Probabilistic modeling of weld fracture in steel frame connections. Part II: seismic loading

This work develops and applies a probabilistic model to study the dynamic, nonlinear fracture behavior in the beam lower-flange to column welds found in moment resistant steel frames. The effort focuses on connection designs used prior to the 1994 Northridge earthquake (A36 beams, A572 columns and E70T-4 welds with access holes, shear tabs and continuity plates). The study applies the same probabilistic model to examine and compare fracture behavior in the simpler pull-plate specimen developed after the Northridge event to simulate conditions at the lower-flange connection. 3-D finite element analyses, coupled with an advanced micro-mechanical fracture model based on the Weibull stress, are used to assess the relative significance of loading rate, residual stresses, plasticity, access hole geometry, beam yield strengths, and various weld (backup bar) modifications. The present work considers only the initiation of brittle fracture triggered by a transgranular cleavage mechanism typical of that exhibited by ferritic steels (and welds) operating in the ductile-to-brittle transition region. Key computational results express the cumulative probability of fracture in terms of the beam moment at the face of the column and an equivalent beam moment for the pull-plate specimen. Part I of this study examines these same issues for connections and for pull-plate specimens under static loading as used in recent laboratory studies.