Probabilistic modeling of weld fracture in steel frame connections part I: quasi-static loading

This study applies an advanced micro-mechanics model of cleavage fracture in ferritic steels to examine the nonlinear fracture behavior of welded, moment resistant steel frames of the type widely constructed prior to the 1994 Northridge earthquake. The Weibull stress model for cleavage, coupled with 3-D analyses of connections containing crack-like defects, provides a quantitative estimate of the cumulative failure probabilities with increasing beam moment. The 3-D models incorporate the complete geometry of a welded joint (access holes, shear tabs, continuity plate, weld geometry, backup bars). A set of previously conducted, 15 full-scale tests on T-connections of the pre-Northridge design (A36 beams, A572 columns, E70T-4 welds, backup bars left in place) provide fracture moments to calibrate parameters of the Weibull stress model. The present work considers quasi-static loading typically imposed in large-scale testing of the connections. Once calibrated, the model is used here to examine the importance of welding induced residual stresses in the lower-flange weld, the effects of stronger (A572) beams and modified access hole geometries, and a variety of proposed changes in the weld detail (backup bars, fillet reinforcements). The model predicts the cumulative failure probability as a function of beam moment for these various configurations. Using this same approach, the simplified “pull-plate” specimen is examined from a fracture mechanics viewpoint as a suitable replacement for full connection testing to evaluate alternative welding details.