Both detailed and reduced models have been used to study the susceptibility to collapse of steel gravity frame systems (i.e., frames designed to carry only vertical loads) with simple shear connections under column loss. The composite floor slab was found to significantly enhance the capacity of the system relative to the bare steel framing system. However, even accounting for this enhanced capacity, the composite floor systems in the prototype buildings were found inadequate to sustain the applicable gravity loads under sudden column loss. Additional reinforcement in the floor slab was found to be effective at preventing collapse under column loss, and required tie force levels were investigated.
The response of steel beam-column assemblies with moment connections under monotonic loading conditions simulating a column removal scenario has been investigated computationally. Two beam-column assemblies were analyzed, which incorporate (1) welded unreinforced flange, bolted web (WUF-B) connections and (2) reduced beam section (RBS) connections. Detailed models of the assemblies have been developed, which use highly refined solid and shell elements to represent nonlinear material behavior and fracture. Reduced models were also developed, which use a much smaller number of beam and spring elements and are intended for use assess the susceptibility of complete structural systems to disproportionate collapse. Computational results were compared with the results of full-scale tests described in the companion paper, and good agreement was observed, demonstrating that both the detailed and reduced models are capable of capturing the predominant response characteristics and failure modes of the assemblies, including the development of tensile forces associated with catenary action and the ultimate failure of the moment connections under combined bending and axial stresses.
The response of cast-in-place concrete beam-column assemblies under monotonic loading conditions simulating a column removal scenario has been investigated computationally. Two beam-column assemblies were analyzed; one assembly was part of an intermediate moment frame and the other was part of a special moment frame. Two types of models were developed: (1) detailed models with highly refined solid and beam elements to represent the nonlinear material behavior of concrete and reinforcement, and (2) reduced-order models with significantly fewer beam and spring elements to represent the nonlinear behavior of structural components. The computational results were compared with experimental data from full-scale tests, and good agreement was observed, which demonstrates the capability of the detailed and reduced models to capture the primary response characteristics and failure modes, including the successive development of compressive arching action and catenary action in the beams and the fracture of reinforcing bars at the beam-column interface.