Effects of Postelastic Stiffness Ratio on Dynamic Increase Factor in Progressive Collapse

Prediction of structural response at the sudden removal of gravity load-bearing element in structures is an important issue in assessing progressive collapse potential of structures. In this paper, the effect of the postelastic stiffness ratio, as an effective parameter on the dynamic increase factor (DIF), in nonlinear static analysis of structures at the column removal is investigated, and a modified empirical DIF is presented. For this purpose, a series of low-and midrise moment frame structures with different span lengths and various numbers of stories are prepared, and the effect of the postelastic stiffness ratio on DIF is investigated, using several nonlinear static and dynamic analyses. For each postelastic stiffness ratio, a nonlinear dynamic analysis and a step-by-step nonlinear static analysis are carried out, and modified empirical DIF formulas are derived. Finally, an empirical formula is recommended that relates to the postelastic stiffness ratio. The new modified DIF can be used with a nonlinear static analysis instead of a nonlinear dynamic analysis to assess the progressive collapse potential of steel moment frame buildings having different postelastic stiffness ratios. (C) 2017 American Society of Civil Engineers.