ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/532296
PROJECT Extradosed Prestressed Concrete Bridges Proportioning guidelines and a design example by Dr. Steven L. Stroh, AECOM Extradosed prestressed concrete bridges are a relatively new development in bridge engineering, with only about 60 of these completed worldwide. The extradosed prestressed concrete bridge has the appearance of a cable-stayed bridge with “short” towers, but behaves structurally closer to a prestressed concrete girder bridge with external prestressing. Based on work by the author in developing the design for the first extradosed prestressed concrete bridge in the United States, the Pearl Harbor Memorial Bridge, and from reviewing existing extradosed prestressed concrete bridge designs worldwide,1 this article discusses proportioning guidelines for this bridge type. These guidelines are then applied to the design for the Pearl Harbor Memorial Bridge. Span Length Range Extradosed prestressed bridges can be considered in the transition region of span lengths between traditional girder bridges and the longer-span bridge types such as truss, arch, and cablestayed. Sources in Japan, where most of the extradosed bridges have been constructed, have set the applicable span range for extradosed prestressed bridges to be generally between 100 and 200 m (328 and 656 ft). A review of extradosed prestressed bridges worldwide shows span lengths ranging from 172 to 902 ft. Based on these data, a range from 300 to 600 ft is shown to be a common span range for typical bridges of this type, with span range applicability of 200 to 900 ft. Side Span Ratios The ratio between the main span length, L, and the side span length, L1, has an influence on the vertical reactions or anchoring forces at the anchor pier, the moment demands on the girder, and stress changes in the stay cables. For concrete cable-stayed bridges, an economical side-to-main span ratio (L1/L) is about 0.42. For concrete girder bridges, L1/L should range from about 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. 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 balanced-cantilever-constructed concrete girder bridges. Multi-span Bridge Application Cable-stayed bridges are typically either two-span or three-span arrangements. These span arrangements are ideal for cable-stayed bridges because back stay cables can be provided from the anchor piers to the top of the towers to provide stiffening of the towers. A recent review of more than 1200 examples of cable-stayed bridges worldwide revealed that only seven cable-stayed bridges are multi-span bridges (meaning more than three spans).1 Design of a multi-span cable-stayed bridge presents a special challenge, in that there is no opportunity for backstay cables in the central spans, and special design considerations must be made to address the resulting flexibility of the structural system. Solutions for multi-span cable-stayed bridges include the provision of very stiff towers or providing crossing backstay cables that are anchored from the central tower to the base of adjacent towers.