Distortion-induced fatigue cracking has become very prevalent in steel plate girder bridge members. Distortion-induced cracks have occurred in girder webs and gusset plates, usually in the short gaps between the end of a gusset plate or transverse stiffener and the girder flange. NCHRP Project 12-28(6) was initiated with 3 primary objectives. They were to (1) categorize the kinds of fatigue cracks that have occurred because of out-of-plane distortions, (2) develop comprehensive guidelines and criteria that define the conditions that are likely to cause fatigue cracking induced by out-of-plane distortions, and (3) evaluate possible retrofitting techniques for cracked girders that have been subjected to out-of-plane distortions.

Research has been completed and the final report published as NCHRP Report 336, "Distortion-Induced Fatigue Cracking in Steel Bridges." In addition to guidelines on preventing distortion-induced cracking and recommended retrofit techniques, the report also contains recommended revisions to the fatigue design provisions in the AASHTO Standard Specifications for Highway Bridges. It is expected that AASHTO will consider the recommended design provisions for adoption in 1992.