Keywords: progressive collapse, inelastic analysis, framed structures, plastic hinges, alternate path, failure modes.

Strengthening of structural systems for the prevention of progressive collapse is an expensive or unrealistic goal for most buildings. This is in part because of the difficulty in quantifying 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 the 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) identification of important failure modes and progressive collapse mechanisms for framed structures subjected to loss of primary load-carrying members, and 2) identifying modeling considerations for better representation of the structural response.

To accomplish the two goals, linear and nonlinear static analyses are used to model a case-study reinforced concrete framed structure and examine and 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 design standard titled "Unified Facilities Criteria (UFC) Design of Buildings to Resist Progressive Collapse" [1]. The alternate path (AP) method is used to select columns for progressive collapse analysis.

The paper concludes that the use of the AP method for progressive investigation involves a high degree of uncertainty in terms of quantifying the causes; therefore highly refined nonlinear dynamic analysis may not be justified. Accounting for dynamic effects related to the removal of load-carrying members can be done reasonably by carefully selecting dynamic magnification factors. Nonlinear elastic analysis is sufficient to understand the collapse mechanisms and nonlinear hinge locations. Nonlinear hinges should be carefully designed and detailed to ensure ductile behavior.

1

"Unified Facilities Criteria (UFC): Design of Buildings to Resist Progressive Collapse", United States Department of Defense, Washington, D.C., United States, 2005.

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