Lessons Learned from the Design of Steel and RC Structures against Progressive Collapse Using Nonlinear Alternate Path Analysis

Abstract

Lessons learned from designing against disproportionate collapse relevant to an international audience are discussed. Two structurally dissimilar buildings, designed against sudden column loss using two different methods of nonlinear alternate progressive collapse analysis, are selected for discussion. The first, a commercial medium-rise office building steel braced frame with reinforced concrete cores, was designed using nonlinear static (pushover) methods with dynamic energy balance, and used simple (non-moment resisting) connections in the design. Such a structural arrangement is typical of non-seismic areas and is recognized as typically presenting challenges when designing against sudden column loss. The use of an energy-based approach permitted the fundamental behavior to be explored efficiently and different mechanisms of resistance in resisting the removal of corner, penultimate and typical perimeter columns to be studied in detail. The second structure, a seismically-designed transit center in California, comprised a steel superstructure above-grade with a heavy park at roof level and a reinforced concrete three-story substructure. Detailed nonlinear dynamic time history analysis in LS-DYNA was undertaken to assess resistance to sudden column loss. The model accounts for material and geometric nonlinearities and the inertial effects following column loss. Structural framing members were modeled using fiber elements in LS-DYNA. Material models were selected to model the concrete fibers to account for concrete cracking in tension, crushing in compression, post-peak strain softening, and strain rate effects. Element-level acceptance criteria in UFC 4-023-03 were translated to material-level failure criteria and incorporated into the analysis. The design of secondary beams and placement of slab reinforcement were shown in both structures to be critical to resisting collapse through the development of compressive arching and membrane action. Composite action of the slabs with the secondary beams was shown to play an important role in developing resistance against collapse and heavily influence the demands on connections. The ductility of the connections in the moment-resisting frame influenced the ability to develop plastic hinges, but the selection and design of ductile shear connections in the steel braced frame was key to providing resistance against collapse: catenary action, commonly held to be the principal mechanism of resistance, was found to be of limited significance.