Progressive Collapse Behavior of RBS Joints in Steel Frames Involving CFRP-Steel Composite Plates

Although a reduced beam section (RBS) joint has an effective seismic design, it also requires good robustness to resist the progressive collapse caused by different unexpected disasters. In this study, a novel RBS joint was introduced, which arranged carbon fiber–reinforced polymer (CFRP)–steel composite plates at the reduced section along the inner surface of the beam flange. First, experiments were conducted on a basic RBS specimen, an RBS specimen with CFRP-steel composite plates (RBS-CS), and an RBS specimen with steel-CFRP-steel sandwich composite plates (RBS-SCS), investigating their experimental phenomena, failure modes, deformation shapes, and resistance mechanisms. Second, validated numerical simulations were conducted to further investigate the stress and deformation of the CFRP composite plates, and the results confirmed the rationality of the simplified model. Third, the interaction between the CFRP sandwich composite plate and RBS section beams is discussed. The results indicated that the specimens with CFRP composite plates demonstrated good robustness, with both their deformation capacity and catenary action stability being superior to those of the basic RBS specimen. It was also found that the straightening displacement ws of the composite plate is significantly linked to the fracture displacement wcrack of the RBS section. When ws>wcrack, both the ultimate load and displacement increase with decreasing composite bending height, leading to a ductile failure mode. Conversely, when ws<wcrack, the ultimate load and displacement increased as the composite bending height increased, resulting in brittle failure.