Segmental Bridge Construction in Florida — a Review and Perspective
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Special Report Segmental Bridge Construction in Florida — A Review and Perspective by Alan J. Moreton, P.E. State Structures Engineer Florida Department of Transportation Tallahassee, Florida 36 SYNOPSIS This paper offers an overview of the precast concrete segmental bridges designed and built in the state of Florida during the last ten years. The article summarizes various statistical structural parameters, segment manufacturing and erection methods, construction times, costs, and reviews problems typically encountered. Also included is a discussion of current industry and nationwide design and construction practices and some suggestions for possible improvements. CONTENTS Synopsis............................................37 1. Introduction ......................................38 2. Precast Segmental Bridges ........................38 3. Florida's Segmental Bridges .......................44 4. Structural Parametrics .............................44 5. Casting Yard Operations ...........................48 6. Rejected Segments .............................. 49 7. Erection Operations ...............................50 8. Some Typical Problems ...........................52 9. Time ............................................55 10. Costs ...........................................57 11. Administration Processes — Design, Construction andShop Drawings ...............................60 12. Actions by the Florida Department of Transportation ...63 13. Benefits of Segmental Bridges ......................64 14. Summary ........................................65 15. Conclusions .....................................66 References..........................................66 Acknowledgments....................................67 Appendix............................................68 PCI JOURNAUMay-June 1989 37 1. INTRODUCTION ode 01 concrete segmental bridges Precast segmental bridges were a nat- M were made possible as a result of ural development for efficiency, stan- postwar developments in post-tension- dardized mass production, speed of ing systems and materials technology. erection, the elimination of expensive This was spurred on by the need for formwork in deep valleys and over much reconstruction and new infra- navigable waterways and, particularly, structure in Europe following World to afford solutions for restricted con- War 11. After some initial development strnction access in congested urban or in J urope, these systems were intro- environmentall y sensitive areas. The duced into North America in the '60s most notable example of the latter is the and '70s and have now become quite Linn Cove Viaduct' on Grandfather commonplace. 17ountain in North Carolina. This Early post-tensioned concrete seg- bridge was constructed entirely From the mental bridges were cast-in-place in top, including piers and foundations, in cantilever using traveling formwork order to preserve the delicate environ- with spans of up to 400 ft (122 m.). This ment of this scenic region. It is the "pre- remains the preferred method for large cast type of segmental bridge that has spans such as the Houston Ship Channel found many applications in Florida over [at 750 ft (229 in)] and the planned con- the last 10 years. So far, 31 major struc- crete alternate for the Acosta Bridge in tures have been built, including the Jacksonville [at 630 ft (192 m)1. Sunshine Skyway Bridge. 2. PRECAST SEGMENTAL BRIDGES Precast segmental bridges are so Bridge over '1 ampa Bay.'.s called because they are made of indi- (h) Balanced Cantilever — Where vidual precast units or "segments" segments are erected sequentially in carefully manufactured in a precast con- cantilever on each side of an initial seg- crete plant, either on or off the site. The ment placed on top of the pier. Stability segments are later erected and secured is provided by a temporary tower or together by longitudinal post-tensioning other support near the permanent pier. to form each span or cantilever. Cantilevers are joined by cast-in-place They fall into the following general closures at niidspan. There are many categories according to their method of examples of this throughout the state, construction: including Ramp I at the Florida "l urn- (a) Span-by-Span — Where all the pike/1-75 Interchange (Fig. 3).8 segments of'one span between piers are (c) Progressive Cantilever - Where erected on a special supporting truss or segments are erected in cantilever in gantry and are longitudinally post-ten- one direction, starting at one end of the sioned together after making small bridge and progressing overall the piers cast-in-place closure joints at one or in sequence. Additional intermediate both ends next to the pier segments. temporary piers or towers with cable Examples include the Long Key and stays are needed to facilitate construc- Seven Mile Bridges in the Florida Keys tion in cantilever from one pier to the (Figs. 1 and 2) 2.3 and the high level ap- next. Examples include the Linn Cove proaches to the new Sunshine Skyway Viaduct in North Carolina and the Fon- 38 Fig. 1. Long Key Bridge — A precast segmental, post-tensioned, span-by-span structure. (Courtesy of Figg and Muller Engineers Inc., Designer) PC i JOURNAL/May-June 1989 39 M Fig. 2. Seven Mile Bridge — A precast segmental, post-tensioned, span-by-span structure. (Courtesy of Figg and Muller Engineers Inc.. Designer) tenoy Bridge in France.7 Temporary post-tensioning bars are In all these systems, the precast seg- used to secure each segment tightly to ments are usually made in a special form its neighbor prior to installing and or "casting cell" where a new segment stressing the permanent longitudinal is cast against its older neighbor to post-tensioning tendons which provide achieve a perfectly mating or "snatch the structural capacity and continuity of cast" joint. To date, only the "short line" the superstructure. Schematic illustra- or "single cell" casting machine has tions of typical span-by-span and bal- been used (as opposed to the "long line anced cantilever construction are bed" system) in Florida. In the long line shown iii Figs. 4 through 8. bed system, the entire soffit of the Other types of bridge construction bridge is laid out in the casting yard and systems are sometimes referred to as each segment is made in turn in its "segmental." These include: proper place. • Incremental launching When erected in the bridge, the joints • Partial precast and cast-in-place between segments are coated with cousin iction epoxy to fill any surface imperfections • Post-tensioned segmental I-girders and provide a tightly bedded joint. This • Cast-in-place post-tensioned also helps to seal and protect internal bridges post-tensioning tendons. Because exter- • Precast wet joint segmental nal post-tensioning tendons were used These systems share many common in the first span-by-span bridges in the features, especially post-tensioning and Florida Keys, the segments were not special erection systems of falsework, jointed with epoxy but were left dry. towers or travelers, etc. However, they However, all subsequent structures, in- are different in techniques from precast chiding the similar span-by-span ap- segmental bridges and, with the excep- proaches to the Skyway, have epoxy fil- tion of post-tensioned I-girders, have led joints between segments. found few applications in Florida. 40 Fig. 3. Ramp I — A precast segmental, post-tensioned, balanced cantilever viaduct. (Courtesy of Beiswenger Hoch and Associates, Designer) continuous deck P.T. at pier ,_ P.T. at midspan Section Fig. 4. Typical span-by-span superstructure. PCI JOURNALMay-June 1989 41 o - :-r-- L.driiied Shafts C.I.P. footer V-pier column pier = =_ - 4 drilled shafts- precast box pier Fig. 5. Typical span-by-span substructures. P.T.at pier T. at midspan Section Fig. 6. Typical balanced cantilever superstructure. 42 r C.I.P. joint ^caat-In-place piers ( __ pler with features - driven piles or spread footings II Fig. 7. Typical balanced cantilever substructures. Span-by-Span anchorage external P.T. deviation saddle Balanced Cantilever cantilever P.T. Ifl nsrI ii /1 mldspan closure ) continuity P.T. top and btm. Fig. 8. Typical post-tensioning. PCI JOURNALMay-June 1989 43 3. FLORIDAS SEGMENTAL BRIDGES The Long Key Bridge was the first used only on straight structures, over precast segmental bridge constructed in water with spans between 118 and 143 ft Florida; since then, a total of 31 bridges (36 and 44 m). For larger spans up to 225 have been built, including the cable ft (69 m) and especially for curved inter- stayed Sunshine Skyway Bridge over change viaducts on land sites, balanced Tampa Bay (Fig. 9). Several more have cantilever has been used. been and are being designed. (The Sun- Substructure and foundation types shine Skyway is an exceptional structure follow a similar trend, with lighter pre- in its own right and will not be dis- cast and cast-in-place substructures in cussed in detail here. However, certain span-by-span applications, as opposed information and experience from the to more massive cast-in-place substruc- Skyway project have been incor- tures required by balanced cantilever porated — particularly that which per- construction. The difference arises from tains to the high level span-by-span ap- the span lengths and the fact that proaches which are typical of other cantilever construction usually requires segmental, rather than cable stayed, significant out-of-balance erection ef- construction. Also, the Sunshine Sky- fects be carried into the foundations. way