Impact of structural modifications on progressive collapse vulnerability of seismically designed buildings

This study investigates the progressive collapse resistance of a mid-rise reinforced concrete structure imperilled to critical column loss scenarios. A seven-story RC frame structure, with plan dimensions of 36 meters by 36 meters and uniform 6-meter bays in both orthogonal directions, was modelled using ETABS. The structural model incorporated both material nonlinearity and geometric nonlinearity to capture realistic behaviour under sudden load redistribution accurately. A total of six simulation cases were analysed. The initial three examples consisted of the immediate elimination of a vital column, to be precise, an interior column, a corner column and an edge column that did not undergo any additional structural support. The other three cases were the same column removal situations but with the introduction of X-type steel bracing systems at the outer rim of the first story. These bracing systems were modelled with Fe250 grade I-section steel members, intended to enhance the frame’s lateral stiffness and provide alternative load transfer paths in case the core structural element is lost. A nonlinear time-history analysis was used to model the triggering events of a progressive collapse in the real world. Axial stiffness of the target column was rapidly reduced to near zero at 0.1 seconds, thus commencing the collapse mechanism. Parameters of structural response, including joint displacements, rotational deformations, the development of plastic hinges, and Demand-Capacity Ratios (DCRs), were seriously considered to determine collapse behaviour. The analysis results indicate that the provision of steel bracing will considerably enhance the resistance of the structure against progressive collapse. Bracing presence significantly minimised the lateral displacements and minimised the spread of plastic hinges throughout the frame. It was also observed that the DCR values were significantly smaller in braced models, which implies that it have a better stress distribution and increased load carrying capacity.