Modeling for progressive collapse mitigation using nonlinear static analysis procedures

Abstract

Strengthening of structural systems for prevention of progressive collapse is an expensive or unrealistic goal for most buildings. This is in part because of the difficulty in quantifying precisely the potential causes of progressive collapse. Therefore, most design specifications and standards address progressive collapse mitigation independent of hazards. As demonstrated by many studies, nonlinear dynamic analysis of gravity and lateral force resisting systems is necessary for accurate assessment of design response quantities including forces and deformations. However, for framed structural systems, mitigation of progressive collapse through ductile design is a more realistic goal that can be accomplished by understanding the collapse mechanisms and failure modes associated with the loss of a primary load-carrying member. Toward this goal, nonlinear elastic analysis provides good understanding and quantitative assessment of the structural response that is needed for ductile design. This paper has two objectives: 1) identify important failure modes and progressive collapse mechanism for framed structures subjected to loss of primary load-carrying members, and 2) identify modelling considerations for better representation of structural responses. To accomplish the two goals, nonlinear elastic analysis is used to model a case-study reinforced concrete framed structure and examine the pattern of nonlinear hinging that leads to the formation of a mechanism and the effect on load-carrying capacity. The process of internal and external column removal and load pattern used in the models are based on United States Department of Defense (DoD) design standard titled "Unified Facilities Criteria (UFC) Design of Buildings to Resist Progressive Collapse." The Alternate Path (AP) method is used to select columns for progressive collapse analysis.

Keywords: Progressive collapse analysis, General Services Administration, Column removal scenarios, Alternate load path method, Demand capacity ratio, Nonlinear staged construction, Catastrophic failure