Fatigue strength of steel bridge girders with distortion induced stresses, PhD dissertation (September) 1987, 170p.

A fairly large number of welded steel bridge girders have developed fatigue cracking due to out-of-plane distortion. These occurances of cracking are often found at unstiffened gaps at the ends of connection plates of transverse and longitudinal members. This includes diaphragm connection plates in multigirder bridges, floorbeam connection plates and gusset connection plates in floorbeamgirder bridges, and diaphragm connection plates in box girder bridges. In this dissertation both multigirder and floorbeam-girder bridges are studied. The field studies include field inspections and stress measurements. The stresses at the gap region were further analyzed by a finite element analysis model which shows good agreement with the field test results. These high outof-plane bending stresses are highly localized in the vicinity of the gap regions. The stress redistribution at the gap region after the development of a throughthickness crack is studied analytically. Parametric study on multigirder bridge systems using the calibrated analytical model is also carried out to examine the effects of the global structural parameters on the stress distributions in these gap regions. A finite element program with singularity elements at the crack tip is utilized to determine the stress intensity factors. A parametric study on loading and geometric conditions is also conducted. Subsequently, the S-N curves corresponding to different loading and geometric conditions are established utilizing the Paris rule and integating through the half thickness of the web plate. The results show that the S-N curves for out-of-plane bending are very