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 i n ferritic steels to examine the nonlinear fracture behavior of welded, mom ent 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 quan titative estimate of the cumulative failure probabilities with increasing b eam moment. The 3-D models incorporate the complete geometry of a welded jo int (access holes, shear tabs, continuity plate, weld geometry, backup bars ). A set of previously conducted, 15 full-scale tests on T-connections of t he pre-Northridge design (A36 beams, A572 columns, E70T-4 welds, backup bar s left in place) provide fracture moments to calibrate parameters of the We ibull stress model. The present work considers quasi-static loading typical ly imposed in large-scale testing of the connections. Once calibrated, the model is used here to examine the importance of welding induced residual st resses in the lower-flange weld, the effects-of stronger (A572) beams and m odified access hole geometries, and a variety of proposed changes in the we ld detail (backup bars, fillet reinforcements). The model predicts the cumu lative failure probability as a function of beam moment for these various c onfigurations. Using this same approach, the simplified "pull-plate" specim en is examined from a fracture mechanics viewpoint as a suitable replacemen t for full connection testing to evaluate alternative welding details. (C) 2001 Elsevier Science Ltd. All rights reserved.