reinforced concrete section.
• Flexural and cracking check of a prestressed concrete section.
C-2. Demand Calculations
First, the demand moments and shears must be found over the segment. A finite element model of thesegment is created to find the appropriate values.
The finite element analysis is performed by using the SAP2000 nonlinear version 6.11 program fromComputers and Structures, Inc. – Berkeley. The nonlinear features in the program are not used during theanalysis.
Grid models constructed by beam elements simulate the behavior of the dam. The properties of theelements are changed as the dam structure changes for different stages. The centerline of the frameelements is located at the centerline of the webs (Figures C-2 and C-3).
Figure C-1. Diagram of dam being lowered onto foundation
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Figure C-2. Labels of equivalent frame members
Figure C-3. Diagram of frame sections used during analysis
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Figure C-4. Bending moments on frame model during float-out load case
The buoyancy while the dam is floating is modeled by springs located at each node location. The springstiffnesses are equivalent to the tributary area multiplied by the density of the water and thereby simulatethe buoyancy.
The demand for the three cases:
A load factor of 1.7 has been used for all moments and shears as it is a hydrostatic load case (seeFigure C-4).
1) Flexural strength check of critical section - Line 2, segment NPMu = 3640 kN-m
2) Shear strength check of critical section - Line 2, segment NPVu = 863 kN
3) Flexural and cracking strength check of prestressed concrete section –Line 7, segment GHMu = -2422 kN-m
C-3. Capacity Calculations
1) Flexural check of reinforced concrete section
Use line 2, segment NP. The flexural strength of this segment must be checked against the demandmoment calculated during the float-out process (LC1). The capacity will be calculated according toAASHTO section 5.7.3 – Flexural Members. The segment is simplified as a wide-flange beam with thefollowing assumptions:
EC 1110-2-60521 Jan 01
C-4
The tension steel consists of 60 #8 bars. The steel is lumped at its centroid, 6 in. from the edge of theflange. The steel in the web and the compression steel are ignored, a conservative estimate.
Calculate effective flange width. Taken as the least of three values (4.6.2.6.1):
• one-quarter of the effective span length = ¼ x 135’ = 33.75 ft
• 12.0 times the average thickness of the slab, plus the greater of web thickness or one-half thewidth of the top flange of the girder= 12 x 1’ + ½ x 7.5’ = 16 ft
• the average spacing of adjacent beams = 16 ft
Use 16 ft.
The beam has the following properties:
Property English Units SI UnitsAs 47.4 in.2 30581 mm2
f 60 ksi 414 MPabeam depth 21 ft 6401 mmf 5 ksi 34 MPab 15.92 ft 4852 mmbw 0.83 ft 253 mmhf 1 ft 305 mmcover 0.5 ft 152 mmds 20.5 ft 6248 mm
Calculate β1, stress block factor (for concrete strength greater than 28 MPa, β1 shall be reduced from 0.85at a rate of 0.05 for each 7 MPa of strength in excess of 28 Mpa):
80407
)2834(050850
7
)28(0508501 .
..
f..
'c =−⋅−=
−⋅−=β (5.7.2.2)
Assuming the section behaves as a rectangular beam, calculate c, distance between the neutral axis andthe compressive face:
f'c
yh
..
,
bf.
fsAc <=
⋅⋅⋅⋅=
⋅⋅⋅
⋅= mm111
4852804034850
41458130
850 1β(5.7.3.1.1-4)
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C-5
The assumption of behavior as a rectangular beam rather than a t-beam is correct because c is less thanthe hf, and the compression zone does not extend into the web.Calculate a, depth of equivalent stress block:
mm8980401111 =⋅=⋅= .ca β (5.7.2.2)
Find nominal flexural resistance:
mm-N1084872
89624841458130
210⋅=
−⋅⋅=
−⋅⋅= .,
adfAM sysn (5.7.3.2.2-1)
Find factored flexural resistance:
900.=ϕ for flexure and tension of reinforced concrete beams (5.5.4.2.1)
mm-N1006710848790 1010 ⋅=⋅⋅=⋅= ...MM nr ϕ (5.7.3.2.1-1)
Check demand versus capacity:
OK,,
?MM ur
m-kN6403m-Nk60070 >>
2) Check of shear in line 2, segment NP:
The cross-section is the same as example 1. Shear reinforcement consists of #8 stirrups spaced at 9 in.Calculate Av, steel area:
22 .in2012.in600 ..Av =⋅=
Find bv, effective web width, and dv, effective shear depth:
ft4202
ft830
.a
dd
.thicknesswebb
sv
v
=−=
==
ϕ, the resistance factor for shear, is 0.9 for normal density, reinforced concrete(5.5.4.2.2).
Property English Units SI UnitsAv 1.2 in.2 774 mm2
s 0.75 ft 229 mmEs 29000 ksi 2x105 MPabv 0.83 ft 253 mmdv 20.4 ft 6204 mmϕ 0.9 0.9
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C-6
To find Vn, nominal shear resistance, take the lesser of:
vv'cn dbf.V ⋅⋅⋅= 250)1 (5.8.3.3-1)
pscn VVVV ++=)2 (5.8.3.3-2)
Calculate first term:
kN52713620425334250250 ,.dbf.V vv'cn =⋅⋅⋅=⋅⋅⋅=
Calculate second term:Vp, the vertical component of the prestressing force = 0
In order to calculate Vs and Vc, assume a value for θ , angle of inclination of transverse reinforcement tolongitudinal axis, and β, factor indicating ability of diagonally cracked concrete to transmit tension:
θ 32β 3.5
Calculate Vc and Vs:
kN26776204253345308300830 =⋅⋅⋅⋅=⋅⋅⋅⋅= ..dbf.V vv'cc β (5.8.3.3-3)
kN90913229
(32)cot6204414774cot,
s
dfAV vyv
s =⋅⋅⋅=⋅⋅⋅
=θ
(C5.8.3.3-1)
Find v, the shear stress on the concrete:
MPa6110620425390
090746862.
.
.,
db
VVv
vv
pu =⋅⋅
⋅−=⋅⋅⋅−
=ϕ
ϕ(5.8.3.4.2-1)
and
018034
6110.
.
f
v'c
==
Calculate εx, the strain in the reinforcement on the flexural tension side:
45 1012
58130102
)32(cot746862506204
0006403
cot50
−⋅=⋅×
⋅⋅+=
⋅
⋅⋅+=
.,
,.,,
AE
V.d
M
ss
uv
u
x
θε
(5.8.3.4.2-2)
Checking the values on the charts: (see following page)
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C-7
Table 5.8.3.4.2-1. Values of θθθθ and ββββ for sections with Transverse Reinforcement
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C-8
If 0180.fv 'c = and 41012 −⋅= .xε , then β = 3.6 and βθ = 28°. Need to guess new values for β and θ
Try: β = 3.6 and θ = 28°
Thus,
41032
MPa6110
kN34716
kN6772
−×=
===
.
.v
,V
,V
x
s
c
ε
If 0180.fv 'c = and 41032 −⋅= .xε , then β = 3.6 and θ = 28°, so estimates are good.
kN52713Use
kN52713kN02419
kN02419kN0kN34716kN6772
,
,,
,,,VVVV pscn
>
=++=++=
kN17412kN5271390 ,,.VV nr =⋅=⋅=ϕ
Check demand versus capacity:
OK,
?VV ur
kN863kN17412 >>
3) Check bending moment and crack width in prestressed beam - line 7, segment GH:
• Calculate effective flange width. Taken as the least of three values (4.6.2.6.1):
• One-quarter of the effective span length = ¼ ! 40 ft = 10 ft
• 12.0 times the average thickness of the slab, plus the greater of web thickness or one-half thewidth of the top flange of the girder = 12 ft + ½ ! 5.75 ft = 15 ft
• the average spacing of adjacent beams = 16 ft
Use 10 feet.
EC 1110-2-60521 Jan 01
C-9
The prestressed beam has the following properties:
Property English Units SI UnitsAs 36.3 in.2 23445 mm2
fy 60 ksi 414 MpaEs 29000 ksi 2x105 Mpabeam depth 8 ft 2438 mmf'c 5 ksi 34 Mpab 10 ft 3048 mmbw 1.0 ft 305 mmhf 1.0 ft 305 mmcover 7 in. 178 mmds 7.42 ft 2260 mmAps 1.72 in.2 1110 mm2
fpe 162 ksi 1117 MPadp 7.56 ft 2303 mm
Calculate β1, stress block factor:
80407
)2834(050850
7
)28(0508501 .
..
f..
'c =−⋅⋅=
−⋅⋅=β (5.7.2.2)
Find fpy, the yield strength of the prestressing tendons:
MPa16751860900900 =⋅=⋅= .f.f pupy (C5.7.3.1.1-1)
Find k:
2801862
167504120412 ..
f
f.k
pu
py =
−⋅=
−⋅= (5.7.3.1.1-2)
Assuming the section behaves as a rectangular beam, calculate c, distance between the neutral axis andthe compressive face:
mm163
2303
186011102803048804034850
4144552318611110
850 1
=⋅⋅+⋅⋅⋅
⋅+⋅=
⋅⋅+⋅⋅⋅
⋅+⋅=
...
,
d
fAkbf.
fsAfAc
p
pups
'c
ypups
β(5.7.3.1.1-4)
Find fps, the average stress in prestressing steel:
MPa18242303
363280118611 =
⋅−⋅=
⋅−⋅= .
.d
ckff
ppups (5.7.3.1.1-1)
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C-10
Calculate a, depth of equivalent stress block:
mm13180401631 =⋅=⋅= .ca β (5.7.2.2)
Find nominal flexural resistance:
m-N10582
2131
2303182411102
131226041445523
22
10⋅=
−⋅⋅+
−⋅⋅=
−⋅+
−⋅⋅=
.
,
adfA
adfAM ppspssysn
(5.7.3.2.2-1)
Find factored flexural resistance:
01.=ϕ for flexure and tension of prestressed concrete
m-Nk800251058201 10 ,..MM nr =⋅⋅=⋅=ϕ (5.7.3.2.1-1)
Check demand versus capacity:
OK,
?MM ur
m-kN2422m-kN80025 >>
The moment capacity is sufficient, but must also check that there is no cracking or small cracking in thebase of the slab due to the bending moment.
In order to calculate the stress in the bottom flange of the beam:
pbpadtotal σσσσ ++=
where:
The tensile stress due to the demand bending moment:
MPa93010193
12191042212
9
..
.
I
yM dd =
⋅⋅⋅=
⋅=σ
where
y = the distance from the centroid to the extreme fiber = 1219 mm
I = the moment of inertia of the section
The compressive stress due to the axial force applied by the prestressing tendons:
MPa51010422
102416
6
..
.
A
Ppa −=
⋅⋅=−=σ
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where
P = the axial force applied by the prestressing tendons = 1240 kN
A = the area of the section = 2.42 ! 106 mm2
The compressive stress due to the bending moment applied by the prestressing force:
MPa51010193
12192610841024112
6
..
..
I
yePpb −=
⋅⋅⋅⋅=⋅⋅−=σ
where
e = the distance from the prestressing tendons to the centroid of the section
σtotal = 0.93 – 0.51 – 0.51 = -0.09 MPa (compression)
Thus, because there is no tension in the bottom of the slab, there is no cracking.If there is tension, then it would have to be checked according to provision 5.7.3.4 (not included herein).
