Progressive Collapse Assessment of Reinforced Concrete Buildings with Setbacks Irregularities

Setback structures, characterized by their irregular geometric shape and height setbacks, introduce vulnerabilities into the structural system. These vulnerabilities hold significant implications in scenarios of progressive collapse directly impacting human safety. Furthermore, it is anticipated that forthcoming research will extensively investigate the heightened susceptibility of high-rise irregular setback structures to progressive collapse. In this study, three buildings of 9, 6, and 4 stories, featuring both regular and irregular setback concrete frames equipped with a specialized moment frame system, were selected for evaluation and comparison regarding progressive collapse. Loading guidelines outlined in GSA2016 and the FEMA273 guide for the allocation of loads and the placement of functional plastic hinges on structures were employed. Our findings reveal that the removal of key column elements, particularly in locations where the column is situated within varying plan layouts, poses the greatest risk for frame failure among the 26 column removal scenarios analyzed using pushdown and nonlinear time history analysis. According to our data, in most instances of intermediate column removal, the demand-to-capacity ratios (DCR) for beams and columns, as per the GSA2016 code, remain below 2 and 1, respectively, thus adhering to code stipulations. Joint rotations fall within the range of 0.005–0.02, with joint conditions typically classified within the safe zones A to IO and A to LS, indicative of high energy absorption and minimal damage in these regions. However, in scenarios involving the removal of corner columns and columns from upper floors, there is reduced energy absorption, leading to increased DCR values.