The Just-in-Case Factor

by Charles Seim, (F.ASCE), Principal; T. Y. Lin, Int'l., San Francisco, CA,
Richard Parmelee, (F.ASCE), Vice Pres.; Alfred Benesch & Co., Chicago, IL,
Harold R. Sandberg, (F.ASCE), Chrmn.; Alfred Benesch & Co., Chicago, IL,

Serial Information: Civil Engineering—ASCE, 1987, Vol. 57, Issue 10, Pg. 55-58

Document Type: Feature article


Computer analysis allows bridge designers to take a more precise look at the old structural principle of redundancy. No one can define redundancy, but everybody knows what it means. Two engineering firms have recently dealt with redundancy in different ways. At T. Y. Lin International, engineers have added cables that prestress steel beams and steel plate girders. In a design review of another bridge, they used a computer simulation to add redundancy to a three-span continuous two-girder bridge. The cost of adding reinforced cross frames and lateral bracing was less than adding another girder. Engineers at Alfred Benesch & Co. made a computer study of a four-girder bridge that was redesigned as a three-girder bridge. Typical cross frames were designed to function normally under service loads but yield under a redundant load, which would be picked up by heavy crossframes. The capacity of the three-girder system is not as great as the original four-girder system, but with the addition of the heavy cross frames, it provides a reliable alternate path of support.

Subject Headings: Bridge design | Girder bridges | Cables | Steel beams | Steel plates | Frames | Computer analysis | Plate girders

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