Stephen B. Quinn, PE Project Manager Howard Needles Tammen & Bergendoff Houston, Texas M. Jack Kopetz, PE Chief Engineer Howard Needles Tammen & Bergendoff Kansas City, Missouri Design and Construction of the Houston Ship Channel Bridge T he Houston Ship Channel Bridge is a prime example of the "old" and the "new" in prestressed concrete bridge design. The approach spans use Texas DOT prestressed 1-beams consid- ered standard today. The main spans are prestressed segmental construction, new in the United States where all such bridges have been built within the last decade. Further, in a variation from tradi- tional bidding practice, "open specifi- cations" were used to allow the low bidder to make modifications to the de- sign compatible with his anticipated stressing techniques. The bidder was NOTE: This paper is based on a presentation given at the Segmental Concrete Bridge Confer- ence in Kansas City, Missouri, March 9-10, 1982. The Conference was sponsored by the Associated Reinforcing Bar Producers—CAST, Federal High- way Administration, Portland Cement Association, Post-Tensioning Institute, and Prestressed Con- crete Institute. also permitted to modify the bridge cross section to suit his system of traveling forms. This article will address the design, design modifications under the "open specs" system, and construction of the Houston Ship Channel Bridge includ- ing the 1500-ft (457 m) segmental con- crete main structure which has the American record span of 750 ft (229 in). This paper is adapted from one pre- sented at the 1982 Segmental Bridge Conference (see footnote) in the session on construction which addressed con- stniction problems and their field so- lutions. LOCATION Houston, Texas, is the third Iargest city in the United States, and one of the most unusual. The city had no natural access to the Gulf of Mexico until the 30
Transcript
Stephen B. Quinn, PEProject ManagerHoward Needles Tammen
& BergendoffHouston, Texas
M. Jack Kopetz, PEChief Engineer
Howard Needles Tammen& Bergendoff
Kansas City, Missouri
Design and Constructionof the Houston ShipChannel Bridge
The Houston Ship Channel Bridge isa prime example of the "old" and
the "new" in prestressed concretebridge design. The approach spans useTexas DOT prestressed 1-beams consid-ered standard today. The main spans areprestressed segmental construction, newin the United States where all suchbridges have been built within the lastdecade.
Further, in a variation from tradi-tional bidding practice, "open specifi-cations" were used to allow the lowbidder to make modifications to the de-sign compatible with his anticipatedstressing techniques. The bidder was
NOTE: This paper is based on a presentationgiven at the Segmental Concrete Bridge Confer-ence in Kansas City, Missouri, March 9-10, 1982.The Conference was sponsored by the AssociatedReinforcing Bar Producers—CAST, Federal High-way Administration, Portland Cement Association,Post-Tensioning Institute, and Prestressed Con-crete Institute.
also permitted to modify the bridgecross section to suit his system oftraveling forms.
This article will address the design,design modifications under the "openspecs" system, and construction of theHouston Ship Channel Bridge includ-ing the 1500-ft (457 m) segmental con-crete main structure which has theAmerican record span of 750 ft (229 in).
This paper is adapted from one pre-sented at the 1982 Segmental BridgeConference (see footnote) in the sessionon construction which addressed con-stniction problems and their field so-lutions.
LOCATIONHouston, Texas, is the third Iargest
city in the United States, and one of themost unusual. The city had no naturalaccess to the Gulf of Mexico until the
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Describes the major design and constructionhighlights of the $60 million Houston Ship ChannelBridge, a prestressed concrete segmental structurehaving a record length 1500 ft (457 m) main span.The approach spans use Texas DOT standardizedprestressed I-beams.
In February 1977, Howard Needles Tammen & Bergendotf portation, the summary report included order-of-magnitude esti-(HNTB} presented a report to the Texas Turnpike Authority sum- mates of construction cost, and estimates of annual operational andmarizing the results of a preliminary engineering review of a pro- maintenance expenses. Estimates of potential traffic and generatedposed crossing of the Houston Ship Channel. The proposed bridge revenues were included and the feasibility of operating the projectwould be located in eastern Harris County about 7 miles east of as a toll facility was reviewed.existing Interstate 610 on the Beltway 8 Alignment and would con- These preliminary studies indicated an exceptionally high cross-nect State Highway 225 with Interstate 10 (see map above), channel travel demand. The anticipated toll revenues indicated
Based on preliminary plans, reports, and other data prepared probable financial feasibility when compared to estimated construc-previously by the Stale Department of Highways and Public Trans- tion costs and operating expenses.
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Corps of Engineers widened and deep-ened Buffalo Bayou by dredging the 25miles (40 km) to Galveston Bay. The jobwas completed in 1914 and Houstonwas officially established as an ocean-going port.
Today, the Houston Ship Channelruns 25 miles (40 km) along the bed of'Buffalo Bayou and another 25 miles (40km) across Galveston Bay to the Gulf ofMexico (see Fig. 1). The channel ac-commodates a wide variety of ocean-going ships and is the center of Hous-ton's vast petrochemical industry. Be-fore the new bridge was opened to traf^tic, severe traffic congestion existed inthe area. An estimated increase of 250vehicles a day poured onto Houstonand Harris County streets and freeways.Much of the cargo destined for shipsusing the Houston Ship Channel wereshipped by truck and heavy truckscomprised the bulk of the commercialtraffic stream.
Prior to the opening of the HoustonShip Channel Bridge, there were fourchannel crossings — two tunnels, aferry and the Interstate Route 610Bridge. All crossings except the I-610Bridge imposed severe restrictions oncargo and vehicle type permitted.
The Houston Ship Channel Bridgewas envisioned as part of the outer cir-cumferential route for the City ofHouston (see Fig, 2). The 87.5-mile(141 km) roadway was planned to circlethe city's central business district on a12-mile (19 km) radius. By the end ofthe 1960's, about 36 miles (60 km) ofthe outer belt was either in place orunder construction.
In 1972, the Texas Highway Depart-ment developed a preliminary designreport for that portion of the beltwayacross the ship channel. In 1976, theTexas Turnpike Authority was au-thorized to investigate the possibility ofconstructing that 4.2-mile (6.8 km) seg-ment as a toll facility.
In 1977 the Turnpike Authoritycommissioned HNTB, as its' consulting
engineer, to do a preliminary feasibilitystudy for the route as a toll road. Theproject was deemed to be feasible andthe turnpike authority then requestedHNTB to set tip both a design and con-struction management team composedof local consultants and to set up formaldesign criteria to be utilized by thevarious consulting engineers in imple-menting the project's design concepts.
PRELIMINARY DESIGNIn the preliminary design phase,
eight structure types were studied forthe main channel crossing. The purposewas to determine the best structuretype or types to meet the conditions. Amajor consideration, of course, was cost,but constructability, interference withnavigation, construction time, availabil-ity and price stability of materials andmaintenance were also important con-siderations.
ted girder and steel tied arch typeswere the three most competitive alter-nate designs. These three designs werecompared in terms of interference withnavigation during construction, long-term maintenance and constructionscheduling.
Ultimately, the concrete box girderdesign was selected for the main spans.The construction required no restric-tions on the channel, and could becompleted in an estimated 4 monthsearlier. The structure will require avery low level of long-term mainte-nance. Also, when the design alterna-
(c) Steel cantilever through truss, concrete and steel cable-stayed girder. HOUSTON SHIP CHANNEL BRIDGE
tives were considered, it was judgedthat the price of concrete was more sta-ble than steel.
Steel delivery time was questionablewith the volatile labor conditions at thetime and the long-term maintenancecosts were evaluated as being less witha concrete structure. Additional reasonsinvolved the unskilled labor force inHouston and the fact that the balancedsegmental construction technique per-mitted erection without adversely af-fecting traffic in the heavily traveledHouston Ship Channel.
DESIGNThe design is predicated on an initial
four-lane bridge with provisions foradding a parallel twin bridge in the fu-ture. At that time, the first four-lane,two-way bridge would be converted tothree lanes one way with full shoulders.
Revenue and traffic projections indi-cate the initial bridge will reach capac-ity in about 8 years and be financiallyable to support construction of theparallel bridge. Right-of-way has beenacquired in the first stage to accommo-date the future bridge and roadway.The interchange connections, toll col-lection facilities and drainage havebeen designed to serve both stages ofthe project.
The lanes are split to accommodateconstruction of the future through lanesand connecting ramps with minimuminterference to 1-10 or the outer belttraffic. The parallel structure will bewest of the initial bridge. The diamondinterchange at 1-10 will be a directionalinterchange when the project is ex-panded.
The approaches are 54 and 72 in,(1370 and 1830 mm) Texas DOT pre-stressed concrete I-beams of 94 to 120 ft(28.6 to 36.6 m) spans. The south ap-proach is 6000 ft (1829 m) long, thenorth approach just under 3000 ft (914m). The contractor was given the optionto use precast prestressed deck forms
which were designed to function com-positely with a cast-in-place topping.This system was used on most of theapproaches. The rest of the projectcomprises about 2 miles (3.2 km) ofroadway and another 1300-ft (396 m)I-heam bridge. The total constructioncost of the project is approximately$60,000,000. The main span bid wasapproximately $19,000,000.
A special study was made to ascertainthe effect of higher fuel costs and fuelshortages on the revenue. With reason-able allowances for these items, the an-ticipated need is still sufficient to sup-port the project.
The Houston Ship Channel is a busywaterway in this area. It serves for pas-sage of all types of craft includingocean-going vessels. Because of this,the Coast Guard required a clear chan-nel 700 ft (213.4 m) wide with verticalclearance over the central 500 ft (152.4m) at 175 ft (53.3 m). This will providetwo-way traffic as well as maneuveringroom into the many dock areas. Themain bridge is a three-span unit withthe navigation span 750 ft (228.6 m)long.
It appears under such marine trafficconditions that it would be unfeasibleand dangerous to obstruct the channelwith falsework or erection bents. It wasconsidered costly, risky and not too de-sirable to have erection barges in thenavigation portion of the channel. Thiswas a major consideration in the selec-tion of the structure type.
Borings and geological studies indi-cated that the site was overlaid with aplastic, poorly bedded clay interbeddedwith lentils of sand. This led to a selec-tion of piling foundations supportedprimarily by friction in cohesive typematerials. The main span is supportedon 24-in. (620 mm) diameter pipe piles,the approaches mostly on prestressedconcrete piles.
For the past several decades, heavypumping of the ground water in theHouston-Galveston region has resulted
38
TYPICAL HALF SECTIONTYPICAL HALF SEC_T1.21 AT PIER AT CENTER 4FSAt
Fig. 4. Bridge cross section as designed.
in water level declines in wells of 200to 375 ft (61 to 114 m). This has causeda general subsidence of the land overthe region. The maximum subsidencehas occurred at Pasadena, an incorpo-rated area just southeast of the bridge.The subsidence contours for the maxi-mum subsidence are almost centeredon the bridge. A total subsidence of 7'ft (2.29 m) was recorded between 1943and 1973. From 1964 to 1973, the last 9years of that period, subsidence was ata rate of 0.4 ft (0.12 m) per year.
Restrictions on pumping are now ineffect and some attempt at returnpumping into the wells has been made.The subsidence rate is expected totaper off, but will continue for sometime. The design of the bridge and se-lection of structure types gave consid-eration to the anticipated differentialsubsidence over the life of the project.The design criteria provided for a dif-ferential movement of 9 in. (2290 mm)between the main and side piers. Thiscovers the predicted differential subsi-dence plus foundation settlement, andwill assure the safety of the bridge.
The plans prepared for bid provideda cross section with two 27-ft (8.23 m)roadways, a 2 ft 3 in. (0.68 in) New Jer-sey type median barrier and 1 ft 6 in.(0.46 m) wide parapets. The medianbarrier was designed to be removablein the future for conversion to one-waytraffic. The main span is 750 ft (228.6m), the side spans 375 ft (114.3 m). Thebridge was designed for an HS-20 traf-fic loading and, although maximum op-erating speeds were projected to con-tinue to be 55 miles per hr (88.5 km/hr)the design criterion was based on ahigher design speed of 60 miles per hr(96.6 km/hr). The Houston design crosssection consisted of a two-cell box witha slab overhang of 10 ft 71/2 in. (3.24 m)and three 14-in. (0.36 m) wide websspaced at 19 ft (5.79 m). A two-web al-ternate was studied but the balance ofslab versus web tipped slightly towardthree webs. The box was 12 ft (3.66 m)deep at midspan with a haunch at thepiers 42 fl 7s in. (13.1 in) deep. The topslab varied from 8 to 18 in. (0.20 to 0.46m); the bottom slab from 10 in. to 3 ft9 in. (0.25 to 1.14 m) at the pier (Fig. 4).
PCI JOURNAL/May-June 1982 39
The design provided for cantilevererection from both piers concurrently,with an imbalance of one segment. Be-cause of the subsidence problem, atemporary hinge was provided in theside spans with a 95-ft (29 m) simplespan from the hinge to the pier. Thehinge would be "locked" during thecantilever erection with temporary pre-stressing tendons for the erection ofthose six segments. After closure withthe landing at the pier, the prescribedpositive moment prestressing would beplaced and the proper reaction jackedin at the pier. The hinge would then be"unlocked" and the temporary negativemoment tendons would be removed.
During design it was recognized thatthe hinges would be costly but theywere considered less costly than de-signing a continuous structure for thesettlement, Contractor costs in such atrade-off are difficult to forecast so thecontractor was allowed the optionunder the redesign provisions to elimi-nate the hinge provided he designedfor the full 9-in. (229 mm) differentialsettlement.
The outside parapets used a uniquedesign. They were faced with the con-figuration of the New Jersey type bar-rier but were designed higher andstronger to contain an 80,000 lb (356kN) truck traveling up to 50 miles perhr (80 km/hr.) The loading and designwere established with the help of bar-rier performance studies made at theTexas Transportation Institute as wellas data from the FHWA. The designadopted was based on the balancedcantilever method using prestressedsegmented cast-in-place concrete con-struction.
The controlling condition for estab-lishing the section dimensions was thecantilever moment. The superpositionof other moments gave the final mo-ment diagram to which a tendon ar-rangement was closely matched by an-choring some tendons at each segment.The stepped arrangement was achieved
by providing empty ducts and layingtendons as needed.
Three sets of longitudinal tendonswere used: cantilever tendons, spantendons and continuity tendons. Thecantilever tendons carry the loads dur-ing construction. These stresses are"locked in" and remain except for re-distribution due to creep and shrinkage.
The cantilever tendons were an-chored at each segment in the top filletof the webs and thus were completelyencased when the next segment wasplaced.
The continuity and span tendonsprovide for positive moments that occurunder live load and redistribution ofmoments. These were placed andstressed when the closure pours weremade. At that time the structure becamea three-span continuous frame.
Because the effect of long-term creepand shrinkage can only be estimated, itis current design practice on long-spanbridges to make provision for "con-tingency" tendons. These were merelyducts through the diaphragms and an-chor plates which will permit addingpositive moment tendons in the futureif desired. The concern is not for safety,but rather to correct excessive creepdeflection, if desired.
As is customary, additional ductswere called for in the design. This pro-vided for construction problems in caseof a blocked duct or broken tendon.
The contractor provided for partialprestressing before the concrete hadreached full design strength to allowhim to advance his forms and speed upthe cycle. This was done by the addi-tion of prestressing bars. These barsbecame part of the design prestressing.
CONTRACTOR'S DESIGNMODIFICATIONS
Because cost is the most critical itemon a toll project, an attempt was madeto give the contractor maximum flexi-bility in his bid. Many hours were
40
TYPICAL HALF SECTIONTYPICAL MALF SECTION AT PIER AT CENTER OF SPAN
Fig. 5. Bridge cross section as modified by contractor.
spent to obtain the exact language inthe contract documents to give thisflexibility and still retain some controlto exclude some possible unknown op-tions not wanted. Value engineering,per se, was not compatible with the fi-nancial and bonding requirements butit was obtained, in effect, during thebidding stage.
In brief, the contractor was requiredto maintain the roadway cross section,the navigation clearances and the gen-eral type and shape of the bridge. Hewas allowed complete redesign as he sochose, subject to preliminary planssubmitted with the bid and review ofhis final design.
Three bids were submitted. All threeincluded designs which eliminated thehinge. The contractors felt the addedmaterial required to accommodate thesettlement was not enough to offset thedisruption and the continuity of the re-petitive operation of casting segments.
The contractor changed the boxshape in cross section to sloping webs.Some material savings resulted but the
forming and steel fabrication and con-crete placement became more complex.The variable depth required by thehaunch made adjustment of the travel-ing forms more difficult and increasedthe cycle time somewhat (see Fig. 5).
The shape of the soffit haunches waschanged from circular to parabolic witha deeper section at the piers but thegeneral longitudinal shape differed toolittle to be visually apparent (see Fig.6).
The most significant change was inthe design strength of the concrete. Thepiers were designed for 5500 psi (37.9MPa) and the superstructure for 6000psi (41.4 MPa) versus 3600 and 5000 psi(24.8 and 34.5 MPa) in the original de-sign.
The pier, a hollow box section, wasredesigned, necessarily, to match therevised cross section. Because of theincreased concrete strength the con-tractor decreased the wall thickness.This again provided substantial mate-rial savings but made placement of re-inforcing steel and concrete difficult
PCI JOURNALMay-June 1982 41
Fig. 6. Artist's rendition of Houston Ship Channel Bridge.
and consolidation virtually impossible.Changes in the concrete mix and theuse of supplementary external vibrationwere necessary.
The contractor elected to use partiallongitudinal prestress of the segment topermit advancing the form travelersearlier. The partial prestress was pro-vided by bars, the full prestress by ten-dons added later. To reduce the cycletime the partial prestress required 4000psi (27.6 MPa) concrete in 40 hours.This in turn required a rich mix withType III cement (high early strength).This combination of amount and type ofcement, low water-cement ratio andambient temperature made pumpingimpossible and placing by any meansvery difficult.
Other changes in redesign wereminor and incidental to those de-scribed. The segment lengths, forexample, were adjusted to fit thecapacity of the contractor-designedtravelers.
Bids were taken for the main span inMarch, 1979. Low bidder for this con-tract at $19.6 million was WilliamsBrothers Construction Company,Houston, Texas with Prescon Corpora-
tion, San Antonio, Texas, as the super-structure subcontractor, Prescon Corpo-ration's engineer for the proposed de-sign modifications was Figg & MullerEngineering, Inc., Tallahassee, Florida.Bids were taken for all the approachesand remaining portions of the projectwithin a few months.
CONSTRUCTION OF BRIDGEAPPROACH SPANS
Alternate modes of construction werepermitted on the foundations, eithersteel piles, concrete piles or drilledshafts. With the exception of the lowspans on the south approach, where thedrilled shaft alternate was part of thelow bid, the prestressed concrete pilealternate was chosen by all the con-tractors. National Soils Service, Hous-ton, Texas, performed the soils analysis.Their recommendation as to embed-ment length was shown on the plans.
In addition, each contract includedprovisions for test piles. The use of testpiles to determine the actual bearingcapacity of the piles resulted in the re-duction of pile length of at least 25 per-cent. Breakage of piles was under 2
42
Fig. 7. Piers on land approaches.
percent for the overall project. Thedrilled shaft construction employed 8-ft(2.44 m) diameter drilled shafts. Ap-proximately 20 percent had to be tre-mied, with the remainder concreted inthe dry.
The piers were built in up to threetiers, as shown in Fig. 7. For purposesof economy these piers were made thesame from the top down. The upper tierconsisted of the pier cap and approxi-mately 55 ft (16.8 m) of column. Themiddle tier employed a diaphragm andanother 55 ft (16.8 m) of column. Thelower tier employed trapezoidal col-umns connected by a web wall, Thus, a40-ft (12.2 m) high pier would have apier cap and straight columns. An 80-ft(24.4 m) pier would have the entireupper tier, the diaphragm and the re-maining columns.
Once the pier reached a height in ex-cess of 60 ft (18.3 m), the entire upper
portion remained the same. Similarly,when the height exceeded 120 ft (36.6m) the entire upper portion remainedthe same. This allowed the contractor'smaximum economy in reusing theforms which was reflected in their bids.
Superstructure construction startedwith beam erection. On this project54-in. (1.37 m) AASHTO type beams forthe 120-ft (36.6 m) spans were set onelastomeric bearing pads. Each con-tractor had his own method of settingbeams, Most contractors utilized twocranes in setting beams, as shown inFig. 8. One contractor developed adelta frame consisting of tubular steelfor the delta and flat plates for stiffen-ers. This approximately 25-ft (7.6 in)lifting frame permitted him to lift mostof the beams with a single crane. Steeldiaphragms were used to provide stiff-ness and Iateral support during con-struction.
PCI JOURNAUMay-June 1982 43
Fig. 8. Placing AASHTO type beams on land approaches.
Similarly, some contractors elected touse precast prestressed concrete deckpanels in lieu of conventionally placingthe deck (see Fig. 9). These 4-in. (102mm) panels were prefabricated on siteand put into place with a 3½-in, (89mm) cast-in-place deck placed on top ofthem. This composite deck paneleliminated the wood formwork. Finally,after the decks were placed, the medianand parapets were placed. The parapetwas a conventional "Jersey Barrier"type. It was, however, 1 ft (0.3 m)higher than the conventional barrierand heavily reinforced. On some con-tracts this barrier was cast in place inconventional forms, on others it wasslipformed. The median was either pre-cast, poured in place in forms or slip-formed, depending on the contractor.
MAIN BRIDGECONSTRUCTION
FoundationWhile foundation design modifica-
tions were not permitted in the specifi-cations, this bridge, like most bridges
built over a waterway, had some diffi-culty during foundation construction.The main channel piers are supportedby 85x79x15ft(25.9x24.1x4.6m)thick footings which are founded on255 — 24-ft (6.1 m) diameter r -in. (12.7mm) thick wall, open ended steel pipepiles.
The specifications anticipated prob-lems with pile heave caused by drivingthrough and into various clay layers.Redriving of all heaved piles was aspecification requirement. The con-tractor's cofferdam consisted of PZ 36sheet piles driven to a tip elevation of–50 ft (-15.2 m). Pile tip elevation was–110 ft (-33.5 m) with the bottom offooting elevation being –25 ft (-7.6 m).Due to the stability of the various claylayers encountered, no tremie seal wasrequired. This permitted pile drivingoperations to take place after dewater-ing and excavation.
The anticipated heaving did, in fact,occur. The piles heaved as much as 18in. (0.46 m) and caused as much as 9 in.(0.23 m) of differential deflection in thewales of the cofferdam. The eccentric-
44
A }. LL 6&
rei s
Fig. 9. Precast prestressed concrete deck panels used on land approaches.
ity of the loading caused stresses in ex-cess of 30 ksi (206.7 MPa) on the wales.In order to assure a safe working envi-ronment, an additional wale was addedto the cofferdam.
Redriving of heaved piles could havebeen a never-ending operation. Be-cause the area influenced by each pilewas so large and because the contrac-tor's superstructure modifications light-ened the loads on the piles by about 10percent. the contractor was advised thathe could test load the piles in order todetermine their bearing capacity in theheaved condition. The contractorelected to follow this option. The testload revealed a carrying capacity of thepiles to be 350 tons (3100 kN), as op-posed to the nominal 140 tons (1245kN) design load. As the factor of safetywas greater than two, no redriving wasnecessary.
Pier ShaftThe contractor's modifications to the
superstructure involved sloping websrather than a rectangular box. Thischange required that the pier shaft also
be revised to take the new box girdershape. These modifications were sub-mitted with the bid package. The spec-ified strength of the concrete in theoriginal design was 3600 psi (24.8 MPa)while the modified design required5500-psi (37.9 MPa) concrete. This in-creased strength, while more costly anddifficult to attain, enabled the contrac-tor to reduce the wall thickness of theland piers from 24 to 16 in. (610 to 406mm) and reduce the transverse wall ofthe channel pier from 33 to 24 in. (838to 610 mm). The reduction in the wallthickness required the contractor to re-detail the reinforcing steel for the piershaft.
This decision, while providing asavings in material cost, greatly in-creased the difficulty of placing theconcrete. The contractor elected toplace the wall lifts for the pier shafts in27-ft (8.2 m) increments. The relativelythin walls on the land piers causedmajor concrete placing problems. Theamount of room remaining after placingthe reinforcing steel cages only per-mitted a 6-in. (152 mm) chute to he
PCI JOURNALJMay-June 1982 45
utilized for concrete placement. Therewas insufficient space for a man to fitinside the walls to vibrate the concrete.
This constraint resulted in all theconcrete placed being vibrated from aheight of approximately 30 ft (9.2 m). Inorder to make the system work thecontractor then utilized a combinationof external form vibration and highcycle internal vibrators. This involvedstrengthening up the forms to take theadditional loads caused by the externalform vibrators. This combination aidedin solving the placing problem.
The comer reinforcement was dif-ficult to place. Steel from the walls in-tersected each other and with the addi-tion of corner reinforcing, caused agood deal of congestion. It becamenecessary to shift the splice bars inorder to allow concrete to bond aroundthe steel. The specified corner bars, abox bar with tails extending througheach wall, were changed to double Jbars for placing purposes. Upon com-pletion of all of these modifications tothe details, the concrete could beplaced in the walls in a proper mannerin order to produce well consolidatedconcrete.
Pier TableAs noted, the contractor's design
modifications changed the rectangularwebs to sloping webs for the box girderconstruction. In addition, to take ad-vantage of the design modification pro-visions of the specifications, hespecified 6000-psi (41.3 MPa) concretein lieu of the 5000-psi (34.5 MPa) con-crete required. To eliminate partialpost-tensioning a high early strength of4000 psi (27.6 MPa) in 40 hours wasalso specified. All of these decisionshad major ramifications concerning theconstruction and, in fact, the decisionconcerning the high early strength ofthe concrete resulting in significantplacing difficulties for the concrete.
To achieve the high early strength,the contractor specified a Type III ce-
ment mix that proved to be highlyreactive. The first placements madewith this mix involved the 180 cu yds(137.7 m3) bottom slab of each of thetwo pier tables. The slabs werepumped with difficulty but were suc-cessfully completed. The next place-ment involved the first wall lift of the48 ft (14.6 m) pier table. In attemptingto place concrete in the 180 lineal ft(54.9 m) of walls that rest on the bottomslab, two pumps malfunctioned and theplacement had to be aborted. The workwas halted for a period of approximately2 months while various experimentswere made in order to modify the con-crete mix design so that a workableconcrete could be found that wouldproduce an acceptable product.
The basic problem experienced wasthe rapid slump loss versus time whichpermitted only 45 minutes from thetime the concrete was hatched to whenit was placed. With an off-site batchplant 20 to 30 minutes away from thesite, this left insufficient time to placethe concrete. Various trial batches weremade with different proportions for theingredients. The contractor, the testinglaboratory (Southwestern Laboratories,Houston, Texas) and HNTB indepen-dently investigated the problem. Theultimate mix design was a joint effort ofthese three parties.
This solution consisted of asuperplasticizer as well as a more pow-erful retarder in order to better controlthe mix design. The intent initially wasto leave the water-cement ratio as de-signed at 0.39 and increase the slump ofthe concrete utilizing the superplas-ticizer to give additional time to handlethe concrete. This worked extremelywell and 9 to 9V2 in. (229 to 241 mm)slump concrete was utilized at a slightincrease in strength to place the re-mainder of the walls.
A fourth and final placement in thepier table was the top slab. Ninety-two4-in. (102 mm) longitudinal ducts wentthrough the top slabs as well as the 14
46
Fig. 10. Traveler set ready for concrete placement.
transverse ducts. These ducts wereutilized for the 270-ksi (1860 MPa)0.6-in. (15.24 mm) 7 -wire, low-relaxation strands. In addition, verticalpost-tensioning in the web walls had tobe placed along with blockouts to makeroom for the jacks over the center. Thecontractor reduced the top slab from 18to 10 in. (152 to 254 mm). The contrac-tor's redesign failed to take into accountthe actual diameter of the elementsbeing used and when the materialswere placed, an 11Vs-in. (280 mm) deckwould he required.
The solution to this problem was toslightly reduce the clearance require-ments and use a 10 1/2-in. (267 mm) slab.It is imperative that design engineerstake into account the actual dimensionof material utilized in this type of con-struction. Just as important is to leavesufficient space for fabrication andplacing tolerances.
Traveler ErectionThe contractor designed the travelers
for the project. The original concept in-
dicated a 240,000-lb (1070 kN) travelercapable of supporting a 400,000-lb(1780 kN) concrete load. Problems de-veloped in deflections and the align-ment beams. After the travelers weremodified their weight was increased toapproximately 315,000 lbs (14(10 kN).Due to the designed configuration ofthe traveler, it was approximately 6 in.(152 mm) too long and therefore bothtravelers could not be placed on thepier table simultaneously.
This problem was solved by settingthe first traveler in its final pour posi-tion. After the concrete was placed andthe segment stressed, this traveler wasmoved. At that point, the secondtraveler, which had been erected be-yond its final position was pulled backinto its pour position thus allowing thework to continue. Once the contractorwas able to establish a working cycle hewas able to turn over each set of formsin as little as 5 days. This rapid rate ofprogress allowed a very efficientworking system to be developed. Thetravelers, as field modified, served theproject very well (see Fig. 10).
PCI JOURNAUMay-June 1982 47
Fig. 11. Top slab formwork.
Fig. 12. Bulkhead for extension of top slab.
48
Fig. 13. Webwall forms.
Segmental SuperstructureSegmental construction for the pro-
ject involved very exacting quality con-trol by both the contractor and the in-spection team. This part of the projectcan be divided into a number of distinctparts.
1. Form Alignment and Construction.The contractor's decision to change therectangular webs for the outside of thebox to sloping webs provided a savingsin material at a cost of additional labor(see Figs. 11, 12 and 13). Because thesoffit of the structure is a third-ordercurve, the walls are cut at different an-gles; this makes the forming of theinterior walls more difficult.
In addition, the thickness of the ex-terior walls was given as horizontal andnot normal to the slope. This simplefact caused problems during erection ofthe forms because the natural tendencyof the workmen is to measure normal tothe forms. Support and alignment ofsloping webs is a complex item of con-struction. The changing width of the
bottom slab required changes in thetransverse dimension of the soffit formevery time the traveler was moved (seeFig. 14).
2. Construction of Embedded Items.The reinforcing steel and the ducts thatare placed for future insertion of thestrands must be located exactly as de-tailed on the plans for structural pur-poses (see Fig, 15). Each segmentplaced is distinctly different from thepreviously placed segment. Exactingquality control is necessary. Problemsthat arose included interference andconflicting call-outs on the plans. A de-cision had to be made on which em-bedded items had priority. Closeliaison with HNTB and the contractor'sengineer was established and the finaldecision on the priorities was set by thedesigners.
Fig. 14. Changing dimension of widthof bottom slab (looking up).
PCI JOURNAL/May-June 1982 49
, t\ v
Fig. 15. Bottom slab construction.
A major effort is necessary in thistype of construction concerning planpreparation. Extra care must be takenby the detailers to avoid conflicts in lo-cation in this three-dimensional struc-ture. In addition, some thought must begiven during the detailing phase toplacement of concrete in the segments.For example, in the 15-ft (4.6 m) seg-ments, no physical location was avail-able to place a tremie hopper and chuteinto the center web wall.
Because of this it was necessary totemporarily cut a transverse duct inorder to place a chute for concretingoperations. This then required haltingthe concrete placement while the chutewas pulled and the transverse ducts re-paired. For the last ten segments thethickness of the walls was reduced to12 in. (305 mm). This left only 5 in. (127mm) clear between the reinforcing steelto place a chute for the concrete. Thisled to the use of flexible chutes that hadto be cut off each time the height wasadjusted.
3. Concrete Placement. Concreting ofthe segments is a major operation. Theoriginal design mix as modified by thesuperplasticizer met the contractor'srequirement of 4000 psi (27.6 MPa)concrete in 40 hours and 6000 psi (41.3MPa) concrete in 28 days. Because ofthe lost time in construction of the piertable, the contractor asked HNTB andthe testing laboratory if there was anyway to obtain the 4000 psi (27.6 MPa)required strength for stressing in 16 to20 hours in Iieu of the 40 hours re-quired.
This was achieved by reducing thewater-cement ratio to as low as 0.33, orapproximately 29 gallons (110 litres)per cubic yard of concrete, and in-creasing the superplasticizer to obtain aflowable concrete. The average 28-daystrength was increased to 8300 psi (57.2MPa) from 6500 psi (44.8 MPa) and anumber of cylinder breaks were in ex-cess of 10,000 psi (68.9 MPa). Exactingrequirements concerning the produc-tion of concrete were required and, at
50
Fig. 16. Concrete placement with crane and bucket.
the contractor's request, this functionwas handled by HNTB's staff in con-juction with the testing laboratory'spersonnel.
Placing of the concrete was difficult.The entire cross-sectional area wasplaced monolithically. The first place-ment included a 46-ft (14 m) drop in thewalls as well as the floor of the box. Be-cause of the height of the drop and theslope of the web walls, a remixing hop-per was placed at the bottom of thechute which solved the segregationproblem and helped produce well con-solidated concrete. In accordance withan option in the specifications, HNTBrequired external vibration of the forms.Part of the modifications to the travelerpreviously discussed involvedstrengthening the forms to take theextra load caused by the external vi-brators. External vibration combinedwith internal vibration through win-dows in the walls aided in producingwell consolidated, honeycomb-freewalls.
The tower crane for the project wascapable of directly serving only 20 ofthe 58 segments to be placed (see Fig.16). Beyond the reach of the towercrane the contractor devised a system oflifting the 2 cu yd (1.53 m s) bucket tothe top of the pier, unloading approxi-mately 7 cu ft (0.2 m') to a motorizedbuggie, running the buggie to a rampand dumping it into a conveyor whichtook the concrete into the forms. Thiswas later modified to a forklift opera-tion which eliminated the ramp andmotorized huggie (see Fig. 17). Thismodification reduced the concreteplacing time from 12 hours to 7 hoursand greatly enhanced the quality of theconcrete poured.
4. Stressing. The entire system ofsegmental construction depends on thepost-tensioning system. Tension isplaced in the longitudinal strands andthen locked into position in order tohold the segments together. Thestrands themselves are composed of avery high strength steel which must be
PCI JOURNALJMay-June 1982 51
Fig. 17. Concrete placement with forklift and conveyor.
handled with extreme caution. Thestressing of the 270 ksi (1860 MPa) lowrelaxation strands was controlled onthis project by achieving the desiredelongation of the strand within thelimits of force allowed in the specifica-tions as measured by the jacking pres-sures. In order to minimize differentialstress on the structure, simultaneousstressing was required in the longitudi-nal tendons when stressing the largetendons. This required two-way com-munication between the contractor'spersonnel and the inspection team. Be-cause of the amount of friction losses,dead-end stressing was a necessity.Stressing was a complex operation thatrequired close monitoring by the en-gineer and a thorough knowledge of theforces and reactions involved in everyphase of the stressing operation (seeFig. I8).
5. Survey Controls. One of the moredifficult items of the segmental con-struction was survey control. At the
start of the project concrete tests wererun to determine the modulus of elas-ticity of the concrete and its propertiesin relation to creep and shrinkage.These tests required a lead time of 12months. These concrete characteristics,together with the anticipated time ofconstruction of the cantilever construc-tion, were programmed into a castingcurve for the project. Accurate fieldmeasurements and constant communi-cation with the design office wasmaintained to provide them with up-dated information on how the structurewas reacting.
In order to minimize the effects ofdifferential thermal stresses in thestructure, surveying was done at day-break in the balanced condition. A jointfield party of the contractor and the en-gineer surveyed the project with twoindependent sets of readings to certifythe results. The use of the same instru-ment and the same set-up served as anexcellent check.
52
Fig. 18. Stressing of vertical tendons.
CREDITSOWNER: TEXAS TURNPIKE AUTHORITYGENERAL CONSULTANT: HOWARD NEEDLES TAMMEN & BERGENDOFF
CONTRACT CONTRACTOR ENGINEER
South Approach Grading, Steel Construction Co.Drainage and Paving Turner
South Approach Low Spans Austin Bridge Company Collie &South Approach High Spans Williams Bros. Constr. Co. Braden
Main Span Williams Bros. Constr. Co.(Prescon Corp. Superstructure
Subcontractor) HNTBFigg & Muller Engineers, Inc.(Contractor Engineer for
Design Modifications)
North Approach Spans Gardner B/H Contractors BernardJacintoport Boulevard Bridge Brown & Root, Inc. Johnson, Inc.
Grading and Drainage for Williams Bros. Constr. Co. Lockwood,North Approach Andrews &
North Approach Paving R. W. McKinney & 1. L. James Newnam
Lighting Alder Electric Company Bovay EngineersAdministration Building T-Iowcon, Inc. Bovay Engineers
& HNTBFencing Universal. Services, Inc. HNTB
PCI JOURNAL/May-June 1982 53
Fig. 19. Construction in October, 1981.
SUMMARYThe Houston Ship Channel Bridge
project is a 1500 ft (457.5 m) long seg-mentally constructed concrete bridge(see Figs. 19 to 23). Segmental bridgeconstruction is a relatively new indus-try in the United States. This pre-stressed concrete structure is the
longest of its type in North America.Being at the edge of the state of the artit was anticipated that some problemswould develop during the constructionof the project that would require on-siteexpertise as well as close cooperationand communication between the de-signers and field engineers. The majorconstruction features discussed were:
54
Fig. 20. Construction in December, 1981.
1. Foundation construction.2. Thin wall concreting.3. Problems associated with sloping
web construction.4. Problems associated with the use
of high strength concrete.5. A need for a thorough set of work-
ing drawings including a layout ofembedded items using the actual
dimensions of these items in orderto minimize construction problems.
6. Problems associated with stressingrequirements.
7. The lead time necessary for sur-veying controls.
The contractor realized the complex-ity of the project. His design modifica-tion engineer provided an on-site as-
PCI JOURNALJMay-June 1982 55
Fig. 21. Panoramic view of completed structure.
sistant during the construction of thesuperstructure to facilitate decisionmaking. The inspection team for HNTBlikewise was well versed in the designand construction ramifications in thisdifficult type of bridge construction.
A communications channel with allconcerned was opened up early whichaided in keeping the problems en-countered to a minimum.
The anticipated time for completionof the project was 900 calendar days.The project exceeded this time byslightly over 10 percent. The majorityof the delays in construction were as-sociated with three main factors:
1. Foundation construction problems.2. Utilization of the high strength
concrete.3. Difficulties in placing details.
The superstructure construction ex-clusive of the pier tables was antici-pated to take approximately 57 weeksand in actuality took 60 weeks. In orderto maintain this schedule it was neces-sary to work 7 days a week and, in ad-dition, provide a supporting night shiftfor such items as grouting, strandplacement and tensioning.
The open specifications under whichthis project was bid is a variation of tra-ditional bidding practices generallyutilized in this country. It permitted theowner to reap the benefit of innovativedesign and construction practices whilestill retaining the owners relationshipto the design engineer of his choice.While minor problems with this con-cept arose, the system generallyfunctioned well.
