I36

Design, Fabrication, and Erection of a Curved,

Prestressed Concrete Bridge with Continuous GirdersROBERT M. BARNOFF, GORDON NAGLE, MARIO G. SUAREZ,

LOUIS F. GESCHWINDNER, JR., H. WILLIAM MERZ, JR., and HARRY H. WEST

ABSTRACT

À five-span, prestressed concrete bridge hasbeen constructed for thê Pennsylvania De-partment of Transportation at the Phila-delphía Airport. The five curved, post-tensioned¡ box girders for the bridge arecontinuous over two and three spans, respec-tively. Approximate lengths are I39 and 126ft for the two-span girders and 92, 135, and92 ft for the three-span girilers. Radius ofcurvature is 478 ft for the two-span girdersand 326 ft for the three-span girders. Cur-vature of the girders was achieveil by incre-mental chords 20 ft long' anrjl field splicingwas ¿lone only at piers with cast-in-placecliaphragms. The brídge was analyzed assum-ing that the girders and cliaphragrns act as atwo-dímensional grid s-ystern. A conventionalprogram based on a matrix nethod of analysiswas used to find the bending nonents. shearforces, torsional nonents, and displace-¡nents. Post-tensioning forces were analyzedusing a space frarne matrix nethod of analy-sis. Girders were prefabricated in lengthscorresponcling to the five spans and weretransported by truck to the job site. Eachspan length of the girder vras partly post-tensioned for shípment, and final post-tensioning for continuity over the piers gras

clone in the field. The design, fabrication,transportation, erection, and final post-tensioning of the continuous curved girdersare described in this paper. Details of thecomposite ¿leck are also discussed.

À five-span, prestresse¿l concrete bridge has beenconstructed for the Pennsylvania Department ofTransportation on LegislatÍve Route (LR) 795 (Inter-state 95) in Philadelphia, Pennsylvania. Thisstructure, which is located on a conpound horizontalcurve, is identifiecl as ranp K over LR 57054. therarnp provides vehicular access to the PhiladelphiaInternational Àirport.

The original design of ranp K was a curved,steel-plate-girder briclge, but an alternate designfor a continuous, curved, prestressed, concretebritlge was subnitted and approved. I.A. Construc-tion Corporation was the general contractor for thisprojectr and the prestressed concrete componentswere fabricated and supplied by Schuylkill Products.Inc., Cressona, Pa. Schupack Suarez Engineers,Inc., rdere the dlesigners for the project with as-sistance from R.M. Barnoff and Associates, fnc., whoperformed the analysis of the continuous structure.

DESCRIPTION OF STRUCTURE

The conpound horizontal curvature of ramp K dictatedthe use of two separate structures as shown on Fig-ure 1. À two-span continuous structure, with spans

FIGURE I General layout of ramp K at PhiladelphiaInternational Airport.

of l-38 f.E J-J-:3/4 in. and 126 ft 4-3/8 ín., v¡as se-lected for the portion of the curve that has aradius of. 477.4I f.t. Three continuous spans of 92ft 5 in.r 134 ft 9 ín., and 91 ft 6 in. were usedfor the remainder of the ramp that is located on acurve with a radius of 326.13 ft. One end of eachof these structures has a conrnon bearing on pier 6

as shown in Figure 1. AIl di¡nensions shown ín fig-ure I and given previously are on the constructioncenterline.

Ranp K has a roadvray width of. 24 ft plus a lo-ftshoulder on the outside of the curve and a 6-ftshoulder on the inside. The overall width of thestructure is 43 ft 6 in., and a slope of 0.05 ftlftis maintained across the roadway. A vertical curvealong the entire length of the bridge further com-plicates the geonetry.

Five precastr post-tensioned, concrete box gircl-ers were selected as the main structural conponentsfor the two struetures. Figures 2 anil 3 are framingplans showing the geonetry of the girders and theIocation of the exterior cast-in-place díaphragns.The girders were curved by fabricating then withincremental chords 20 ft in length. End chords ofall girders deviated frorn the 20-ft length to accoÍr-modate the required center-to-center bearing lengthof the girders. Radially curved girders were notfeasible because of fabrication diffículties.

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Barnoff et aI.

FIELOSPLICE

FIGURE 2 Framing plan for spans 5 and 6.

CENTERLINE P:ER 5

CENTERLiNE BRIOOE EXP.

CAS'I _IN-PLACE I]IAPHRAGHMS'-TYPICAL

137

tion are shown in Figure 4. Voids were formed inÈhe girders with Styrofoa¡n, and internal diaphragrnswere provided at the junctíon of each 20-ft seg-nent. Polyethytene ducts vtere provide¿l for thenultistrand post-tensioning ten¿lons. Each girderreceived sone post-tensioning at the fabricatingplant Èo counteract the ilead l-oad stresses proiluce¿lby the weight of the girders. Final post-tensioningof the continuous structure was done in the fieldafÈer the diaphragms vtere constructed. The ductsfor the tendons were in the voí¿ls of the box beamand the tendons were not bonded to the gírder.

FIGURE 4 Typical girder details.

External cast-in-place diaphragrns were heavilyreinforced and tied into the girders. Post-tension-ing was used to provide continuity in the directiontransverse to the span of the girders. Essentially,the diaphrâgns were constructed so that the conti-nuity between the gírders and diaphragrns resulted ína structural grid in the horizontal pLane.

A composite deck was placed over the gridsr andPennsylvania Department of Transportation (PennDOT)

stanalard parapets ancl safety curbs were constructedto conplete the structure. The composite ¿leck con-sisted of 2.5-in.-thick precast, prestressed con-crete paneLs that span the box girders and a5-ín.-thick cast-in-place topping over the panels.Details of the deck are sho\dn on the typical crosssection in Figure 5. Mild steel reinforcement usedin the cast-in-place portion of the slab was epoxycoâted in accorclance with PennDOT specifications.

ANALYSIS AND DESIGN

The location of the structure, the methoal selectedfor fabrication, the transportation of the girders,and the exístence of roadways and utilities underthe bridge created several design constraints. Someof these erere

1. The five girders for each span were fabrj-cated individually for the fuII span length betweensupports.

2. During construction the girders Ìrerê erecteiland functioned as sinple beans between supports tocarry their ovrn dead load plus the dead load of theexteríor cliaphragrns. P1ant post-tensíoning vras sup-plied to accon¡nodate the dead load stresses in thesirnple beans. Obstructions at the site preventedthe use of temporary shoring.

3. A constant radius of. 477.4I ft was used forall girders on spans 5 and 6, the two-span continu-ous structure. Girders in spans 7, 8¡ and 9, thethree-span continuous structure, \{ere fabricatedwith a radius of 326.13 ft.

4. Pier 5K, the interior support for spans 5 and

CENTERLIÑE PIER 1

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AEUTMENT 2

FIGURE 3 Framing plan for spans 7, B, and 9'

The girclers were fabricated and shípped inlengÈhs corresponding to the five different spans.Continuíty for the two- and three-span bridges nasprovided by spticing the gÍrders at the piers withexterior cast-in-p1ace diaphragrns and applying post-tensioning for the full tength of the spliced gird-ers. For ease of fabrícation all girders in spans 5

and 6 were fabricated vrith a co¡ilnon railius at thehorizontal centerline of the 20-ft chordsr and allgirders in spans 7, 8, and 9 also have a conìmonradius.

Dimensions and details of the giriler cross sec-

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FIGURE 5 Typical cross section anddeck details.

6, was skewed in relatíon to a radius of the curve.This resultecl in aII girders for these tvro spanshaving sígnificantly differenÈ lengths. ÀI1 otherpiers, and the abutrnent at the far end of span 9twere orienteil so that their transverse centerlinewas on a radius.

5. Curvature of the girders caused a torgue ateach end yrhen the si¡np1e spans were erected and be-fore the ¿liâphragns were placeil. Struts were caston the ends of the girders and jacks were used dur-ing erection to counteract this torque. The strutsbecane part of the diaphragms at the supports afterthe cast-in-place concrete construction nas com-pleted.

6. The post-tensioned grids consisting of thecontinuous girders and the exterior diaphragns weredesi.gned so that the deck could be re¡noved and re-placecl wlthou! overstressing any of the structuralcor¡ponents. rhis conclition required that the post-tensioning of the continuous girders be done beforethe deck concrete hras placed.

The five individual girders for each span wereanalyzecl in the conventional manner as simplebea¡ns. Stresses were computed for the dead load ofthe girders and dead load of the exterior dia-phragns. Stresses due to the liftlng and tran8port-ing operation were also evaluated. The magnitu¿le ofpost-tensioning required to counteract thesestresses was founil and applied at the fabricatingplant before the girders were moved from the castingbed.

Analysis of the two-span and three-span gridsconsisting of the girders anil their connectfng ex-ternal diaphragms nas acconplished using â stiffness¡natrix nethod of analysis. Each straight beam seg-nent and iliaphragm were consiileredl to be a struc-tural member of the grid, resulting in 114 membersand 75 joints for the tv¡o-span structure and 133menbers and 90 joints for the thrêe-span structure.Coorclinates for each joint erere conputedl and beca¡nepart of the input data for the analysls, along withassu¡ned cross-sectional propertles.

ltornents, shears, torque, and displacernents at

Transportation Research Record 950

each end of each ¡ne¡nber were found for the followingloadling conditions.

I. Dead load of beatn haunches¡2. Dead loacl of 2.5-in.-thick prestressedl panels

over the entire length of the bridge, and3. Dead load of cast-in-place dleck concrete in

the positive no¡nent portion of the girder spans.

All of these data were found using a STRITDL gridanalysls program. Af,ter the dead load actions werêdetermined, the section propertles in the girderlrere modified to account for the colnposíte action ofthe deck slab. The grids were then analyzed for thefollowing loading condltions.

1. Dead load of cast-in-place deck concrete innegative ¡nonent portion of the girder spans,

2. Dead load of parapets,3. Dead load of future wearlng surface, and4. Live loaals plus inpact.

Prelininary analyses were conducted to determinethe positions and types of Live loadings that wouldproduce the naximu¡n actlons in the varloug netnberEof the grid. ÀÀSH1IO Rs20 loadlng was u6ed in thelive load analysis. Truck loads, lane loads, andoverloadts were investigated; and the naxi¡num in-ternal ¡nenber actions at the jolnts were used in thefinal analysis for stresses.

Analysis of the continuous grids for Etresses dueto fíeld post-tension, which had a variable eccen-tricity, nas done by rnodeling the structures asthree-dimensional rlgid franes. Tl¡pícal profiles ofthe field post-tensioning tendons are shown in Flg-ure 6. Forces fror¡ the fíeldl post-tensloning wereappliedl at the interior cliaphragns and at each endlof the grlds. At each end of each nember the ap-pliecl forces conaisted of an axial force (adjustedfor friction loss), a vertical forcer and a ¡no¡nentabout the horizontal axis of the cross section. Àtthe ends of each structure the appliedl actions con-sisted of the axial prestresslng force and a nomenttdue to the eccentrlcity of the prestressing force,about the horizontal centroialal axis. Five separateanalyses were done for each spân hrith an asaumedvalue of the prestressing force applied to individ-ual girders for each of the five analyses. Results

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Barnoff et aI.

fro¡n the five analyses were combineil using variouspercentages of prestressing force for indívidualgirders and conbining the prestress with stressesdue to the varíous loailing condítions until the re-quíred prestressing force in each girder was deÈer-¡nined. Nornal losses due to shrinkage and creepwere also considered.

Special attention Ìras given to the torsionalnoments ín the girders that were caused by the grav-ity loads acting on the curved' continuousr gridsystems and by "the eccentricíty and change in direc-tion of the prestressing force. Additional mildsteel reinforcement, in the form of stirrups anillongitudinal reinforce¡nentr was used ethere nee¿led toaccommodate the stresses causecl by the torsíona1noments.

Conventional ilesign procedures were use¿l in se-lecting the reinforcement an¿l prestressing tendonsfor the deck cornponents. The prestressed deck pan-els were designed to support the full dead load ofthe deck ancl to act in a conposite manner with thecast-in-place topping to carry the superixnposetl dleadand live loads. AÀsHTo HS20 loading was used as thedesign live load for the deck. PennDOT specifica-tions and design standards were used to select con-crete components and reinforce¡nent for the deck.

FÀBRICATION AND ERECTION

The twenty-five individual girders were fabricatedby SchuylkilL Products fnc., at their Cressona,Pennsylvania, plant. steel forms were used on thetr¡o sÍdes and botton of the girders and Styrofoarnwas used to forn the internal voids. Reinforcementrras preassernbled into cages and secured ín theforrns. Polyethytene post-tensioning ducts were useilfor the rnultistrand tendons. Because the tendonscould not be bonded to the girder concrete except atthe internal and cast-in-p1ace diaphragms' care wastaken to electricatly insulate the tendons to reducethe possibí1ity of corrosion. Fabrication nethodsused in producing the girders are shown in Figures 7

through 10.Lifting hooks were provided at the internal dia-

phragns that were closest to the balance Points ofthe girders. After the plant Post-tensioning wasappLied the girders were noved from the casting bedand stockpiled. À1I girders were shippe¿l by truckfro¡n the Cressona plant to the construction site atPhlladelphia' a distance of approximately 94 miles.Large tractors with hydraulically steerable dollieshrere used to transport the girders, sorne of whichweighed I15 tonsr were 147 ft longr and had a maxi-mum shipping width of t3 ft 6 irf. and a shippingheight of 12 ft. To equalize tire loads'and elini-nate static overturning monent' the girders wereloadert so that their wèight was transferred to thevehicle through their balance points. the size anilrnagnitude of the loaals dictateal use of a vehiclewith eight axles and thirty wheels.

The individual girders etere set ín position onthe piers and tetnporary torque forces \rere appliedat the girder ends to counteract the ¡îoment due tothe curvature of the girders. Àfter the girders hadbeen set in place the external díaphragns vtereptaced. Post-tensioning was then applied to thegritl framework consisting of the continuous girilersand the external diaphragrns. The conduits housingthe tendons were filled with grout to act as corro-sion protection for the prestressing steel.

construction of the deck follol{ed the normal Pro-cedure for decks with partial depth prestressedpanels. The 2.5-in.-thíck 8-ft-wide panels were setto gratle on grout haunches. The haunches wereplaced using tirnber forms bolted to the sides of thebox girdlers and inserts cast into the concrete.

FIGURE 7 Steel forms with reinforcement and voids in place.

139

FIGURE B Placing tendons for post-tensioning in plant.

I40

FIGURE 9 Post-tensioning in plant before shipping.

FIGURE 10 Completed girder being prepared for shipment tobridge site.

Transportation Research Record 950

After the prestressed panels were placed, the topmat of reinforcing steel was set anal the cast-in-place concrete topping was placed using conven-tional screedlng and finishing equipment.

The cast-in-pIâce concrete topping was placeal insegnents along the length of the contlnuous spansvrith the fírst plaeenent being ¡nade in the positivemoment portions of the continuous structures. Thiswas follor+ed by placement of the deck concrête overthe piers in the negative noment portions. Addi-tional longitudinal reinforcing steel was used inthe deck slab in the negative moment areas to reilucecracking ¿lue to the tensile sÈresses in the slab an¿ltops of the girders. Concrete and reinforcê¡nent forthe parapets and safety curbs were placed in theconventional manner using standard forrning tech-niques.

SU}IMARY AND CONCLUSTONS

Continuous. curved, prestresse¿l concrete bridges canbe built econonicalJ.y using a combination of plantand fleld post-tensioning. Prefabrication of largegirders is necessary to reduce field labor, con-struction time, and costs. Current technology an¿lmodern equipnent allov, plant fabrication and ship-ment of girders that are 160 ft long and weigh 90tons.

Proper analysis and design of curvedr continuous,prestrêssed concrete bri¿lges is essential to achieveproper behavior of the completed structure. CIosêcooperation among the fabricator, erector, anil ile-signer is necessary to prêvent overstressing any ofthe cornponents during the fabrication ancl erectionprocess. Ingenuity is.required of all members of thedesign¡ fabrication, and erection teams to developtechniques that will reduce costs ancl assure goodstrucÈural behavior of the bri¿lge.

Conventional nethods of ¡natrix anaLysis can beused to fínd the dead and live loacl nonents, shearstand torques in the prestressed grid. Care is re-quired in evaluating the prestressing forces appliedas loads to the structural franework. It was ob-served that the internal rnonents and torques !.eeresensitive to slight discrepancies in the appliedloads. The complicate¿l geonetry of the frarneworkanplifies slight imbalances in the loads and resultsin conputed torques and rnonents, of small nagnitude,that do not agree with recognize¿l theory.

htblícation of this paper sponsored by Committee on Construction of Bridgesand Structures-