DISTORTION-INDUCED FATIGUE CRACKING IN STEEL BRIDGES

The report is the result of a review of existing experimental field studies and experimental work performed under NCHRP Project 12-28(6). It provides a detailed review of the experimental studies undertaken on a variety of bridges that had experienced fatigue cracking as a result of web gap distortion. Field measurements indicated that floorbeam-girder connection plates provided about the highest cyclic stresses in the web gap adjacent to the top flange. Floorbeam connected to box tie girder members in tied arch bridges also resulted in relatively large web gap cyclic stresses because internal diaphragms were not attached to the flanges. Multigirder bridges without staggered diaphragms were found to have cyclic stress about half the magnitude of the floorbeam-girder bridges. Structures with lateral bracing web gusset plates that were not provided with a positive attachment between the gusset plates and the transverse stiffeners were found to be very susceptible to distortion in the gap between the gusset and transverse connection plate. The laboratory data from the transverse connection plate web gaps and the lateral gusset plate web gaps demonstrated that the fatigue resistance from the web gap was equivalent to Category C of the AASHTO fatigue resistance curves. Recommendations are provided in this report for changes in the provisions for end connections for stringers and floorbeams. These recommendations provide an extension compatible with the provisions adopted by AASHTO for transverse connection plates at diaphragms.