This study applies an advanced micro-mechanics model of cleavage fracture in ferritic steels to examine the fracture behavior of welded, moment resistant steel frames of the type widely constructed prior to the 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. A set of previously conducted, 15 full-scale tests on T-connections of the pre-Northridge design provide fracture moments to calibrate parameters of the Weibull stress model. Once calibrated, the model is used to examine the importance of welding-induced residual stresses in the lower-flange and the effects of seismic loading rates. The model predicts the cumulative failure probability as a function of beam moment for these various configurations.