Robustness of Prototype Steel Frame Buildings against Column Loss: Assessment and Comparisons J. A. Main 1 and J. Liu 2 1 Engineering Laboratory, National Institute of Standards and Technology (NIST), 100 Bureau Drive, Stop 8611, Gaithersburg, MD 20899-8611; (301) 975-5286; joseph.main@nist.gov 2 Associate Professor, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051; (765) 494-2254; jliu@purdue.edu ABSTRACT Modeling approaches for analyzing the robustness of steel moment-frame and braced frame buildings against column loss are presented, including the nonlinear behavior and failure of the shear and moment connections and the effect of the composite floor slab. For the braced-frame buildings, modeling of the buckling behavior of braces incorporates nonlinear rotational springs representing the out-of-plane behavior of the gusset-plate connections. These approaches are used to analyze the nonlinear system behavior of 10-story prototype steel moment frame and braced frame buildings under column loss scenarios. Analysis results are presented, and the behavior of braced frame and moment frame buildings under column loss is compared. INTRODUCTION As part of ongoing research on structural robustness at NIST, a number of prototype 10-story buildings have been designed for the purpose of assessing their susceptibility to disproportionate collapse. The buildings were designed in partnership with a panel of industry experts to ensure that they are representative of current design practice in the United States. Full-scale beam-column assemblies from the prototype buildings, including steel and reinforced concrete moment frames, have been tested under column removal scenarios to characterize the connection behavior and to provide experimental data for validation of detailed and reduced connection models (Sadek et al. 2011). Reduced connection models (also known as macromodels or component- based models) are assemblies of beam elements, spring elements, and rigid links that represent the nonlinear behavior and failure modes of the connections, facilitating efficient collapse analysis of large structural systems. Full-scale tests of seismically designed steel moment connections under column removal (Lew et al. 2012) have demonstrated that these connections can develop significant vertical load-carrying capacity through a combination of flexural and catenary action, sustaining rotations almost twice as large as those observed in previous seismic tests before fracture. Reduced models of these connections have been developed (Sadek et al. 2012), and computational analyses of 10-story buildings using these reduced connection models (Main et al. 2011, Alashker et al. 2011) have shown that seismically designed moment frames can sustain the sudden loss of multiple columns without collapse. Planar analyses by Khandelwal et al. (2009)