An Overview of Precast Prestressed Segmental Bridges
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An Overview of Precast Prestressed ♦ Segmental Bridges Walter Podolny, Jr. Bridge Division Office of Engineering Federal Highway Administration U.S. Department of Transportation Washington, D.C. he seventies will be recorded by Construction in this manner may T engineering historians as the de- also have a serious impact upon envi- cade in which prestressed concrete ronment and ecology. Prestressed segmental bridge construction came of segmental construction has extended age in North America. Segmental box the practical span of concrete bridges girder bridges have attracted the at- to approximately 800 ft (244 m). tention and captured the imagination Where segmental construction is used of bridge engineers and designers in conjunction with the cable-stay across the continent. bridge concept, the span range can be Because of practical limitations of extended to 1300 ft (400 m) and handling and shipping, the precast perhaps longer.' prestressed I-girder type of bridge Because construction of the super- construction is limited to an approxi- structure is executed from above, i.e., mate range of 120 to 150-ft (37 to 46 at deck level, the use of extensive m) spans. Beyond this range of span, falsework is avoided. Thus, there is no post-tensioned cast-in-place box gir- effect upon navigation clearance from ders on falsework are more attractive. falsework during construction and the However, in certain instances the ex- cost of extensive formwork is elimi- tensive use of falsework can prove to nated. Segmental viaduct type bridges be an economic disadvantage. Where provide a method whereby the impact deep ravines or navigable waterways of highway construction through en- must be crossed, extensive formwork vironmentally sensitive areas can be may be impractical. minimized. 56 Utilization of an elevated viaduct evolution of prestressed concrete type structure requires only a rela- bridges to this point in the state-of- tively narrow path along the align- the-art and to present a few basic de- ment to provide access for pier con- finitions. struction. Once the piers have been Precasting of elements or members constructed, all construction activity is of a structure implies that the concrete from above. Thus, the impact on the is cast in forms at some location other environment is minimized. Also, be- than the final position of the member. cause the structure is elevated, as op- The member may be cast at a perma- posed to an at-grade highway, there is nent precasting plant at some location no interference with wild life migra- other than the construction site; then tory habits. transported by truck, rail, or barge to Prestressed concrete segmental the site; and eventually erected to its bridges have proven to be esthetically final position. The member may also appealing and, because various con- be cast at some location in close struction methods can be used, the proximity to the construction site structures are cost effective and en- eliminating the transportation from vironmentally adaptable. precasting plant to construction site. In either situation the member is cast at a location other than its final posi- Evolution of tion in the structure. Segmental Bridges Segmental construction has been defined as "... a method of construc- Before discussing the various facets tion ... in which primary load carry- and variants of precast segmental box ing members are composed of indi- girder bridges, it may enhance the vidual segments post-tensioned to- understanding of this type of con- gether."2 struction to briefly trace the historical As early as 1948, Eugene Freyssi- PCI JOURNAL/January-February 1979 ҟ 57 Fig. 1. Marne River Bridge near Paris, France. (Courtesy: Figg and Muller Engineers, Inc., Tallahassee, Florida.) net, the great French prestressing was made using a series of machine- pioneer, used prestressed concrete for made blocks (strung like beads on a the construction of five bridges over string) which were prestressed to- the Marne River near Paris, with gether to form a beam. spans of 240 ft (73 m) having an ex- The construction method (although ceptionally light appearance (Fig. 1). very crude by modern standards) is Construction of these bridges, as indi- similar, in principle at least, to today's cated in Fig. 2, utilized precast seg- precast prestressed segmental bridges. ments which were post-tensioned to- The Tennessee prestressed block gether. Thus, by definition, these five beam bridge was followed very structures can be called precast pre- stressed segmental bridges. shortly by the famed Walnut Lane Bridge in Philadelphia, Pennsylvania, Prestressing of bridges in North America did not start until about in 1950. The 160-ft (48.8 m) long 1949.* The first prestressed bridge in beams were cast-in-place and post- tensioned. the United States was built in Madi- son County, Tennessee. This bridge Soon thereafter, precast preten- sioned bridge girders evolved result- *An interesting historical account of the early prestressed ing from inherent economies and bridges in America is given by Charles C. Zollman in "Reflections on the Beginnings of Prestressed Concrete quality control of plant fabricated in America—Part 1: Magnel's Impact on the Advent of elements. With few exceptions, during Prestressed Concrete; Part 2: Dynamic American En- gineers Sustain Magnel's Momentum," PCI JOURNAL, the fifties and early sixties, most V. 23, Nos. 3 and 4, May-June and July-August 1978, pp. multi-span precast prestressed bridges 22-48 and pp. 30-67. See also Ross Bryan's article on "Prestressed Concrete Innovations in Tennessee," pub- built in the United States were de- lished in the current PCI JOURNAL, pp. 14-31. signed as a series of simple spans. 58 They were designed with standard 100 to 120 ft (30.5 to 36.6 m) in length AASHTO-PCI girders of various cross depending upon local regulations. sections in spans ranging up to about As a result of longer span require- 100 ft (30.5 m), but more commonly ments, a study was conducted by the for spans of 40 to 80 ft (12 to 24 m). Prestressed Concrete Institute in The advantages of a continuous cast- cooperation with the Portland Cement in-place structure were abandoned in Association. 4 This study proposed favor of the more economical con- simple spans up to 140 ft (42.7 m) and struction offered by plant produced continuous spans up to 160 ft (48.8 m) standardized units. be constructed of standard precast During the middle sixties a growing girders up to 80 ft (24 m) in length concern with regard for safety of the joined together by splicing and post- highways asserted itself. An AASHTO tensioning. To obtain longer spans the Traffic Safety Committee report in use of inclined or haunched piers was 1967 called for: proposed. In general, these concepts "Adoption and use of two-span utilized precast I- or box girders with bridges for overpasses crossing di- field splices and post-tensioning for vided highways ... to eliminate continuity. the bridge piers normally placed This type of construction, using adjacent to the shoulders." long standard precast prestressed It soon became apparent that the units never quite achieved the popu- conventional precast pretensioned larity that it merited. Despite some AASHTO-PCI girders were limited by limitations, the method is adaptable their transportable length and weight. for spans up to 200 ft (61 m). Transportation over the highways The concepts developed by the limits the precast girder to a range of PCI-PCA studies fall into the defini- Fig. 2. Marne River Bridge near Paris, France. (Courtesy: Figg and Muller Engineers, Inc., Tallahassee, Florida.) PCI JOURNAUJanuary-February 1979 59 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 construction the first to apply cast-in-place seg- and might be described as "longitudi- mental prestressed construction in a nal" segmental construction. The in- balanced sequence to a bridge cross- dividual elements are long with re- ing the Lahn River at Balduinstein, spect to their width. Germany. This system of cantilever As spans increased, designers segmental construction rapidly gained turned toward utilization of post- acceptance in Germany, especially tensioned cast-in-place box girder after the successful completion of a construction. The Division of High- bridge crossing the Rhine River at ways, State of California, including Worms in 1952.5 Since then, the con- several other states, have been quite cept has spread across the entire successful using cast-in-place, multi- world. cell, post-tensioned box girder con- Concurrently, precast segmental struction for multi-span structures construction was evolving during this with spans of 300 ft (91.5 m) and period. In 1952, a single span county longer. However, this type of con- bridge located near Sheldon, New struction has its disadvantages; it re- York, was designed by the Freyssinet quires extensive formwork during Company. Although this bridge was casting with its undesirable impact constructed using longitudinal seg- upon the environment and/or ecology. ments, rather than transverse seg- Meanwhile in Europe, segmental ments, as was being done in Europe, construction proceeded slightly dif- the structure represents the first prac- ferently in conjunction with box gir- tical application of match casting. This der design. Segments were cast-in- technique has become an important place or precast in relatively short development in precast segmental lengths, providing full roadway width construction. and depth. Today, "segmental con- The bridge girders were divided struction" is generally recognized as into three longitudinal segments that having been pioneered in Europe. were cast end to end. The center seg- Ulrich Finsterwalder, in 1950, was ment was cast first and the end seg- 60 Fig. 4. JFK Memorial Causeway, Corpus Christi, Texas. ments were cast directly against the was built in 1967.