Stringer Longitudinal Bending–Induced Fatigue Failure of Stringer-to-Floor Beam Welded Connections in Orthotropic Steel Railway Bridge Decks

Abstract

This article focuses on the fatigue failure of stringer-to-floor beam (SF) welded connections, which are sensitive to fatigue cracking in the orthotropic steel decks (OSDs) of railway bridges. Two full-scale SF welded connections were fabricated and tested. Eccentric loads were introduced to the specimen so that the adverse conditions of longitudinal bending of the stringer could be considered. Static loading was first conducted to measure the elastic strain distributions at potential hot spots where high stress levels are expected. Structural hot-spot stresses at weld toes and stress-concentration factors (SCFs) were quadratically extrapolated, and the critical locations of stress concentrations were subsequently identified. Constant-amplitude cyclic loading was subsequently carried out, by which the fatigue cracking process, fatigue failure mode, characteristic fatigue life, and rigidity degradation were obtained. All cracks were observed on the web of the floor beam and near the weld toes under the stringer’s bottom flange. It was determined that the propagation of cracks can be divided into three main stages, and the variations of crack length on the surface and of crack depth in the plate thickness were also revealed. The fatigue life before the crack ran through the plate thickness was found to be dominant for the whole life of crack propagation. Comparison between experimental numbers of cycles and standard S-N curves indicates that two of the recommended curves provided in current bridge design specifications can be used to estimate the fatigue resistance of such SF welded connections where double-sided fillet welds are adopted. Double-sided groove welds with partial or full penetrations are also recommended to improve the fatigue behaviors of such connections further.