LONG-SPAN PRECASTPRESTRESSED GIRDERBRIDGES

James R. LibbyJames R. Libby & AssociatesSan Diego, California

This paper reviews contemporarypractice in long-span precast pre-stressed concrete girder bridge con-struction. There have been manyprojects on which this mode of fram-ing has been used or proposed foruse. This has normally been the re-sult of cost estimates and practicalconsiderations indicating this type offraming being the most economicaland practical for the particular con-ditions of each individual project.

GENERAL DESIGN CONSIDERATIONS

For long-span bridges, a well-pro-portioned precast girder must havea cross-section which is efficient inresisting bending moments butwhich does not have excess cross-sectional area that contributes un-necessary dead load. It is also impor-tant that the finished profile of abridge is very near the theoreticalprofile if the riding characteristics ofthe structure are to be good. Theengineer must determine the shortand long time deflection characteris-tics of the flexural members specified

and provide details to permit fieldadjustment during constructionwhen the anticipated deflections dif-fer from those that were predicted.The fabricator of the precast bridgemembers must use materials of highuniform quality and good workman-ship in order to minimize variationsin camber and deflections betweenthe members.

Advantages and disadvantages ofusing continuity in the design ofbridge superstructures must be con-sidered. Roadway joints are oftenundesirable from the "ridability"standpoint and frequently result inmaintenance problems. Continuousbridges have fewer joints and hencefewer sources for maintenance prob-lems. Continuity in bridge structuresresults in reduced deflections and,because of their greater stiffness,continuous members are much lesslikely to vibrate to an annoying de-gree under service conditions thanare simple structures. However, pro-viding continuity, or simulated con-tinuity through the use of canti-

80ҟ PCI Journal

Fabrication, handling and temporary construction loads, as well asstructural requirements for shear and flexure, influence long-spanprecast prestressed girder dimensions and details. An analysisof the distribution of wheel loads to girders is presented.Various types of erection procedures are described.

levered members from which simplebeams are suspended, may result inundesirable restrictions on the con-struction methods. In addition, theseschemes may require constructionprocedures and details that areeither expensive or time-consumingand hence extend the constructionperiod.

At times the finished appearanceof a proposed bridge is also a signifi-cant design consideration.

GIRDER DIMENSIONS AND DETAILS

In selecting the dimensions andproportions for a precast prestressedconcrete girder, the designer mustconsider the practical problems ofgirder fabrication, handling andtemporary construction loads as wellas the structural requirements of de-sign shear and flexural stresses. Theproportions of a girder should be se-lected to facilitate constructionwhere possible. For example, itshould be confirmed that a practicaltendon layout can be obtained with

the contemplated web thickness andend block dimensions, giving due re-gard to the space that will be oc-cupied by reinforcing steel. Onemust be certain that sufficient spaceremains for proper concrete placingand consolidation. Providing a rela-tively wide top flange frequentlyeliminates the need for temporarylateral bracing during handling ofthe precast girders as well as whenthey are subjected to temporary con-struction loads. A wide top flangemay also permit the use of a con-crete having a moderate rather thana high 28-day strength. However, theoverall weight of a girder is greaterwhen a wide top flange is provided.This must be given careful consider-ation both from a transportation anderection viewpoint.

Modern lightweight aggregateconcretes with strengths of 5000 psi(35 kg/cm) or more generallyweigh about 110 lb. per cu. ft. (1750kg/ms). The reduction in deadload that results from the use of suchlightweight concrete can be very sig-

July-August 1971 81

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porary deck must be provided overthe tops of the girders when theerection of multispan bridges is to bedone by launcher (see page 94) un-less the permanent deck is placedprior to advancing the launcher. Suf-ficient prestress to withstand theconstruction loads often will notexist in the bottom fibers of thegirders if the permanent deck isplaced before the launcher is ad-vanced.

Girders having the proportionsshown in Fig. 2 normally do not re-quire temporary lateral bracing dur-ing handling and may only requiretemporary bridging or diaphragmsbetween girders to resist construc-tion loads even when launchers areused.

Fig. 1. AASHO-PCI Type IV standardbridge beam

nificant in long span girder bridgeconstruction. This weight reductioncan result in savings in prestressingsteel, reinforcing steel, and substruc-ture costs as well as in transportationand erection costs.

Girders having proportions similarto those shown in Fig. 1 have fre-quently been used in bridge con-struction. The narrow top flange fre-quently results in these girders beingunstable during handling and underthe action of constuction loads un-less they are provided with someform of temporary lateral bracing.This is especially true for the longergirders. This type of girder is cus-tomarily used with a cast-in-placedeck that extends across the tops ofthe girders. Reinforcing steel nor-mally extends from the tops of theprecast members into the cast-in-place slab. The result is that a tem-

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Fig. 2. Precast girder with a wide topflange

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BRIDGE DECK DETAILS

Narrow top flange. The most fre-quently used detail for the connec-tion of the bridge deck to the top ofnarrow-flanged bridge stringers isshown in Fig. 3. This detail providesa thickened section of the deck im-mediately over the girder. The thick-ened section is frequently called ahaunch. The haunch is normallygiven a specific dimension at thecenter lines of bearings and is al-lowed to vary between bearings.With this detail, assuming thehaunch dimension is sufficientlylarge, variation in cambers betweenthe precast girders can be accommo-dated easily and without the girdersextending into the bottom of thedeck. Occasionally the deck-girderconnection detail shown in Fig. 4 isspecified; strict conformance to thisdetail is impossible due to cambervariations between individualstringers. When this detail is used,the finished grade of the deck willgenerally require field adjustment inorder to accommodate the cambervariations and still obtain a satisfac-tory profile. For this reason the de-tail in Fig. 4 is not recommended.Wide top flange. A number of fac-tors must be considered in determin-ing the deck details that are to beused with girders that have wide topflanges. The principal decisions thatmust be made include the methodsof handling the transverse slope ofthe deck and the camber variationsbetween adjacent girders. If thedeck is to be cast-in-place over thetop of the precast girders, using ahaunch of suitable dimension asshown in Fig. 5 will afford a satisfac-tory solution just as it does for gird-ers with a narrow top flange. Thedead load of the structure and thedepth of the haunch increase withthe width of the top flange because

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Fig. 3. Recommended slab haunchdetail

the minimum thickness of thehaunch is controlled by the haunchdepth at the edge of the girderflange on the low side. This is a sig-nificant disadvantage in using thissolution with girders having a widetop flange. On the other hand, if theengineer wishes to use the top flangeof the girder in bending as a portion

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Fig. 4. Poor slab haunch detailJuly-August 1971ҟ 83

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Fig. 5. Typical bridge cross-section with cast-in-place deck

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Fig. 6. Bridge cross-section with asphalt wearing surface of variablethickness

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Fig. 7. Bridge cross-section with separate leveling and wearing courses

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Fig. 8. Bridge cross-section formed with girders of asymmetrical cross-section

of the structural slab, several solu-tions, none of which are ideal, arepossible. One solution is to place thegirders on the same level as shownin Fig. 6 and use the wearing surfaceto provide the necessary transverseslope as well as to accommodatecamber variations. The obvious dis-advantage to this solution is theamount and weight of material re-quired in the wearing surface.

Another method is to place thegirders on bearings set at differentelevations as dictated by the desiredtransverse slope. This is illustratedin Fig. 7. The amount of material inthe leveling course and wearing sur-face is less than that required whenthe girders are placed on the samelevel, but it is still a sizeable amount.Furthermore, the actual dimensions

of the leveling course and wearingsurface must be field adjusted to ac-commodate camber variations be-tween the individual girders aftertheir erection. An additional unde-sirable feature of this detail is thedifference in slope between girderflange and the deck closure, whichresults in the steel being bent aroundreentrant corners.

The transverse slope can also bebuilt into the girders, but this detailresults in an asymmetrical girdersection and is considered undesir-able for this reason. A wearing sur-face is required with the slopinggirders (Fig. 8) to eliminate the un-evenness resulting from camber var-iations and to provide a suitable pro-file.

The scheme shown in Fig. 9 has

Fig. 9. Bridge cross-section with slab of variable depth July-August 1971ҟ 85

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Fig. 10. Reinforced concrete intermediate diaphragm

the advantages of the normal haunchdetail and yet uses the top flange ofthe precast girder structurally as acomponent of the deck. The variabledepth of the deck results in a reduc-tion in the transverse positive bend-ing moments and an increase in thenegative bending moments both ofwhich are advantageous for slabshaving thickened sections near thesupports. Proper detailing and con-struction methods will render awearing surface unnecessary withthis solution.

A manufacturing disadvantage ofthe schemes shown in Fig. 6, 7 and 8is that the reinforcing steel must beextended from the girder into thecast-in-place portion of the deck.This complicates the edge forms forthe girder top flange and increasesthe labor required to assemble andstrip the top flange edge forms. Inaddition the deck reinforcing costsare higher; the many short bars

which must be used with these de-tails result in more material (manylap splices) and higher placing costs.

The decks shown in Fig. 5 and 9can be effectively done in reinforcedor post-tensioned concrete. In addi-tion, with these details it is not nec-essary to extend reinforcing barsthrough the edges of the top flange.

INTERMEDIATE DIAPHRAGMS

Moment resisting intermediatediaphragms are provided in concretebridges to distribute the effect ofconcentrated loads over a number ofgirders. The deck effectively distri-butes a wheel load to the adjacentgirders but the intermediate dia-phragms, being very stiff, cause thegirders to deflect together. In orderto perform as intended, it is neces-sary that the diaphragms have thecapability of carrying bending mo-ments and shear forces. In bridgesuperstructures which are narrow

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when compared to their span, the in-termediate diaphragms can be con-sidered to be infinitely stiff and sub-ject to positive bending moments. Inbridges which are wide when com-pared to their span, the intermediatediaphragms are not infinitely stiffbut deflect elastically and may besubject to negative as well as posi-tive bending moments.

The most commonly used detailfor intermediate diaphragms is illus-trated in Fig. 10. The diaphragmsare generally from 8 to 10 in. (20 to25 cm) thick depending upon theirdepth. To facilitate the removal ofthe girder forms after casting, holesare normally formed through the in-terior girders and inserts are pro-vided in the exterior girders at thelocations of the intermediate dia-phragms, instead of casting the gird-ers with reinforcing steel dowelsprotruding from their sides at thediaphragm locations. The holes andinserts are used as shown in Fig. 10;

the detail facilitates constructionand yet provides the diaphragm withthe capability of transmitting mo-ment and shear forces.

Occasionally post-tensioning inthe intermediate diaphragms is spe-cified. Although post-tensioned dia-phragms are excellent from a struc-tural standpoint, they are not in gen-eral use because they cost more thanreinforced concrete diaphragms.

END DIAPHRAGMS

End diaphragms perform the im-portant function of providing lateralbracing of the girders at the ends. Todo this properly, they must be cap-able of resisting nominal axial loadsand moments. End diaphragms ongirders that are supported by metalbearings often are similar to thatshown in Fig. 11, where the dia-phragm does not extend the fullheight of the girders. With this typeof bearing, the forces from the

Fig. 11. End diaphragm detail when metallic bearings are usedJuly-August 1971ҟ 87

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superstructure are transferred to thesubstructure through the metal bear-ing assemblies. When elastomericbearings are used, the end dia-phragm may be extended to the topof the pier and be provided withshear keys that extend into the pierbetween the girders (Fig. 12). Withthis detail the lateral forces can betransmitted to the substructure with-out restraint against rotation.

The use of elastomeric bearingpads has significantly increased inbridge construction in recent years.This can be attributed in part to thefact that the AASHO "Standard Spe-cifications for Highway Bridges" nolonger restricts the use of elastomer-ic bearing pads to spans of 80 ft.(24m) or less. In addition, their lowercost and simplicity of constructiondetails have enhanced their use.Elastomeric bearing pads have theadvantage of providing the intendedrotational and shear deformations asa function of the physical propertiesand shape of the principal compo-nent materials from which the bear-

ings are made rather than relyingupon a frictional coefficient that mayvary with the •passage of time due tocorrosion or other causes as is thecase with metallic bearings.

DISTRIBUTION OF WHEEL LOADTO GIRDERS

The "Standard Specifications forHighway Bridges" adopted by theAmerican Association of State High-way Officials in 1969 (10th Edition)in Division 1, Section 3, stipulatesthat "in view of the complexity ofthe theoretical analysis involved inthe distribution of wheel loads tostringers, the empirical method here-in described is authorized for the de-sign of normal highway bridges."The live Ioad bending moment fromeach interior stringer with this em-pirical method is then given to bethe same for bridges having con-crete decks on steel I-beam stringersand on prestressed concrete girders.For bridges of two or more trafficlanes having an average girder spac-ing of 14 ft. (about 4m) or less, the

88ҟ PCI Journal

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Fig. 13. Cross-section of a prestressed concrete stringer bridge at an intermediatediaphragm (top); loading and deflection relationships for the loaded girder (lowerleft); loading and deflection relationships for the unloaded girders (lower right)July-August 1971ҟ 89

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Fig. 14. Loading and elastic curve forthe intermediate diaphragm

cm4)° and an elastic modulus of 3,-000,000 psi (210,000 kg/cm2). If aload of 20,000 lb. (9,060 kg) is placedat midspan of the central girder, andthe intermediate diaphragms are assumed to be infinitely stiff, the loadsand deflections of the third pointsand midspan of the loaded and un-loaded girders would be as shown inthe lower left and lower right corn-ers, respectively of Fig. 13.

Equating the relationships forthird-point deflections given in Fig.13, one finds that P' = 0.115 P and ifP = 20,000 lb., P' = 2300 lb. (1050 kg).Using these values of P and P', thedeflections of the unloaded andloaded beams and diaphragm (seeFig. 14) are calculated.

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empirical facor is S/5.5, where Sis girder spacing in feet. The productof this factor and the moment due toone wheel load is the moment to beused in the design of an interiorgirder. If the factor of S/5.5 is rea-sonably accurate for a bridge of nor-mal steel stringer construction, itshould be very conservative for atypical prestressed concrete girderbridge because the latter would havemoment-resisting intermediate dia-phragms whereas the former wouldnot.

As an example of this, consider thecross-section of the prestressed con-crete girder bridge shown in Fig. 13.Assume the bridge is a simple spanof 100 ft. (30 m) with intermediatediaphragms at the third points. Thetransformed composite girder sec-tion will have a moment of inertia ofthe order of 530,700 in.4 (22,000,000em4) and an elastic modulus of 4,-000,000 psi (280,000 kg/cm2). Thediaphragm will have a moment ofinertia of 189,700 in.4 (7,900,000

For unloaded beam:

8 (third point)_ 5 x 2300 x 1003 x 1728

162 x 4,000,000 x 530,700

= 0.578 in.8 (midspan)

_ 23 x 2300 x 100,3 x 1728648 x 4,000,000 x 530,700

= 0.0665 in.

For loaded beam:

S (third point)_ 23 x 20,000 x 1003 x 1728

1296 x 4,000,000 x 530,70020x2300x1003x1728162 x 4,000,000 x 530,700

= 0.2889 — 0.2311= 0.0578 in.

°Ba ed on a T-section with flange 6 x 72 in.(15 x 183 cm) and stem 8 x 45 in. (20 x 115cm).

90 PCI Journal

8 (midspan)20,000 x 1003 x 1728

48 x 4,000,000 x 530,70092 x 2300 x 1003 x 1728

- 648 x 4,000,000 x 530,700

= 0.3392 — 0.2658 = 0.0734 in.For diaphragm:

(max.)_ 21 x 2300 x 29.03 x 1728

384 x 3,000,000 x 189,700

= 0.00932 in.

The diaphragm deflection is small(16 percent) in comparison to thegirder deflection and hence the as-sumption that the diaphragm is in-finitely stiff is not unrealistic. How-ever, the assumption that the inter-mediate diaphragms are infinitelystiff is conservative in the analysisof the exterior girders and not con-servative in the analysis of the gird-ers located near the center of thebridge. The deck is not as efficient indistributing the loads laterally as arethe intermediate diaphragms. Thisshould be apparent when one rea-lizes that a 6 in. (15 cm) thick deckhaving a width of 33 1/3 ft. (10 m) hasa moment of inertia of 7200 in.4(300,000 cm4 ) which is less than 4percent of that of an intermediatediaphragm.

Consider the design live load mo-ments one would use based upon theassumption that the intermediatediaphragm is infinitely stiff, in com-parison to those obtained using theempirical AASHO requirements.Consider the same bridge cross-sec-tion as shown in Fig. 13 with twolanes of truck loading placed as ec-centrically as possible (according toAASHO requirements). With thisgirder arrangement the proportionof the wheel loading per girder isshown in Fig. 15 for both designmethods. The AASHO method gives

higher live load design moments forthe interior girders but a much lowerdesign moment for the exterior gird-er.

Note that AASHO permits [Sec-tion 3-1.3.1 (B) (2)] the loads dueto curbs, railings and wearing sur-faces to be distributed equally to all.roadway stringers when such loadsare applied after the concrete deckslab has cured.

It seems logical in this day of theprogrammable electronic calculatorsand computers that bridge engineerswould be designing bridge struc-tures based upon elastic and plasticanalysis rather than with empiricalmethods. The prestressed concreteindustry might well enhance its po-sition in the area of bridge construc-tion by sponsoring some research in-to more sophisticated methods of de-signing prestressed concrete bridgesuperstructures, which take the stiff-ness of the transverse members (dia-phragms or deck) into account.

It should also be mentioned thatgirder bridges without intermediatediaphragms are being used in Eu-rope. In these bridges the deck is re-lied upon to distribute the wheelloads to the girders. The decks ofbridges of this type are subjected tomoments resulting from the differen-tial deflection between the support-ing girders. Hence, the decks mustbe thicker and more heavily rein-forced than are the decks of bridgeshaving moment-resisting intermed-iate diaphragms. The savings in con-struction cost resulting from theelimination of the intermediate dia-phragms can be significant in someinstances. The deck design require-ments of AASHO are for bridgeswithout moment resisting interme-diate diaphragms.Provision for decksof variable depth should also begiven in the bridge design criteria

July-August 1971ҟ 91

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Fig. 15. The bridge cross-section of Fig. 13, eccentrically loaded with moment distribution factorsbased upon an infinitely stiff diaphragm and upon AASHO standards

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Fig. 16. Floating crane shown erecting a 230-ton girder for the CrossbayParkway Bridge in New York

since the distribution of moment isaffected by the variable depth. Theprestressing industry would probab-ly benefit from such provisions be-cause they would enhance the use ofpost-tensioning in bridge decks.

PRECAST GIRDER ERECTION

Use of cranes. Precast prestressedconcrete bridge girders have beenerected with a variety of equipment.Truck cranes have been used exten-sively in the erection of grade sepa-ration structures and bridges acrossdry river beds. The principal appealof truck cranes lies in their ability tobe moved relatively quickly and atlow cost as well as the fact that theyare available in a variety of sizes inalmost all locations. The principaldisadvantage of truck cranes is theirrelatively low capacity when han-dling loads on large radii or whenhandling loads at heights requiringlong booms.

Crawler cranes having rated capa-cities larger than those of truckcranes are available. As with truckcranes, the actual capacity of crawl-

er cranes depends upon the loadradius on which the crane is operat-ing as well as the length of boomwith which it is equipped. A princi-pal disadvantage of crawler cranesis that they must be moved from jobto job with other equipment.

With both large truck cranes andlarge crawler cranes, a significantamount of dismantling must be donebefore they can legally be movedover the highways. This contributesto the cost of using this equipment.

A limited number of floatingcranes having very high lifting capa-cities are available to constructionprojects in some localities. Some ofthe larger floating cranes have capa-cities as high as 600 tons (545t). Theuse of floating cranes is normallylimited to projects across deep waterand the availability of such equip-ment should be confirmed before aframing scheme requiring suchequipment is adopted. A floatingcrane erecting one of the cantilever-ed girders of the Crossbay ParkwayBridge in New York City is shownin Fig. 16.

July-August 1971ҟ 93

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Fig. 17. The principles of erection with a cantilevered launcher

Use of launchers. Specially con-structed girder launchers have beenused to a limited extent in theUnited States, and to a large degreein other countries. Launchers are de-signed as steel or aluminum trusseswhich are capable of being movedacross a body of water or deep can-yon, erecting the girders in eachspan as the launcher progressesacross the obstacle. Launchers mustbe capable of cantilevering from anerected span to the next pier or abut-ment preparatory to the erection ofanother set of girders. Additionally,the launcher must be capable of lift-ing and erecting the girders. Anotherrequirement for the use of launchersis that the erected girders must besufficiently strong (this may requiretemporary bracing) to support thelauncher when it is being moved for-ward as well as the girders that arebeing moved into position for erec-tion.

The basic sequence of erectingprecast bridge girders with a launch-er is as follows:

1. Using a precast girder as acounterweight and the hoist atthe rear of the launcher, thelauncher is raised in such amanner that the span from theinboard portal to the outboardportal is cantilevered.

2. The launcher and counter-weight girder are moved for-ward as a unit.

3. When the outboard portal hasreached the next pier, the hoistat the rear of the launcher is re-leased allowing the launcher tobecome a simple span from in-board to outboard portal.

4. The girder that is to be erectedis moved forward.

5. The outboard erection hoist onthe launcher is connected to thegirder.

6. With the outboard and inboardends of the girder being sup-ported by the launcher and adolly respectively, the girder ismoved outboard.

7. When the inboard end of thegirder reaches a point where

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the dolly can no longer be used,the inboard end of the girderis picked up by the inboardhoist.

8. With the girder being sup-ported by both hoists, it ismoved into place.

A launcher in the moving mode aswell as in the erection mode is illus-trated in Fig. 17.

After all of the stringers in anyspan are erected, the launcher ismoved ahead and into position forerecting the next span. Temporarybracing may or may not have to beprovided between the erected gird-ers before the launcher is movedahead.

The launcher may be designedtwo spans long with its own counter-weight as shown in Fig. 18. Withthis configuration it is not necessaryto use a girder as a counterweightwhen the launcher is moved for-ward. The disadvantage to this fram-ing scheme is that more material isrequired in the launcher itself thanif a girder is used for the counter-weight.

For spans of 120 to 150 ft. (36 to45 m), launchers which are designedas cantilevers are feasible. For long-

er spans a launcher can be providedwith a tower at the inboard portaland with ties which extend to theextremities of the launcher. In thismanner the trusses of the principallaunching span are not subject tomoments of opposite sign during thelaunching sequence. The result is aneconomy of materials in the launch-er.

Launchers have been providedwith transverse beams and trolliesfor use in moving the girders trans-versely into their final position afterthey have been moved into placelongitudinally (Fig. 18). In other in-stances the girders are moved later-ally with greased sliding plates andhydraulic jacks after the launcherhas positioned the girders longitu-dinally.Use of falsework. Temporary false-work has also been used for theerection of precast prestressedbridge girders on bridges acrossstreams that flow violently yet inter-mittently. This method is especiallyapplicable in areas where suitablecranes or launchers are not avail-able. The principal advantage of thismethod lies in the reduced risk. Ifthe superstructure were cast-in-

Fig. 18. Cable-stayed launcher equipped to move girders longitudinallyand transversely, used on the Yapacani River Bridge in Bolivia

July-August 1971ҟ 95

place on falsework, the work wouldbe vulnerable to possible flood dam-age for a longer period than is thecase when the stringers are precastoutside of the limits of the river bedand moved into place on falsework.

LONG SPAN BRIDGE APPLICATIONS

The following brief descriptions ofseveral projects illustrate some ofthe factors discussed above.Yapacani River Bridge—Republic ofBolivia. This bridge consists of 11 in-terior spans of 128 ft. (38m) each andend spans of 107 ft. (32m). The spansare all simply supported with pre-stressed girders that were designedto be precast. The typical span con-sisted of four girders 6.7 ft. (2m)deep that weigh approximately 60tons (54 t).

It was originally planned to con-struct steel falsework trusses thatwould span from pier to pier belowthe girder soffit grade and cast thegirders in place on top of this false-work, moving the trusses from spanto span after the girders in each spanhad been stressed.

During construction of the sub-structure it was observed that theriver could rise during floods to alevel which provided only 1 ft. (30cm) of clearance between the riverand the specified girder soffit eleva-tion. For this reason the falseworktrusses were not used. The materialthat had been purchased for thefalsework trusses was fabricated intothe launcher shown in Fig. 18 andthe stringers were successfullylaunched. Temporary bracing of theindividual stringers during handlingwas required due to their narrow topflanges. Top flange bracing and tem-porary X-bracing (bridging) had tobe provided laterally between thefour erected girders in each span be-fore the launcher could be movedahead and before additional girders

could be moved into position forerection.

Because of the very rapid rise inwater level that is possible when theYapacani River floods, erection withthe use of cranes in the river bottomwould have been extremely risky. Inaddition, cranes of the required ca-pacity do not exist in Bolivia andbecause of transportation limitationscould only be brought into the coun-try at great expense.Bolivian Highway Projects 1 and 4.Forty-five bridges were included inthis project. They varied frombridges of one span to those havingeight spans, with span lengths fromas little as 50 ft. up to 130 ft. (15 to39 m). The bridges were scatteredthroughout the 170 miles (270 km)of new highway that was included inthe project. The bridges on the west-ern side of the project are on a sec-tion of road that winds down verysteep mountain sides with the resultthat many of the bridges are on tighthorizontal curves over small creeksthat are normally dry, but which canbe torrents during rain storms. Thebridges on the eastern side of theproject are in the jungle and crosstropical rivers which are wide, me-andering streams that are easilycrossed during the dry season andimpossible to cross during the rainyseason.

These bridges were all designedas simple spans with precast pre-stressed girders having narrow topflanges. A launcher was used to erectthe girders in the longer multispanbridges in the jungle because:

1. The jungle terrain is relativelyflat which facilitates movingthe launcher from bridge site tobridge site.

2. The use of cranes was not con-sidered practical due to theirinavailability in Bolivia and the

96ҟ PCI Journal

risk of working them in the riv-er bottoms if they had been im-ported.

3. The launcher permits girdererection even during the rainyseason.

The bridge girders on the westernside of the project were either cast-in-place or precast and moved intoplace with the use of temporaryfalsework because:

1. The extremely steep terrainmade pioneering the road a ma-jor operation and the bridgeswere needed in the early stagesof the project in order to facili-tate the road construction.Moving large erection equip-ment from bridge site to bridgesite in the early stages of con-struction would have been verycostly and perhaps impossible.

2. Many of the bridges are on hor-izontal curves of small radiuswith super-elevations as greatas 10 percent. Since launchersare not easily adapted to theseconditions, the contractor waswilling to take the risk involvedwith the use of temporary false-work.

Temporary bracing was requiredon all but the shortest girders duringhandling. Substantial temporarybracing of the erected girders wasrequired for launcher erection oper-ations.Cross bay Parkway Bridge—NewYork City. This structure has severalsimple spans of 130 ft. (39 m) and amain channel span of 275 ft. (83 m).The main channel span was accom-plished through the use of two can-tilevered spans of 66 ft. (20 m) with asuspended span of 143 ft. (43 m). Thetypical bridge section has elevenmodified T-shaped precast girdersthat are 8 ft. (2.4 m) wide and 8 ft.deep. The depth of the cantilevered

girders varies from 8 to 11 ft. (2.4 to3.3 m).

The girders were precast at CapeCharles, Virginia, and transportedby barge to the bridge site at Jamai-ca Bay on the south side of LongIsland. The girders were erectedwith a floating crane (see Fig. 16).The largest were the cantileveredstringers which were about 200 ft.(60 m) long and which weighed 230tons (210 t). Because of their widetop flanges, these girders did not re-quire temporary bracing during han-dling and no significant loads wereimposed upon the girders after theirerection and before the diaphragmsand deck closures were in place.

The basic solution used on thisproject was well adapted to thestructure because of the availabilityof precasting facilities on the water,adequate barges and floating cranes.Dacca-Aricha Project—East Paki-stan. This project includes five majorbridges. Two of these have simplespans of 80 ft. (24 m) using girderssimilar to the standard AASHO-PCIprestressed concrete girders, whichwill be erected by floating cranes.

The largest bridge crosses the Kal-iganga River and consists of fivecast-in-place box girder portionshaving main spans of 200 or 220 ft.(60 or 66 m) with cantilevers 50 ft.(15 m) long at each end. This resultsin cast-in-place sections that areeither 300 or 320 ft. (90 or 96 m)long. Suspended spans, composed ofprecast girders with a cast-in-placedeck, span 100 ft (30 m) between thecast-in-place sections. A significantdisadvantage with the cast-in-placebox girders lies in the design re-quirement that the falsework mustremain in place until the suspendedspan girders are erected at each end.This results in a substantial amountof falsework material being required

July-August 1971ҟ 97

since two cast-in-place sections mustbe completed before one precastspan can be erected. The narrowsuspended girders require temporarybracing during handling and will beerected with floating equipment.

The remaining two bridges crossthe Mirpur and Bangshi Rivers andare similar in design. Their main-span framing is similar to that whichwas used on the main span of theCross Bay Parkway Bridge. In eachbridge, I-shaped cantilevered gird-ers, which are designed to be cast-in-place, support I-shaped suspend-ed girders. All of the girders are de-signed to receive a cast-in-placedeck after the suspended spans areerected.

The contract documents providethat the cantilevered girders on oneof the latter two bridges must re-main on falsework until the suspend-ed girders are in place. Temporaryprestressing tendons are provided inthe cantilevered girders of bothbridges. The temporary tendons areto be stressed during the first phaseof stressing permanent tendons andmust remain in place until the sus-pended girders are erected. In eachcase, a portion of the permanent ten-dons must be stressed after the sus-pended spans have been erected. Be-cause the cantilevered girders havenarrow top flanges they are veryflexible laterally and must be pro-vided with significant lateral bracingin order to prevent their bucklingwhen the suspended stringers are

erected.The only feasible means of erect-

ing the suspended span precast gird-ers is with floating cranes. A girderlauncher is not feasible because ofthe loads that would have to be im-posed upon the cantilevered girderswith this erection method. Schedul-ing and site conditions will not per-mit other erection procedures.

Alternate designs for these bridgeswere prepared using simple spanprecast, prestressed girders withoutcantilevers. The same spans as wellas vertical and horizontal clearanceswere maintained. The contractorevaluated these designs and decidedthat a significant saving could havebeen made on these bridges if thesealternate designs had been r sed. Thesaving would have been significantlygreater if lightweight concrete couldhave been used. This is in spite ofthe fact that the largest girderswould have weighed 270 tons (245 t),and the enormous launcher requiredto erect these girders would havebeen completely amortized on thisproject.

There is no doubt that precast pre-stressed concrete members have animportant role to play in futurebridge construction. The maximumeconomy will be derived through theuse of simple, economical details ashave been described above, in com-bination with the proper selection offraming types and construction pro-cedures to fit the prevailing condi-tions.

Discussion of this paper is invited. Please forward your comments to PCI Headquartersby Nov. 1 to permit publication in the Nov.-Dec. 1971 issue of the PCI JOURNAL.

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