Progressive collapse behavior of geometrically similar RC frames at ambient and elevated temperatures

Due to limitations in laboratory capacity, investigations of the resistance of reinforced concrete (RC) frames against progressive collapse often rely on scaled specimens. However, the ability of scaled specimens to accurately reflect the collapse-resistant behavior of real structures is uncertain. Particularly in fire scenarios, the fire resistance of specimens of varying scales fails to maintain a similar relationship in terms of fire resistance. To shed light on this issue, a series of scaled models of RC beam-column assemblies were established to investigate the similarity relationship in progressive collapse resistance of RC frames under both ambient temperature and fire conditions. Numerical findings indicate that the failure of RC frames experiencing minor fire-induced damage is governed by rebar fracture at the beam ends near the side columns, whereas RC frames suffering significant fire-induced damage fail because of rebar fracture at the top rebar cut-off point. Increased fire duration results in a notable reduction in load capacity due to changes in failure modes. While the structural behaviors of scaled RC frames under ambient temperature comply with theoretical geometric similarity criteria, this is not the case under fire conditions. Therefore, a time scale was proposed to build the similarity relationship for fire-exposed RC frames.