HISTORICAL-TECHNICAL SERIES

Precast Concrete SegmentalBridges Americas Beautifuland Affordable Icons

Fora bridge type that would be

come an increasingly popularand successful competitor over

alternative building systems, precastconcrete made a rather unpretentiousdebut in the United States. Indeed, onewonders if the engineers who designed and built Americas first precast concrete segmental box girderbridge in 1973 the John F. KennedyMemorial Causeway Bridge connecting Corpus Christi and Padre Island,Texas could have imagined that thissimple, unadomed structure would bethe harbinger of some of the nationsmost beautiful and affordable bridges.

Indeed, thirty years later, precastconcrete segmental construction hasproduced some of the most technologically advanced, visually dramatic,and environmentally sensitive structures in North America as well as in

many other countries throughout theworld. Typically, the precast, prestressed concrete bridge also proves tobe the most cost-effective alternativefor owners, particularly when life-cycle costs such as maintenance andrepair are taken into account.

EVOLUTION OFPRECAST CONCRETE

SEGMENTAL BRIDGES

The European Experience

Originating in Europe, this versatileand economical bridge type emergedshortly after World War II. With theurgent need to reconstruct bombed-outbridges quickly and efficiently, andwith steel production capacity debilitated by war efforts, European designers and builders turned to prestressedconcrete.

How did precast concrete segmental construction become a majorplayer in the modern American bridge industry? While the continuinggrowth in market share for precast, prestressed concrete systems canbe attributed to numerous competitive advantages, this articlepresents an overview of the proliferation of segmental bridges in theUnited States based primarily on five advantages economy,construction efficiency, durability, design innovation and aesthetics.

Linda FiggPresident andDirector of Bridge ArtFIGG Engineering GroupTallahassee, Florida

W. Denney Pate, RE.Senior Vice President andPrincipal Bridge EngineerFIGG Engineering GroupTallahassee, Florida

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The first precast concrete segmentalbridge, the Luzancy Bridge across theMarne River in France, was erected in1946. It was designed and built by Eugene Freyssinet, the French engineerand pioneer whose technical investigations and ingenious advances duringthe first half of the 20th Century madeprestressed concrete a viable construction material.

While numerous precast, prestressedconcrete bridges were built throughoutEurope between 1946 and 1973,American bridge designers did not immediately embrace this new technology, in part because many conventional bridge engineers of the timequestioned the suitability of using precast concrete segments for major orlong-span structures. However, withfurther advancements in technology,precast, prestressed concrete emergedas a strong competitor to conventionalconstruction.

Lee Roy Selmon Expressway

A compelling example of the phenomenal evolution of precast concretesegmental bridges since 1973 ispresently under construction inTampa, Florida where the Prestressed Concrete Institute (PCI) wasfounded in 1954. The three-lane reversible structure erected span-by-span within the median of the Lee RoySelmon Expressway, on single piersonly 6 ft (1.8 m) wide at the base, willmore than double the toll highwayscapacity within the existing right-of-way while maintaining essential trafficflow during construction (see Fig. 1).Operating in the direction of peak demand (westbound from the suburbs todowntown Tampa in the morning andeastbound in the evening), the 5-mile(8 km) long structure will provide sixlanes in 6 feet, preserving most of themedian for future use.

The 3032 prestressed segments [59ft (18 m) wide, about 70 tons (64tonnes)] are precisely cast at a nearbyfactory and delivered by truck duringoff-peak hours. In the immediate areaof the worksite, the inside lanes of thefour-lane roadway are temporarilyclosed while the segments are liftedonto the self-launching, underslungerection girder. During peak commut

segmental bridges in the last thirty years.

ing hours, all lanes are open whileworkers apply epoxy between the segments and post-tension the tendons,without impeding traffic below. Atypical 140 ft (42.7 m) span requiresabout three days to complete.

Designed by Figg Bridge Engineersand built by PCL Civil Constructors,the sculpted curved forms of the superstructure and piers embody the visionof the Tampa-Hillsborough Expressway Authority for an aestheticallyunique bridge truly, a landmarkstructure. The precast concrete boxgirder has curved geometry in thewebs, corners, and the underside of thebarrier.

The bridge piers also presentsculpted shaping, with curved flares atthe top and a 3 in. (76 mm) recessalong the vertical faces. With a softcolor finish and accent lighting frombelow, the bridge will be a strikingfeature of the Tampa landscape whencompleted in 2006.

Part of a $350 million expansionprogram that includes several miles ofpark-like boulevards providing localaccess to and from the reversible express lanes at each end, the segmentalbridge contains approximately 1.6million sq ft (148,640 m2) of deck.According to the Expressway Authority, the Lee Roy Selmon Expressway

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Fig. 1. Presently under construction over moving traffic, the Lee Roy SelmonExpressway in Tampa, Florida, supports six lanes of traffic on single piers only 6 ft(1 .8 m) wide, and demonstrates the extraordinary evolution of precast concrete

September-October 2004 27

bridge is being constructed for $65 persq ft ($700/rn2), or currently about 30percent less than other major bridgesin Florida.

This rapid transit bridge is only oneexample of several other significantsegmental bridge projects currentlyunder construction.

COASTAL ENVIRONMENTSAmong the advantages touted by its

champions was the ease and economyof maintenance for precast concretebridges, particularly in coastal environments. Bridge maintenance wasone of the concerns of the Florida Departinent of Transportation (FDOT)engineers in the mid-1970s as theyconsidered replacement options for thedecaying bridges of the OverseasHighway, the transportation lifeline ofthe Florida Keys.

Fig. 2. With the firstas cast riding

surface in theUnited States, the2.3 mile (3.7 krn)

Long Key Bridge inthe Florida Keys

was completed at acost of $32.66 persq ft ($352/rn2)in1980, confirming

the economicfeasibility of precastconcrete segmental

technology.

Long Key Bridge

A Tallahassee, Florida-based civilengineer with a penchant for innovation, Eugene Figg, proposed precastconcrete segmental bridges as the solution. And further, he suggested thatFDOT request federal funds to financea demonstration project of precastconcrete technology for bridges incoastal environments. When the Federal Highway Administration agreedto his proposal, Figg designed and inspected the construction of the LongKey Bridge. Michael Construction Co.completed the 12,040 ft (3670 m)structure in 1980 five months aheadof schedule at a cost of $15.3 million, at a remarkable cost of $32.66per sq ft ($352/m2).

Erected using the span-by-spanmethod, the 103 spans, with typicallengths of 118 ft (36 m), were delivered by barge, installed as a unit,aligned, and then post-tensioned. The

Fig. 3. The longestcontinuous precast

segmental bridge inthe world in 1982,

the Seven MileBridge in the

Florida Keys wasthe first use in

North America ofprecast, match-cast

box piers,assembled with

vertical post-tensioning.

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post-tensioning tendons were externalto the box girder web, which reducedmaterial costs and construction time.The Long Key Bridge was the first ascast riding surface in the UnitedStates (see Fig. 2).

Seven Mile Bridge

Soon thereafter, Misener MarineConstruction started the nearby SevenMile Bridge. Spanning 35,867 ft(10,932 rn), it was the longest continuous precast segmental bridge in theworld in 1982. The precast segmentsfor the piers and superstructure weremanufactured in Tampa and barged450 miles (724 km) to the erection sitesouth of Marathon Key.

An overhead gantry lifted each ofthe 268 spans [135 ft (41.2 m) typicallength] into position as a unit. The superstructure was erected on precast,match-cast box piers that were assembled with vertical post-tensioning (seeFig. 3). These design innovations allowed a 60 ft (18.3) tall pier to be constructed in one day and three 405 ft(123 m) spans to be erected per week.The $45 million Seven Mile Bridgewas $7 million less than the alternative solution, and was completed sixmonths ahead of schedule. The projectwon a PCI Design Award in 1982.

ENGINEERING ANDAESTHETICS

Eugene Figgs designs were a decisive demonstration of the constructability, efficiency and economyof precast concrete segmental construction. Founded in 1978, the firm,now known as the Figg EngineeringGroup (FIGG), has continued to pioneer segmental bridge technologythrough advancements in design, construction techniques, and materials.

With numerous signature structures built in 34 states, and with construction values exceeding $7 billion,FIGGs forward-thinking design andconstruction engineers are as focusedon aesthetic achievement as they areon cost and time-saving innovations.The firms vision is to create bridgesas timeless works of art that reflect thenatural environment and the spirit ofthe communities they serve.

As a consequence of the firms vision, its clients projects have earnedmore than 200 awards, including threeof the five bridges ever honored withthe Presidential Award for Design Excell