Service Performance of Steel Bridges Compared to Fatigue‐Life Predictions

Abstract

Ten continuous, rolled‐steel‐beam Interstate Highway bridges containing welded cover plates and welded splices were analyzed for fatigue life according to guidelines issued by the American Association of State Highway and Transportation Officials (AASHTO) in 1990. The bridges were entirely field welded and constructed before 1970, when the extreme fatigue sensitivity of cover‐plate terminations was unknown to design engineers. Each bridge is subjected to high volumes of truck traffic and contains AASHTO Category E′ cover plates at every pier. The fatigue‐life calculations incorporated site‐specific truck‐traffic data and lateral distribution factors smaller than typical design values. Although the results indicated the calculated mean fatigue life has already been exceeded at a minimum of one cover‐plate termination in eight of the 10 bridges, nondestructive inspection revealed that cracking had not occurred. Even though it is very encouraging that the actual service performance of these bridges is superior to analytical predictions, bridge engineers would appreciate guidance on how to make the best use of fatigue‐life predictions. Since funds for bridge repair are difficult to obtain, it would be imprudent to initiate bridge rehabilitation projects based solely on the results of fatigue‐life calculations. The best analytical results will be obtained when site‐specific data are available, and tests conducted on segments salvaged from similar bridges can be justified in certain instances.