The collapse resistance of a seismically designed steel moment frame building is investigated using 3-D nonlinear models. Finite element analyses are carried out using validated models to investigate building response to loss of internal and external columns at various floor levels. Contributing mechanisms to collapse resistance that were studied include composite action between the slab and underlying steel beams and slab membrane action. The results of the simulations suggest that the building is more vulnerable to loss of columns in the upper stories than in the lower ones. It is also vulnerable to loss of interior gravity columns at all floor levels. Although the slab can contribute significantly to the robustness of the structure, it can become detrimental in the final stages of collapse.
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