DANIEL P. JENNY FELLOWSHIP The Inverted Tee Shallow Bridge System for Rural Areas Maher K. Tadros, Ph.D., P.E. Cheryl Prewett Professor Civil Engineering Department University of Nebraska-Lincoln Omaha, Nebraska R ecent publications” 2 have shown that more than 95 percent of all bridges built in the United States between 1950 and 1990 have span lengths less than 100 ft (30 m). A sig nificant number of these bridges must cross waterways or railroads where clearance is critical. The most common type of shallow, short span bridge superstructure is the conventionally reinforced cast-in-place (CIP) slab. It offers the highest possi ble span-to-depth ratio and does not re quire large equipment or a complicated construction procedure. However, its span capacity is limited to about 51 ft (15.5 m). This limitation often results in the need for piers in the waterway. In ad dition, field forming results in slow construction and high field labor costs. The formwork also often requires use of temporary supports in the form of wood piles. When construction is completed, the piles are cut at ground level, which may cause future obstruc Mounir R. Kamel, Ph.D. Research Assistant Professor Civil Engineering Department University of Nebraska-Lincoln Omaha, Nebraska The great majority of the bridges in the United States have spans less that 100 ft (30 m). A large percentage of these bridges are either structurally deficient or functionally obsolete. This paper presents a new precast concrete composite bridge superstructure system called the Inverted Tee System (IT) consisting of inverted tee precast, prestressed concrete beams with a 6.0 in. (150 mm) cast-in-place topping. The IT beams are simple to produce in a single set of forms for various depths. The system is intended to provide an alternate to the cast-in-place slab bridge system that does not require field form work and that can be installed with relatively small construction equipment. The proposed system has a span-to-depth ratio of up to 35, making it shallower than other available precast concrete products. The new system is shown to be structurally efficient, rapid to build, and economically competitive. The results of the analytical and experimental work used to develop the proposed system are presented. Several IT bridges are already in various design stages by the Nebraska Department of Roads and other agencies. 28 PC JOURNAL
Transcript
DANIEL P. JENNY FELLOWSHIP
The Inverted Tee ShallowBridge System for Rural Areas
Maher K. Tadros, Ph.D., P.E.Cheryl Prewett ProfessorCivil Engineering DepartmentUniversity of Nebraska-LincolnOmaha, Nebraska
Recent publications”2have shownthat more than 95 percent of allbridges built in the United States
between 1950 and 1990 have spanlengths less than 100 ft (30 m). A significant number of these bridges mustcross waterways or railroads whereclearance is critical.
The most common type of shallow,short span bridge superstructure is theconventionally reinforced cast-in-place(CIP) slab. It offers the highest possible span-to-depth ratio and does not re
quire large equipment or a complicatedconstruction procedure. However, itsspan capacity is limited to about 51 ft(15.5 m).
This limitation often results in theneed for piers in the waterway. In addition, field forming results in slowconstruction and high field labor costs.The formwork also often requires useof temporary supports in the form ofwood piles. When construction iscompleted, the piles are cut at groundlevel, which may cause future obstruc
Mounir R. Kamel, Ph.D.Research Assistant ProfessorCivil Engineering DepartmentUniversity of Nebraska-LincolnOmaha, Nebraska
The great majority of the bridges in the United States have spans lessthat 100 ft (30 m). A large percentage of these bridges are eitherstructurally deficient or functionally obsolete. This paper presents anew precast concrete composite bridge superstructure system calledthe Inverted Tee System (IT) consisting of inverted tee precast,prestressed concrete beams with a 6.0 in. (150 mm) cast-in-placetopping. The IT beams are simple to produce in a single set of formsfor various depths. The system is intended to provide an alternate tothe cast-in-place slab bridge system that does not require fieldform work and that can be installed with relatively small constructionequipment. The proposed system has a span-to-depth ratio of up to35, making it shallower than other available precast concreteproducts. The new system is shown to be structurally efficient, rapidto build, and economically competitive. The results of the analyticaland experimental work used to develop the proposed system arepresented. Several IT bridges are already in various design stages bythe Nebraska Department of Roads and other agencies.
28 PC JOURNAL
tion. Therefore, there is a need for a Table 1. Precast concrete sections used for short-span bridges in the United States.3precast concrete system that can besubstituted for the CIP slab.
Several precast concrete systems already exist for short span bridges.They are summarized in Table 1. Ascan be seen, the solid slab, voided slaband multi-stem products are very suitable substitutes for the CIP slab system. However, they have a limitedspan capacity. Lack of a CIP toppingand the requirement of relativelyheavy erection equipment may furtherlimit their use in some locations. Otherprecast products shown in Table 1have demonstrated their efficiencyover the past several decades. However, their relatively small span-to-depth ratios cannot always competewith CIP slabs.
This paper presents a new precastconcrete composite bridge system thatcan compete favorably with CIP slabs.It is called the Inverted Tee (IT) system. The system consists of precast,prestressed concrete inverted tees thatare topped with a CIP slab (see Fig.1). The ITs are extremely simple toproduce and light to handle. The topping reinforcement is a single layer ofwelded wire fabric located at midthickness.
The new system emerged after anextensive literature search and national survey of producers and bridgedesigners. It has been found that simi]ar cross section shapes are used inEngland, Germany, the Netherlands,Israel and other countries. Indeed, asimilar bridge section was used inOhio several decades ago.4
The new shape, which has no topflange and has a constant web width,is the simplest of all available precastconcrete shapes. It allows the use ofonly one set of forms for various beamdepths. This simplified section wasmade possible through a recent changein the AASHTO Specifications.5TheSpecifications now allow higher concrete compressive stresses, as discussed later in this paper.
The proposed member is prismaticwith a constant cross section along itslength. It is reinforced with straightprestresssing strands and welded wirefabric only. No draped strands or individual reinforcing bars are used in thissystem. The extreme simplicity of the
product allows it to be produced andhandled in a similar manner as for precast concrete piles. In fact, the weightof the IT-300 is comparable to that ofa 12 x 12 in. (300 x 300 mm) pile.
36to96 lOtol8 upto3o
It has been shown that the compressive concrete stress at service load atmidspan top fibers and member deflection are the two criteria that oftencontrol the design of this system. De
Solid slab
Typical section Width (in.) Depth (in.) - Span range (ft)
Voided slab 36 to 48
.1
Multi-stem
15to23 201o60
48 16 to 23 20 to 60
60 to 96 16 to 23 20 to 60
48 to 72Single stem
1.
24 to 48 35 to 80
Box girder 36 to 48 27 to 42
Deck bulb tee
60 to 100
48 to 84 29 to 41 6Oor 110
1-girder’s 18 to 26 36 and 45 40 to 80
Note: 1 in. = 25.4 mm; 1 ft = 0.3048 m.AASHTO 1-Girders Types LI and 111.
Dimensions are in mm1 inch = 25.4 mm
Fig. 1. Transverse section of inverted tee system.
September-October 1996 29
d. Inverted Single Tee
1 inch=25.4mm
e. Open channel
2.5 PrecastPanels
Fig. 2. Possible new shapes for shallow precast concrete sections.
tails of the various design steps andexperimental verification of the system performance are given. Alsogiven is a worked out design exampleof a three-span bridge, including comparison with the corresponding CIPslab system.
It is shown that the proposed systemis not only faster to construct but canalso be more economical. Some of thedetails of a bridge scheduled to beconstructed in Omaha, Nebraska, inearly 1997 are also included.
SURVEY RESULTS
Five shapes were initially considered in the study. Fig. 2 shows theirshapes and dimensions. Three of theseshapes are all-precast and two areprecast/cast-in-place (composite) construction. The precast sections are apie “it” shape, a butted I-girder, and abulb tee. The composite sections arean inverted tee and an open channel.
Information including properties, dimensions, span capacities, and system
descriptions of the proposed newshapes was sent in a survey form tobridge owners, general contractors,precast concrete producers and consultants around the United States. The respondents were asked to comment onthe dimensions, manufacturing process, and appearance of each shape. Inaddition, an overall ranking evaluationof all the systems was requested. Themajority of the respondents favoredthe open channel shape and the inverted tee [see Figs. 2(d) and 2(e)].
96”511 611 511
-j{4 j h5”
241
6
:a. (Pie) Section
72”
96”
b. Butted I-Girder
c. Deck Bulb Tee
5,’
T
511 CIP Concrete2.5” Precast
I__
2t
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The selected critical variables foroptimization include:
1. High span-to-depth ratio2. Ease of fabrication3. Hard metric dimensions4. Minimal or no field formwork5. Aesthetic appearance6. Lightweight precast concrete
unitsBased on information from the Ne
braska Department of Roads bridgedesigners, local producers, and the resuits of the survey, the IT system wasproposed as the optimum precast concrete system for bridges with shallowsuperstructure depth requirements andwith spans less than 100 ft (30 m).
SYSTEM DESCRIPTIONAND APPLICATION
Fig. 3 shows the system dimensionsand details. Because of the mandatedFHWA decision to adopt the metric(SI) system, the IT system was designed using SI units. The IT precastbeam has a fixed width of 600 mm(23.5 in.) and a standard depth thatvaries from 300 to 900 mm (11. 8 to35.5 in.) in 100 mm (3.9 in.) increments. However, units with any required depth can be fabricated usingthe same set of forms. Properties ofthe proposed standard beam sectionsare listed in Table 2. CIP slab thickness is 150 mm (5.9 in.).
Fig. 3. Inverted tee system details.
The IT beam can accommodate upto 22 prestressing strands in the bottom flange. Two additional top strandscould be used to control the releasestresses at the beam ends. For simplicity, all strands have a straight profile.The recent success of the use of
welded wire fabric in double tees,I-girders and some box girder sectionshas led to consideration of its use inthe IT beams.6
As shown in Fig. 3, the shear reinforcement details are developed usingwelded wire fabric. A spacing of 300
Table 2. Inverted Tee (IT) precast beam properties.
Distance betweenCross-sectional Moment centroid and
Total depth area of inertia bottom fibers Weight ofh, mm A, mm2 1, mm4 Yb, mm IT, kN/m
mm (12 in.) is used throughout theprecast beam length except at the twoends where closer spacing is required.Alternatively, the welded wire fabriccan be made in only one spacing andadditional reinforcing bars are addedat the ends.
The use of welded wire fabricgreatly simplifies production and improves placement accuracy. Due to theshort transverse span of the CIP slab,only one layer of welded wire fabric atmid-thickness is required. It has beenshown that 150x150-D22xD16 (6x6-
D22xD16) is sufficient for HS-25truck loading.
Applications of the new systeminclude:
1. New construction where the superstructure depth must be kept to aminimum as in high flood elevation
Fig. 5. An alternate method of forming for topping concrete.
Superstructure
depth(mm)
Superstructure
depth(in)
500
450
400
Fig. 6. Maximum spans of inverted tee system.
32 PCI JOURNAL
areas with the requirement of a nearlyflat road profile such as overpasses inflat terrain.
2. New construction where conventional site formwork cannot be useddue to existing restrictions.
3. Bridge superstructure replacement where a higher load capacity isrequired for the same superstructuredepth.
4. Applications where structuralcontinuity over the support is required.
CONSTRUCTION ISSUESThe IT system is designed to be es
pecially efficient for construction bysmall contractors in sparsely populatedareas. The weight of the IT beams issuitable for bridges in areas where contractors do not have heavy constructionequipment. The beams can be handledwith the same equipment used for precast concrete pile handling.
A cross slope for drainage, which isnormally 2 percent, can be achieved bysloping the top surface of the pier cap.Fig. 4 shows an example of an end ITbeam for use with this system. The endbeam can be fabricated using the standard IT beam forms with proper inserts. This shape facilitates attachmentof various types of barriers.
Because the bottom flanges of theadjacent IT beams are not connected,it is preferable to cast the pier diaphragms and the intermediate diaphragms, if any, in one stage, beforethe topping concrete is cast. This provides for additional stiffness of the ITsagainst rotation and displacement during construction. It also aids in loaddistribution due to the weight of thedeck concrete placement equipment.The pier diaphragm could be designedto provide beam continuity against thedeck weight. This would significantlyimprove structural performance, espe
cially deflection due to deck weight.Forms for the CIP slab must be left
in place. Several systems can be economically used. The first is expandedpolystyrene (EPS) blocks. They can beused to fill the voids underneath thedeck slab as shown in Fig. 1. The EPSblocks should be snug fit between thewebs in order to avoid leakage between the EPS blocks and the ITwebs. Low density polystyrene is generally sufficient to carry the weight ofthe wet concrete without getting compressed. The cost of the BPS block inthe IT system is approximately $1.50per sq ft ($16.1/rn2).
A second system, which would costless than $1.00 per sq ft ($10.7/rn2),involves using cement board such asthat produced by the U.S. GypsumCompany7for tile floor underlayment.A board thickness of ‘/2 in. (13 mm) isexpected to be sufficient because thespan is only 18 in. (460 mm). The
Fig. 7. Span vs. depth of short span concrete bridge superstructures.
12
38
September-October 1996 33
board would be used in a configuration similar to that shown in Fig. 5.
Discussion with contractors has indicated that providing a constant deckslab thickness would require extensivesurveying and forming of haunchesabove the precast product, similar towhat is normally done with I-girdersystems. It would be much simplerwith the relatively short span IT system to use a variable thickness slabwith no haunches.
SPAN CAPACITIESFig. 6 shows the span capacities of
the IT system for a simple span andthree continuous span configurations.The chart is developed based on thefollowing assumptions:• Precast beam concrete compressive
strength is 7500 psi (51 MPa) at 28days and 6000 psi (41 MPa ) at release of prestress.
• Deflection of precast beams immediately after deck casting must beupward (i.e., positive camber).
• All AASHTO Standard Specification requirements are met.As shown in Fig. 6, a superstructure
with IT-400 (15.7 in.) can span up to48 ft (14.6 m) in a simple span bridge.The span can be increased to about54 ft (16.5 m) if it is made continuousfor live load. If continuity is providedprior to deck slab casting, the span canbe stretched even further.
Fig. 7 compares the span capacitiesof the IT system with other systemsused for short span bridges. The average span-to-depth ratios are also givenin the figure. The graph shows that forthe lower span range, the capacity andthe span-to-depth ratios of the IT system are comparable to those of thesolid slabs, voided slabs and multi-stem beams.
Also, CIP slabs give comparable results for spans up to 50 ft (15 m).However, all these systems are shownto be inferior to the IT system eitherbecause of excessive weight (becauseof their large widths) or the requirement for field forming. The double teesystem is shown to have a muchsmaller span-to-depth ratio than the
34
other systems, and its capacity is limited to about 65 ft (20 m).
CIP slabs cannot span in the upperspan ranges shown in Fig. 7. Onlycomposite I-girder systems and adjacent boxes are suitable for this spanrange. I-girders are known to be veryeconomical, but they are not considered shallow superstructures. Boxesare harder to produce and heavier tohandle than the proposed system.
Providing continuity in the IT system for deck weight plus all superimposed loads produces results represented by the line labeled “ITcontinuous” in Figs. 6 and 7. This lineclearly demonstrates the structural superiority of this system, especiallywhen full continuity is provided.
LIVE LOAD DISTRIBUTIONThis section provides an analysis of
a number of alternatives used to determine the live load distribution factor.These include: AASHTO Specifications, AASHTO LRFD Specificationsand grid analysis. Live load distribution factors in the AASHTO Specifications5 are provided in Article 3.23.Because the ITs are not connected inthe transverse direction, except at thediaphragms, they should behave similarly to a composite I-girder systemwith a relatively narrow spacing.
The AASHTO Specifications indicate that the distribution factor for theinterior girder is Sf5.5 per wheel load,or Sill per lane load. With a spacingof 2 ft (0.6 m), the factor is 0.182.AASHTO LRFD Specifications,8Tables 4.6.2.2.2b-l and 4.6.2.2.3a-lprovide much more detailed calculations for the same system. For an average case of a 60 ft (18.3 m) spanusing IT-400, the distribution factoris 0.258. This value is much higherthan what is indicated in theAASHTO Specifications.
In both the LRFD and AASHTOSpecifications, the range of beamspacing for which the distribution factors are specified is considerablylarger than in the proposed system: 6to 14 ft (1.8 to 4.3 m) and 3.5 to 14 ft(1.1 to 4.3 m), respectively, vs. 2 ft(0.6 m). For this reason, a grid analysis was undertaken to determine an appropriate factor.
The same 60 ft (18.3 m) bridge example with IT-400 was used in the gridanalysis.9The inverted tees were modeled as a series of beam elements, connected with another series of crossbeam elements, representing the deckslab. Conservatively, diaphragms werenot included in the model. The distribution factor per lane was found to be0.178. Accordingly, it is recommendedto use AASHTO Specifications distribution provisions for composite I-girders with the IT system.
CONTINUITY PRIOR TODECK SLAB CASTING
Continuity in the superstructureprior to casting of the deck slab wouldhave significant impact on such a shallow superstructure as the IT system.Stresses and deflections due to slaband construction loads would be significantly reduced. Continuity alsoeliminates open joints and costlymaintenance.
A simple method for creating continuity is shown in Fig. 8. It involvesproviding reinforcing bars and concrete in the voids between the websfor a distance equal to 20 percent ofthe span. As an example, it can beshown that five #5 Grade 60 bars pervoid location would be required toprovide continuity for a bridge ofthree 60 ft (18.3 m) spans.
Continuity could also be achievedby splicing the top strands or highstrength threaded rods. Details of providing continuity by spliced topstrands were developed in Ref. 10. Ithas already been incorporated in theconstruction of a five-span pedestrian!bicycle overpass in Lincoln, Nebraska.” Experimental work is underway at the University of Nebraska toinvestigate the effectiveness of splicedthreaded rods.
FULL SCALE TEST ANDSPECIMEN DESIGN
The objectives of the full scale testing are to:
1. Experimentally validate the newsystem.
2. Investigate the applicability ofthe AASHTO Specifications for itsdesign.
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3. Determine if the AASHTO allowable concrete compressive stressesat prestress release, and at service dueto full load, can be exceeded.
Two IT-400 beams were selectedfor this experiment. It was decided touse 22 strands, the maximum numberthat can be placed in the bottom flangeat 50 mm (2.0 in.) spacing (see Fig. 3).It was further decided to not debond ordrape any of these strands in order toinvestigate the impact of high compressive stresses at release of prestress. To keep the tensile stresses inthe concrete top fibers at the memberends to AASHTO limits, two fullytensioned strands were placed at thetop of the web (see Fig. 3).
This prestressing arrangement resulted in release stresses at transferlength away from the member endequal to 4845 psi (33.0 MPa) compression in the bottom fibers and 500psi (3.4 MPa) compression in the topfibers. The bottom fiber stresses were0.8lf, wheref was specified to beabout 6000 psi (41.0 MPa), and wasdetermined through cylinder testing tobe 5995 psi (40.9 MPa).
The span capacity of the memberwas determined using the AASHTOrequirement for allowable tensionin the bottom fibers at service of6.J or6fiö =520 psi (3.6 MPa).This corresponds to a member lengthof about 60 ft (18.6 m) and a spanlength of 58 ft (18.0 m). The actualconcrete compressive strength at service was determined to be 7700 psi(52.6 MPa). Welded wire fabric wasused to satisfy the AASHTO shear requirements. Two IT-400, 60 ft (18.6m) beams were fabricated according tothe reinforcement details shown inFig. 3. The design of the beam wasdone in such a way so as to answer thechallenges sought by the researchers:
1. Compressive stress at release was35 percent higher than the AASHTOallowable.
2. Compressive stress at service inthe top fibers of the IT was calculatedby the standard elastic analysis to be5991 psi (40.9 MPa), which was about33 percent higher than the 0.6fcAASHTO allowance. This stress reflects HS25 live loading, and the allowance reflects the latest AASHTO allowable stresses, which were increased
in the 1995 Interim Specifications.3. The reinforcement index ex
ceeded the maximum AASHTO requirement for ductility, unless thedeck slab concrete strength was specified atf’= 8500 psi (58.1 MPa). Thedeck concrete strength was thus specified to be 10,000 psi (68.4 MPa).However, actual strength was determined to be 8100 psi (55.4 MPa). Inactual field practice, a 5000 psi (34.2MPa) concrete slab is more realistic.
4. Due to the full tension of the top
strands, camber was expected to beminimal, and the deck weight was expected to produce a net downward deflection. Even though this was not aviolation of AASHTO, it was considered to be aesthetically unacceptable.
Specimen Fabrication and Testing
The two IT precast concrete beamswere fabricated in the PrestressedConcrete Inc. plant in Newton,Kansas. The actual initial prestressing
stirrups #3@ 10’
Plan View
0.4 Li Cast-in-Place Concrete
I— L
k ----_(i•__-
Fig. 8. Methods of creating continuity of inverted tee beams.
I.I
Fig. 9. Reinforcement details of inverted tee beam.
September-October 1996 35
force per strand was estimated fromfield measurements as 31,375 lbs(193.5 kN). Fig. 9 shows the invertedbeam specimen reinforcement details.Fig. 10 shows one of the invertedbeams after stripping the forms. Therewere no reported problems duringstripping of the forms.
The two beams were shipped to theUniversity of Nebraska StructuresLaboratory. Expanded polystyrenefoam blocks were used to fill the voidsbetween the two precast beams andthe deck slab. Plywood forms were attached to the bottom flanges of the inverted tees to form for the deck slabsides.
The specimen was tested as a simplespan beam subjected to two concentrated loads. Fig. 11 shows the flexuretest setup. The two loading locations
were at 18.5 ft (5.6 m) from each endof the specimen. Loading was introduced into two stages. In the firststage, a load of up to 70 kips (311 kN)was reached, then the load was decreased to zero. In the second stage,the specimen was loaded to failure at aload of 103 kips (460 kN). Theseloads were the average measured values of each of the two point loads.
Discussion of Test Results
The average camber at midspan ofthe two beams immediately after prestress release was 0.71 in. (18 mm)compared to a calculated value of 0.60in. (15 mm) by elastic theory. Fig. 12represents the recorded load-deflectioncurves of the test specimen atmidspan. The first visible cracks at the
midspan region started at a load of 32kips (142 kN). Deflection after thefirst stage of loading was 1.0 in. (24mm). The figure also shows the calculated elastic deflection, using standardprocedures. It indicates a good correlation between measured and calculated values.
The observed flexural cracking moment was evaluated by comparisonwith a computed value. Its value wasdetermined as 592 kip-ft (803 kN-m)from the load measurement. The calculated cracking moment Mer based onactual material properties at the timeof the test was determined as 501kip-ft (679 kN-m), which was lessthan the observed value by 15 percent.Both values were greater than the required service load moment of 452kip-ft (612 kN-m) due to HS25 liveload. Thus, the member met theAASHTO requirement of no crackingunder service load.
The observed deflection due toHS25 live load was about 2 in. (50mm), which corresponds to Span/348.This is in excess of the desiredSpanJ800 limit, which would indicatethat the member depth has to be increased or the span reduced or themember made continuous at its ends,to satisfy deflection criteria. It is important to realize that the recommended practice in actual bridge construction is to have a jointless bridgein which the girders are continuouswith the piers and the abutments. Endrotational restraints can reduce deflection by up to 80 percent.
There was no negative impact detected as a result of exceeding the allowable AASHTO concrete compressive stress either at release or due tofull service load. Time-dependentanalysis (discussed in the followingsection) revealed that interaction between the precast and the cast-in-placeconcrete produces favorable redistribution of service load stress. At ultimate load, the measured strand strainwas close to the yield point. This wasexpected from the strain compatibilityanalysis.’2 The factored live load dueto HS25 loading corresponds to apoint load of 53.1 kips (235.6 kN).The specimen failed at a load of 103kips (457 kN), which was almosttwice that required in an actual design.
Hg. 10. Beam after stripping of forms.
36 PCI JOURNAL
AASHTO approximate equationsfor calculating the nominal flexuralstrength produced a moment of 2131kip-ft (2890 kN-m). This comparedvery well with the more detailedstrain compatibility moment of 2193kip-ft (2975 kN-m) and the observedfailure load moment of 2203 kip-ft(2988 kN-m).
After the specimen failed in flexure,a 20 ft (6.1 in.) long piece was cutfrom the specimen and tested in shear.The specimen failed due to a shearingforce of 295 kips (1312 kN). Thiscompared well with the AASHTO predicted capacity of 280 kips (1645 kN).The value far exceeded the requiredshear due to HS25 loading of 100 kips(444 kN). Therefore, the reinforcement shown may be used as a standardshear reinforcement for all bridges ofthis type without concern about shearcapacity. This was an expected behavior because of the shallow depth and
0
Fig. 13. Midspan deflection vs. time for IT-400 composite member.
‘O
C
Fig. 12. Load-deflection curve of flexure test.
8 10 12 14 6 8
rnidspan deflection (in)
1.00 prestress releasedslab cast
upwara0.50 A
0,00
20 40 60 80 100 120 140-0.50 -
V —calculated-i.oo downward
A observed-1.50
A A-2.00
Time (days)
a. Test specimen
4.003.50 slab cast
2.00 —.prestress released
0.50 upward0.00 , I I I
20 40 60 80 100 120 140
Time (days)
b. Condition when the top strands are cut at time of prestress release
September-October 1996 37
the large amount of web in a typicalbridge cross section.
Time-Dependent Analysis
The stresses in concrete and prestressing steel vary continually withtime due to the effect of creep andshrinkage of concrete and stress relaxation of steel. In composite beams,Continuous stress redistribution occursbetween the two concrete types so thatthe compatibility of displacement ismaintained at their interface. The purpose of the time-dependent analysisconducted in this project was to con-
firm the long-term deflections and toassess the impact of the high concretecompressive stresses.
The presence of the top tensionedstrands magnified the impact of thetop fiber compression and the netdownward deflection. It improved theopportunity for accurate assessment ofthese two factors. However, in a functional bridge, fully tensioned topstrands would not be recommended,unless the strands are debonded andlater cut in the midspan zone.
Time-dependent stress and deflectioncalculations were performed using thecomputer program CREEP3.’3The the-
ory of analysis utilized in the program isdiscussed in detail in Ref. 14. It is basedon the “initial strain” theory often usedfor the analysis of concrete membersdue to temperature gradients. Time-stepfunctions and the finite element (stiffness) analysis approach are used to determine the development of stress anddeformation with time. Multistage prestressing, loading, creep and shrinkageas well as variability of concrete properties with time are taken into account.
Fig. 13(a) shows an excellent agreement between theoretical and experimental deflections. Fig. 13(b) showsthe effect of cutting the central portion
1.00
0.00
top stresses
bot. stresses
-1.00cn
V
VI
(ID
-2.00
slab cast
(- = comp. , + = tension)
-3.00
-4.00 :. -
I.
-5.00
20 40 60 80Time (days)
100 120
a. At section 2.5 ft from beam end
140
0.00
-1.00top fibers
bot. fibers
-2.00
slab cast
VCM
V
(ID
-3.00
-4.00
-5.00
-6.00
0 20 40 60 80Time (days)
b.
100
At midspan section
120
Fig. 14. Time-dependent stresses of inverted beam specimen.
140
38 PCI JOURNAL
\ /
Fig. 15. Three-span cast-in-place slab bridge according to Nebraska Department of Roads practice.
of the top strands at prestress release.In this case, the midspan deflectionafter deck placement would be 1.5 in.(38 mm) upward, which is a more aesthetically pleasing option.
The time-dependent stresses areshown in Fig. 14. Fig. 14(a) shows thevalues at a transfer point distance fromthe end, estimated to be 2.5 ft (0.75 m)and Fig. 14(b) shows the values atmidspan. The diagrams in Fig. 14 indicate that creep causes a significant relief of high concrete compressivestresses. The bottom fiber compressionof the member dropped from 0.8f toabout 0.5f while the concrete strengthincreased from 6000 to 7500 psi (41 to51 MPa). The top fiber stress atmidspan experienced a significant dropas well from 0.70ff’ at the time of deckplacement to 0.53f’at time infinity.
It should be mentioned that the apparently high initial compression in relationship to the concrete strength maynever be realized if a more accuratenonlinear analysis is done in place ofthe straight-line elastic analysis commonly employed.
DESIGN EXAMPL.E
The proposed system was developedas an alternative to CIP slabs. It can spanfarther than a conventionally reinforcedCIP slab bridge for the same structuraldepth. The following example comparesthe two design options. Fig. 15 shows
the plan and sectional elevation of a CIPslab design for a three-span bridge of 42,56 and 42 ft (12.8, 17.1 and 12.8 m)spans, respectively. The design wasdone according to Nebraska Departmentof Roads practice’5 for HS25 truck load.Fig. 16 shows the same bridge designedwith the IT-400 system.
30.f [1 Piers’’ HI 42-0” 56’- 0” 42’- 0”
140’- 0’
A
—I
#4 @5”
19 1/2”#9 @5”
Total thickness = 19 5”+6 ‘= 25 5”Longitudinal section A A
Table 3. Cost of the Nebraska lT-400 system.
Cost per 60 ft beam Unit cost
3 cu yd x $100,000 $300.00
Reinforcing bars and bearing plates $300.00
19 /2 in. diameter strands, (19)($0.5 per ft)(60) $570.00
Expanded polystyrene (EPS) blocks $150.00
Total cost per beam $1320.00
Costpersqft $11.00
Cast-in-place Concrete slab per sq ft$2 77
(assume a unit price of $150.00 per cu yd)
Galvanized welded wire fabric for slab reinforcement per sq ft$1 88
(assume a unit price of $0.75 per lb and an average of 2.5 lb per sq ft)
Additional longitudinal reinforcement for negative moment$0 37
(based on a unit price of $0.5 per lb)
Total cost per sq ft $16.02
Note: 1 ft= 0.3048 m; I in. = 25.4 mm; 1 sq ft= 0.093 m’; I cu yd = 0.7646 m3; 1 lb = 0.4536 kg.
September-October 1996 39
The equivalent solid slab thicknessis 14.2 and 19.8 in. (360 and 500 mm)for the IT system and the CIP slab system, respectively. This shows a savings of 28 percent in superstructure
concrete volume. Also, for hydrauliccalculations, the structural depth of theIT-400 system is 20.6 in. (525 mm),compared to 25.5 in. (645 mm) for theCIP slab.
A total of 16 0.5 in. (12.7 rum) diameter low relaxation strands wereused in the design, i.e., 14 bottom andtwo top strands. The two top strandsare to be pretensioned to 50 ksi (345MPa) only. They are intended to control release stresses. All AASHTO requirements are satisfied.
The CREEP3 computer programwas used in this study to calculate thetime-dependent deflections of this example. Fig. 17 represents the deflection of the intermediate beam atmidspan for a bridge made continuousagainst live load. It also shows thecase of continuity being applied beforecasting of the deck slab. In the lattercase, deflection due to slab weight isreduced by almost 65 percent. In bothcases, live load deflection was determined to be 0.6 in. (15.2 mm), whichmeets the AASHTO requirement ofL/800.
COST ANALYSISThe superstructure costs of the two
alternative designs shown in Figs. 15and 16 are used to illustrate the competitiveness of the proposed system.The listed unit prices for the IT systemshown in Table 3 are given by localprecast contractors. The unit prices ofthe CIP slab system per square foot istaken from records of similar projectsby the Nebraska Department of Roadsas $21.50 per sq ft. This average costper square foot does not include thecost of rails, or substructures.
This cost analysis shows savings ofmore than 20 percent in the superstructure cost with the use of the ITsystem when compared to the conventional CIP slab system. It should beemphasized that, due to the muchlarger span capacity of the IT system,a significant savings in substructurecost could also be realized.
IMPlEMENTATION PLAN
The Nebraska Department of Roadshas adopted the new IT system. It isbecoming one of their standard bridgesuperstructures. The Little PapioCreek, Syracuse East & West and theCook Spur South bridges in Nebraska,among other bridges, are now beingdesigned by NDOR engineers with the
new system. Fig. 18 shows construction details of the Little Papio CreekBridge.
Lamp Rynearson & Associates,Omaha, is designing the DahimanAvenue Bridge for the City of Omahausing the IT system. Also, the KansasDepartment of Transportation and theFlorida Department of Transportationare considering adoption of thissystem.
Because the majority of bridges inthe United States have short tomedium spans, the IT system hastremendous potential nationwide. TheUniversity of Nebraska has beenawarded a research project by the Nebraska Department of Roads to develop standard details of the new system, and to instrument the first bridgein Nebraska designed with the IT system for long-term behavior. The two-year project started in July 1996.
CONCLUSIONSBased on this investigation, the fol
lowing conclusions can be made:1. Most of the structurally deficient
bridges in the United States are short tomedium span bridges in the 50 to 100 ft(15 to 30 m) span range. Although CIPslab bridges have a shallow superstructure depth, they are costly and time-consuming to construct.
2. A new precast concrete systemcalled the IT system has been developed for short to medium spanbridges. The IT beam varies in depth
as needed for design, using a single setof forms.
3. The simplicity of the IT systemlies in its light weight, simple forming,exclusive use of welded wire fabricfor auxiliary reinforcement andstraight strand profile.
4. Besides being cost competitive,the new system requires no temporaryfield forming, spans further, and isconstructed quicker than CIP slabbridges.
5. Test results of two full-scale 400mm (15.7 in.) deep ITs with CIP topping confirmed that available designprocedures are sufficient for design ofthis new system.
6. The AASHTO allowable compressive stress, especially at prestressing force release, appears to be tooconservative. Increasing the allowablecompressive stress at release to 0.8ffrom the current AASHTO limit of0.60f would not result in negative impact. This would allow producers toreduce the amount of draped ordebonded strands at the ends, or relievecompressive strength requirements.
7. The new system has superiorshear strength. The minimal amount ofwelded wire fabric shear reinforcement recommended in this papershould be sufficient for all practicalapplications of the IT system.
8. Deflection of the proposed system is an important design consideration. It can be significantly controlledby creating continuity in the superstructure.
9. Top tensioned strands, whenused to control release stresses and/orto achieve beam-to-beam continuity,should be shielded and cut in the central portion of each span shortly afterprestress release. This would helpcreate an aesthetically pleasing upward camber in the system due to fulldead load.
ACKNOWLEDGMENTThis investigation was carried out
under the sponsorship of a Daniel P.Jenny Graduate Research Fellowship.The authors wish to thank the Precast!Prestressed Concrete Institute and theCenter for Infrastructure Research atthe University of Nebraska for providing the funding for this project.
The authors wish to thank MarkLafferty, Prestressed Concrete, Inc.,Newton, Kansas, for his generous donation of the two inverted tee beamspecimens that were tested. Specialthanks go to Lyman Freemon, MoJamshidi, Mike Beacham, Sam Fallaha and Omar Qudus, Nebraska Department of Roads.
Many other individuals deserve aspecial acknowledgment for the guidance they provided throughout thevarious stages of this project. They include Alex Aswad, Anat Y. Deboholkar, Jagdish C. Nijhawan, AminEinea, Mary L. Ralls, Mark Simpson,Panchy Arumugasaamy, Paul Johal,Mantu Baishya, Rick Phillips, GeorgeNasser and Steve Cheney.
42 PCI JOURNAL
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