Distortion-Induced Fatigue Cracking in a Seismically Retrofitted Steel Bridge

Abstract

This study examines the effect of a seismic retrofit on distortion-induced fatigue cracking in a steel girder bridge in the Midwestern United States. The seismic retrofit of the bridge was deemed necessary following a structural review initiated by the Federal Highway Administration (FHWA) in response to the 1989 Loma Prieta Earthquake. Following the review, the bridge was modified using a conventional seismic retrofit strategy to meet the 1995 FHWA standards for survivability and seismic performance. Upon completion of the retrofit, subsequent inspections identified new cracks (at the location of the retrofit) in the webs of longitudinal girders at transverse stiffener locations in an area known as the web gap. This study investigates the influence of the seismic retrofit strategy on crack formation by comparing principal stresses in the web-gap region of the original and retrofitted bridge under dead, live and thermal loading. A three-dimensional linear-elastic finite element analysis of the bridge before and after the retrofit is presented with results obtained from a commercial finite element software package. Comparisons of stresses before and after the retrofit indicate a measurable increase in stress near the web-gap region attributable to thermal and live loads following the retrofit. In addition, several viable repair strategies to limit these web-gap stresses and subsequent distortion-induced fatigue are presented. The results to date clearly demonstrate the importance of considering fatigue-sensitive details in seismic-retrofit strategies.