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PPCO Twist System PROJECT Extradosed Prestressed Concrete Bridges Proportioning guidelines and a design example by Dr. Steven L. Stroh, AECOM 0.8 for conventional cast-in-place-on- falsework construction to about 0.65 for balanced-cantilever construction. A review of worldwide data on extradosed bridges shows that L1/L varied from 0.33 to 0.83 with a mean of 0.57.1 The standard deviation is 0.12, so one standard deviation each side of the mean gives a range for L1/L of 0.45 to 0.69. This places extradosed bridges essentially between the envelopes of concrete cable-stayed bridges and The Odawara Port Bridge, Japan, is the world’s first extradosed prestressed concrete balanced-cantilever-constructed concrete- bridge, completed in 1994. Photo: Steven Stroh. girder bridges. Extradosed prestressed concrete bridges Based on these data, a range from 300 Multi-span are a relatively new development in bridge to 600 ft is shown to be a common span Bridge Application engineering, with only about 60 of these range for typical bridges of this type, with Cable-stayed bridges are typically either completed worldwide. The extradosed span range applicability of 200 to 900 ft. two-span or three-span arrangements. prestressed concrete bridge has the These span arrangements are ideal for appearance of a cable-stayed bridge with Side Span Ratios cable-stayed bridges because back stay “short” towers, but behaves structurally The ratio between the main span length, cables can be provided from the anchor closer to a prestressed concrete girder L, and the side span length, L1, has an piers to the top of the towers to provide bridge with external prestressing. influence on the vertical reactions or stiffening of the towers. A recent review anchoring forces at the anchor pier, of more than 1200 examples of cable- Based on work by the author in the moment demands on the girder, stayed bridges worldwide revealed that developing the design for the first and stress changes in the stay cables. only seven cable-stayed bridges are extradosed prestressed concrete bridge For concrete cable-stayed bridges, an multi-span bridges (meaning more than in the United States, the Pearl Harbor economical side-to-main span ratio three spans).1 Design of a multi-span Memorial Bridge, and from reviewing (L1/L) is about 0.42. For concrete girder cable-stayed bridge presents a special existing extradosed prestressed concrete bridges, L1/L should range from about challenge, in that there is no opportunity bridge designs worldwide,1 this article discusses proportioning guidelines for this bridge type. These guidelines are then Steel Box 200 600 applied to the design for the Pearl Harbor Memorial Bridge. Segmental 200 600 Span Length Range Extradosed 200 900 Extradosed prestressed bridges can be considered in the transition region of Truss 500 1,200 span lengths between traditional girder bridges and the longer-span bridge Arch 500 1,200 types such as truss, arch, and cable- stayed. Sources in Japan, where most Cable Stayed 600 2,800 of the extradosed bridges have been constructed, have set the applicable span Suspension 1,500 range for extradosed prestressed bridges to be generally between 100 and 200 m (328 and 656 ft). A review of extradosed 0 1,000 2,000 3,000 4,000 prestressed bridges worldwide shows Span in Feet span lengths ranging from 172 to 902 ft. Span ranges for common bridge types. Drawing: Federal Highway Administration. 28 | ASPIRE, Summer 2015 range, a typical span-to-depth ratio is about 30. The girder depth at midspan for an extradosed bridge should be similar to a girder bridge constructed in balanced cantilever. Therefore, a recommended midspan depth-to-span ratio for variable-depth extradosed bridges is 50. Case Study—The Pearl Harbor Memorial Bridge Ibi River Bridge, Japan, is a multi-span, extradosed prestressed concrete bridge. Photo: The Pearl Harbor Memorial Bridge Steven Stroh. in New Haven, Conn., is the first extradosed prestressed concrete bridge for backstay cables in the central spans, above the cross section of the girder built in the United States. The Pearl and special design considerations must using a short tower, providing a much Harbor Memorial Bridge is a three-span, be made to address the resulting flexibility larger eccentricity, and therefore a continuous, cast-in-place, segmental of the structural system. Solutions for more efficient use of the prestressing concrete box-girder structure with a multi-span cable-stayed bridges include steel. However, if we continue raising 515-ft-long main span and 249-ft-long the provision of very stiff towers or the tower, at some point the vertical side spans. providing crossing backstay cables that component of the cable reaches a force are anchored from the central tower to level that starts to carry a significant The northbound and southbound the base of adjacent towers. portion of the vertical live load of the roadways are carried on separate structure. parallel structures, each having five Extradosed prestressed bridges do not lanes, a tapering auxiliary lane, and rely on back stay cables. So, unlike cable- A review of extradosed bridges 10-ft-wide shoulders, on a deck that stayed bridges, multi-span extradosed constructed worldwide shows a typical varies in width from 95.4 to 107.6 ft. 1 bridge arrangements do not require range of the H/L to be between ⁄7 and The superstructure consist of a five-cell 1 1 1 special measures. A recent review of ⁄13, with an average value of ⁄10. concrete box-girder section. The girder more than 60 extradosed bridges built depth varies through a parabolic haunch around the world to date showed 19 of Girder Depth from 11.5 ft at midspan to 16.4 ft at these bridges had four or more spans Extradosed prestressed bridges are the towers. The cast-in-place segmental (representing nearly 30% of the bridges typically constructed in balanced concrete box-girder superstructure built).1 The extradosed bridge type is cantilever and their behavior is similar is post-tensioned longitudinally and well suited to long multi-span bridge to that of a girder bridge constructed transversely. arrangements. in balanced cantilever, but with more efficient external prestressing. Therefore, At the two interior supports, the twin Tower Height a reduction in the structural depth at decks are supported by three towers An important parameter for extradosed the support is expected when compared that extend above the deck and an bridges, and one that differentiates to a girder bridge. For extradosed additional intermediate column below them from cable-stayed bridges, is the bridges in the 300- to 600-ft span each deck. The hollow towers have a tower height. The tower height directly range, the span-to-depth ratio at the constant cross section and are elliptical influences the stay stress variation under tower varies from 25 to 35 based on a in shape. Foundations are 8-ft-diameter live load (the fatigue stress range) and the review of existing bridges.1 Within this drilled shafts founded on rock. proportion of loads shared between the girder and the cables. For cable-stayed bridges, the optimal ratio of the tower height, H, to main 1 1 span, L, is typically between ⁄4 and ⁄5. For extradosed bridges, the role of the cables is to act as external post- tensioning tendons and provide prestress to the girder. For an extradosed bridge, the post-tensioning is elevated Pearl Harbor Memorial Bridge is the first extradosed prestressed concrete bridge designed in the United States. Photo: AECOM. Table 1—Recommended Proportioning Guidelines recommended range. At midspan, a depth for Extradosed Prestressed Concrete Bridges of 11.5 ft was selected. This depth was chosen in part to provide a 6.5 ft internal Parameter Guideline clear height within the box girder for Common span range 300 to 600 ft inspection access purposes. This depth gives Side span-to-main span ratio 0.6 a span-to-depth ratio of 45, close to the recommended range. For more information Girder depth at tower* 25 to 35 about this bridge, see ASPIRETM Fall 2012. Girder depth at midspan† 50 ‡ 1 Closing Remarks Tower height above deck ⁄10 Extradosed prestressed concrete bridges * Guideline on ratio of main span length-to-girder depth at tower. are an emerging bridge technology, † Guideline on ratio of main span length-to-girder depth at midspan. ‡ Guideline on ratio of tower height above deck-to-main span length. applicable to bridges in the 300- to 600-ft span range. Basic proportioning parameters for this bridge type are well Application of lower range limit to the side span ratio understood and are summarized in Table 1. Recommendations is an important design parameter. The side span-to-main span length ratio Reference for the Pearl Harbor Memorial Bridge The tower height selected for the Pearl 1. Stroh, Steven L., 2012. On the is 0.48, which is at the low end of the Harbor Memorial Bridge (measured from Development of the Extradosed recommended range. The span length deck level to the uppermost cable) is 60 ft. 1 Bridge Concept, a dissertation was selected for the Pearl Harbor This gives an H/L of ⁄8.6. The tower height submitted in partial fulfillment of Memorial Bridge based on geometric for the Pearl Harbor Memorial Bridge the requirements for the degree of constraints. In final design, the result was selected with a slightly taller tower Doctor of Philosophy, Department of this relatively short side span was height than the suggested value, but well of Civil and Environmental an uplift condition at the anchor piers within the suggested range. This decision Engineering, College of Engineering, under certain live-load conditions.
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