Dynamic increase factor for pushdown analysis of seismically designed steel moment-resisting frames

The independent threat scenario of sudden column loss under localised damage is usually considered in progressive collapse assessment. The effect of the sudden removal of a column is like the sudden application of the gravity load on the structure when significant deformations occur. This conventional approach is based on the simplifying but realistic hypothesis that the peak dynamic response can be assessed with reasonable accuracy using the nonlinear static response. In this approach, amplified gravity loads are applied to the bays that are affected by the removed column to compensate for the dynamic effects corresponding to the real load redistribution. The paper investigates the dynamic increase factor to be considered in the nonlinear pushdown analysis of seismically designed steel moment-resisting frames. The influence of the fundamental parameters involved in progressive collapse analysis was highlighted. The effect of various design variables, such as the number of stories, the number of bays, the location of the removed column and the level of seismic design load was investigated. The dynamic increase factor was estimated in a way to generate the best match of the peak dynamic responses through the nonlinear static analysis. Finally, the values obtained were expressed as a function of the vertical displacement at the location of the removed column and then compared with the GSA formulation based on the ductility factor.