Probabilistic Robustness Assessment of Pre-Northridge Steel Moment Resisting Frames

Abstract

This paper investigates the robustness of moment-resisting steel frames that are typical of building construction in seismic regions before the 1994 Northridge earthquake against progressive (disproportionate) collapse. Uncertainties in the collapse demands and the resisting capacities of the connections in the frames are modeled probabilistically. The dominant connection failure mode, which involves fracture of the weld connecting the beam and column flanges under scenarios involving sudden column loss, is developed using a J-integral formulation of fracture demand and is characterized probabilistically. The connection behavior model is validated using connection test data from the SAC Project on steel frames conducted following the Northridge earthquake. The robustness of two three-story steel frames designed in the SAC Project is assessed by utilizing (a) the requirements in the new Unified Facilities Criteria (UFC), and (b) a system reliability analysis. This analysis reveals that steel moment frames with connections similar to those found in pre-Northridge building construction may not meet the UFC requirements for general structural integrity following notional column removal.