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An Overview ofPrecast PrestressedSegm ental Bridges

Walter Podolny, Jr.Bridge D ivis ionOff ice of En gineer ingFede ra l H ighway A dm in is t ra tionU.S. De par tment of Transpor ta t ionW ashington, D.C.

T he seventies will be recorded byengineering historians as the de-cade in which prestressed concretesegmental bridge construction came ofage in North America. Segmental boxgirder bridges have attracted the at-tention and captured the imaginationof bridge engineers and designersacross the continent.

Because of practical limitations ofhandling and shipping, the precastprestressed I-girder type of bridgeconstruction is limited to an approxi-mate range of 120 to 150-ft (37 to 46m) spans. Beyond this range of span,post-tensioned cast-in-place box gir-ders on falsework are more attractive.However, in certain instances the ex-tensive use of falsework can prove tobe an economic disadvantage. Wheredeep ravines or navigable waterwaysmust be crossed, extensive formworkmay be impractical.

Construction in this manner mayalso have a serious impact upon envi-ronment and ecology. Prestressedsegmental construction has extendedthe practical span of concrete bridgesto approximately 800 ft (244 m).Where segmental construction is usedin conjunction with the cable-staybridge concept, the span range can beextended to 1300 ft (400 m) andperhaps longer.'

Because construction of the super-structure is executed from above, i.e.,at deck level, the use of extensivefalsework is avoided. Thus, there is noeffect upon navigation clearance fromfalsework during construction and thecost of extensive formwork is elimi-nated. Segmental viaduct type bridgesprovide a method whereby the impactof highway construction through en-vironmentally sensitive areas can beminimized.

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Utilization of an elevated viaducttype structure requires only a rela-tively narrow path along the align-ment to provide access for pier con-struction. Once the piers have beenconstructed, all construction activity isfrom above. Thus, the impact on theenvironment is minimized. Also, be-cause the structure is elevated, as op-posed to an at-grade highway, there isno interference with wild life migra-tory habits.Prestressed concrete segmentalbridges have proven to be estheticallyappealing and, because various con-struction methods can be used, thestructures are cost effective and en-vironmentally adaptable.

Evolution ofSegmental BridgesBefore discussing the various facets

and variants of precast segmental boxgirder bridges, it may enhance theunderstanding of this type of con-struction to briefly trace the historical

evolution of prestressed concretebridges to this point in the state-of-the-art and to present a few basic de-finitions.Precasting of elements or membersof a structure implies that the concreteis cast in forms at some location otherthan the final position of the member.The member may be cast at a perma-nent precasting plant at some locationother than the construction site; thentransported by truck, rail, or barge tothe site; and eventually erected to itsfinal position. The member may alsobe cast at some location in closeproximity to the construction siteeliminating the transportation fromprecasting plant to construction site.In either situation the member is castat a location other than its final posi-tion in the structure.Segmental construction has beendefined as "... a method of construc-tion ... in which primary load carry-ing members are composed of indi-vidual segments post-tensioned to-gether. "2

As early as 1948, Eugene Freyssi-PCI JOURNAL/January-February 1979 7


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Fig. 1. Ma rne River Bridge near P aris, France. (Courtesy: Figg and M ullerEngineers, Inc., Tallahassee, Florida.)

net, the great French prestressingpioneer, used prestressed concrete forthe construction of five bridges overthe Marne River near Paris, withspans of 240 ft (73 m) having an ex-ceptionally light appearance (Fig. 1).Construction of these bridges, as indi-cated in Fig. 2, utilized precast seg-ments which were post-tensioned to-gether. Thus , by definition, these fivestructures can be called precast pre-stressed segmental bridges.Prestressing of bridges in NorthAmerica did not start until about1949.* The first prestressed bridge inthe United States was built in Madi-son County, Tennessee. This bridge*An interesting historical account of the early prestressedbridges in A merica is given by Ch arles C. Zollman in"Reflections on the Beginnings of Prestressed Concretein Am erica Part 1: Magnel's Impact on the A dvent ofPrestressed Concrete; Part 2: Dynamic A merican En-gineers Sustain Magnel's Momentum," PCI JOU RNA L,V . 23, Nos. 3 and 4, May-June and July-August 1978, p p.22-48 and pp . 30-67 . See also Ross Bryan's article on"Prestressed Concrete Innovations in Tennessee," pub-lished in the current PCI JOURN A L, pp . 14-31.

was made using a series of machine-made blocks (strung like beads on astring) which were prestressed to-gether to form a beam.The construction method (althoughvery crude by modern standards) is

similar, in principle at least, to today'sprecast prestressed segmental bridges.The Tennessee prestressed blockbeam bridge was followed veryshortly by the famed Walnut LaneBridge in Philadelphia, Pennsylvania,in 1950. The 160-ft (48.8 m) longbeams were cast-in-place and post-

tensioned.Soon thereafter, precast preten-sioned bridge girders evolved result-ing from inherent economies andquality control of plant fabricatedelements. With few exceptions, duringthe fifties and early sixties, mostmulti-span precast prestressed bridgesbuilt in the United States were de-signed as a series of simple spans.

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They were designed with standardAASHTO-PCI girders of various crosssections in spans ranging up to about100 ft (30.5 m), but more commonlyfor spans of 40 to 80 ft (12 to 24 m).The advantages of a continuous cast-in-place structure were abandoned infavor of the more economical con-struction offered by plant producedstandardized units.During the middle sixties a growingconcern with regard for safety of thehighways asserted itself. An AASHTOTraffic Safety Committee report in1967 called for:"A dopt ion and use of two-spanbridges for overp asses crossing di-vided highways ... to eliminatethe bridge piers normally placedadjacent to the sh oulders."

It soon became apparent that theconventional precast pretensionedAASHTO-PCI girders were limited bytheir transportable length and weight.Transportation over the highwayslimits the precast girder to a range of

100 to 120 ft (30.5 to 36.6 m) in lengthdepending upon local regulations.As a result of longer span require-ments, a study was conducted by thePrestressed Concrete Institute incooperation with the Portland CementAssociation. 4 This study proposedsimple spans up to 140 ft (42.7 m) andcontinuous spans up to 160 ft (48.8 m)be constructed of standard precastgirders up to 80 ft (24 m) in lengthjoined together by splicing and post-tensioning. To obtain longer spans theuse of inclined or haunched piers wasproposed. In general, these conceptsutilized precast I- or box girders withfield splices and post-tensioning forcontinuity.This type of construction, usinglong standard precast prestressedunits never quite achieved the popu-larity that it merited. Despite somelimitations, the method is adaptablefor spans up to 200 ft (61 m ).The concepts developed by thePCI-PCA studies fall into the defini-

Fig. 2. Marne River Bridge near Paris, France. (Courtesy: Figg and MullerEngineers, Inc., Tallahassee, Florida.)PCI JO UR NAU January-February 1979 9


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rig. s. unoisey-Le-Roi Bridge over the Seine River south of Paris, France.(Courtesy: Figg and Muller Engineers, Inc., Tallahassee, Florida.)

tion of precast segmental constructionand might be described as "longitudi-nal" segmental construction. The in-dividual elements are long with re-spect to their width.

As spans increased, designersturned toward utilization of post-tensioned cast-in-place box girderconstruction. The Division of High-ways, State of California, includingseveral other states, have been quitesuccessful using cast-in-place, multi-cell, post-tensioned box girder con-struction for multi-span structureswith spans of 300 ft (91.5 m) andlonger. However, this type of con-struction has its disadvantages; it re-quires extensive formwork duringcasting with its undesirable impactupon the environment and/or ecology.

Meanwhile in Europe, segmentalconstruction proceeded slightly dif-ferently in conjunction with box gir-der design. Segments were cast-in-place or precast in relatively shortlengths, providing full roadway widthand depth. Today, "segmental con-struction" is generally recognized ashaving been pioneered in Europe.Ulrich Finsterwalder, in 1950, was

the first to apply cast-in-place seg-mental prestressed construction in abalanced sequence to a bridge cross-ing the Lahn River at Balduinstein,Germany. This system of cantileversegmental construction rapidly gainedacceptance in Germany, especiallyafter the successful completion of abridge crossing the Rhine River atWorms in 1952. 5 Since then, the con-cept has spread across the entireworld.

Concurrently, precast segmentalconstruction was evolving during thisperiod. In 1952, a single span countybridge located near Sheldon, NewYork, was designed by the FreyssinetCompany. Although this bridge wasconstructed using longitudinal seg-ments, rather than transverse seg-ments, as was being done in Europe,the structure represents the first prac-tical application of match casting. Thistechnique has become an importantdevelopment in precast segmentalconstruction.The bridge girders were dividedinto three longitudinal segments thatwere cast end to end. The center seg-ment was cast first and the end seg-

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F ig . 4 . JFK Mem or ia l C ausew ay , Co rpus Ch r is t i, Te xas .

ments were cast directly against thecenter segment. Keys were cast at thejoints so that the three precast ele-ments could be joined together at thesite in the same position they had inthe precasting yard. Upon shipment tothe job site, the three elements of agirder were post-tensioned togetherwith cold joints.6'7

The first major application of matchcast, precast, segmental constructionwas not realized until 10 years later,in 1962, in France. This structure, de-signed by Jean Muller,* was theChoisy-Le-Roi Bridge located south ofParis crossing the Seine River (Fig. 3).Since then the concept has been re-fined and has spread from France tomany other countries.The first precast segmental bridgeto be built in North America was theLievre River Bridge located on High-way 35, 8 miles (13 km) north of NotreDame du Laus, Quebec. The bridge,which had a center span of 260 ft (79.2m) and end spans of 130 ft (39.6 m),

was built in 1967. The Bear RiverBridge, Digby, Nova Scotia, followedin 1972 with six interior spans of 265ft (80.8 m) and end spans of 203.75 ft(62.1 m).The JFK Memorial Causeway, Cor-pus Christi, Texas (Fig. 4) representsthe first precast prestressed segmentalbridge completed in the UnitedStates. It was opened to traffic in1973. Designed by the Bridge Divi-sion of the Texas Highway Depart-ment, this structure has a center spanof 200 ft (61 m) with end spans of 100ft (30.5 m).In the United States, currently(1979), the author is aware of at least24 precast segmental bridge projectsthat are either completed, in con-struction, or in design and planningstages (see Table 1). There are un-doubtedly many more.*Jean Muller, formerly chief engineer with EntreprisesCampenon Bernard, Paris, France, is currently inpartnership with Figg and Muller Engineers, Inc., withoffices in Tallahassee, Florida, Wash ington, D.C., andParis, France.

PC I JOU RN AL/January-February 1979 1


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T able 1. Precast Segm ental Concrete Bridges in N orth A m erica.

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Advantages of P recastSegmental ConstructionIn many instances where pre-stressed concrete segmental bridgedesign alternates have competedagainst structural steel designs theyhave proven to be cost effective. As

previously indicated, when comparedto more conventional methods of con-crete construction, prestressed con-crete segmental construction has ex-tended the span range for concretebridges and is competitive in the in-termediate and long span range. Themethod eliminates the need for costlyfalsework and minimizes its impact onthe environment.In general, the economic feasibilityof cast-in-place or precast segmentswill be determined by site conditions,site accessibility, available erectionequipment, time to construct and/orerect the segments, and the relativeeconomic trade-off in transporting afinished segment as opposed to trans-porting constituent materials.Advantages

The often cited advantages of pre-cast segmental construction are: Fabrication of the segments canbe accomplished while the sub-structure is under construction,

and thus, erection of the super-structure is speeded up. By virtue of precasting andmaturity of the concrete at thetime of erection, the time re-quired for strength gain of theconcrete is removed from theconstruction critical path. As a result of the maturity of theconcrete at the time of erection,the effects of concrete shrinkageand creep are minimized.

Quality control of factory pro-duced precast concrete.

DisadvantagesThe disadvantages of precast seg-

mental construction are: Necessity for a high degree ofgeometry control during fabrica-tion and erection of segments. Temperature and weather limi-tations regarding mixing andplacing epoxy joint material. Lack of mild steel reinforcementacross the joint and therefore alimitation of tension stress acrossthe joint.

It should be noted that for large-sized projects, it is no longer difficultto set-up fully mechanized concretesite mixing equipment. With today'stechnology, it is possible to produce ahigh quality concrete at the site.Therefore, for large projects, it isdoubtful whether factory producedconcrete has an advantage over siteproduced concrete except for the im-portant aspect that loads and pre-stressing forces are applied at a laterage on a more mature concrete.

Ty pes of PrecastSegmental ConstructionIt has been observed' that the

technology for constructing segmentalbridges has rapidly advanced in thelast decade. During the initial de-velopment of segmental bridges theywere constructed by the balancedcantilever method.Currently, such techniques asspan-by-span construction, incremen-tal launching, and progressive placingare also being utilized. Thus, there arenow a variety of design concepts andconstruction methods which may beused to economically produce seg-mental bridges for almost all site con-ditions.

PCI JOURNAL/January-February 1979 3


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Fig. 5. Sallingsund Bridge, Denmark, showing balanced cantilever construction.

Balanced Cantilever ConstructionThe balanced cantilever method of

construction, sometimes referred to asthe free cantilever method, was de-veloped out of a need to eliminatefalsework. Not only is falsework ex-pensive, but it is a temporary structureand, as such, is designed with smallmargins of safety, as has been indi-cated by some falsework failures.

In waterways that are subject tospring flash floods, falsework can bewashed away resulting in potentialdamage to the structure, lost con-struction time, and possible financialruin. In navigable waterways, false-work is either not allowed or is se-verely restricted. With cantilever con-struction, falsework is eliminated be-cause precast segments are erectedand supported from the pier or the al-ready completed portion of the struc-ture.

In this method of construction seg-ments are simply cantilevered fromthe preceding pier in a balanced se-quence on each side until midspan isreached. Then a closure placement ismade with a previous half-span can-

tilever from the preceding pier (Fig.5). This procedure is then repeateduntil the structure is completed.l,s

Unless symmetrical segments aresimultaneously erected, the pier willbe out of balance by one segment.The moment caused by this imbalancecan be accommodated by a momentresistant pier. Where the pier is notmonolithic with the superstructure, atemporary moment resistance may beprovided by temporarily "clamping"the superstructure to the pier, pro-vided the pier is designed to take thetemporary moment. Where feasible,temporary bracing may be provided(Fig. 6). Obviously, the imbalancemust be maintained on the side of thepier where the bracing is located.This concept utilizes a dual systemof prestressing tendons. Cantilever(negative moment) tendons are re-quired at the top of the segments fordead load cantilever stresses [Fig.7(a)] and then after closure, atmidspan, continuity (positive mo-ment) tendons [Fig. 7(b)] are installedto accommodate the positive momentin the continuous structure. Because

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of the high cantilever moments duringconstruction, which are reduced bymoment redistribution in the finalstructure, a slightly larger amount ofprestressing is required compared to astructure supported on falsework.

In continuous structures the finalstresses in the completed structure aresubstantially different from what theywere initially during cantilever con-struction. However, subsequent con-crete creep and steel relaxation willtend to make the initial and finalstresses approach each other. Thismeans that there will be a redistribu-tion in the moments and stresses ofthe structure. In general, the negativemoments over the piers will decreasewhile the positive moments atmidspan will increase by a corre-sponding amount. This redistributionof moments must be accommodatedin the design.Normally in precast balanced can- F i g . 6 . Ko n o sh im a O h a sh i B r id g e , J a p a n .

diaphragmlosure pourR cantilever tendons

main pierentral span100 ft (30.48 m)

(a)

Fig. 7. Balanced cantilever method, system of prestressing tendons(JFK Mem or ia l C ausew ay , Co rpus Chr i s t i, Te xas) .P C I J O U R N A L / J a n u a r y - F e b r u a r y 1 9 7 9 5

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