Cyclic Behavior and Seismic Performance Assessment of Predamaged Straight Mortise-Tenon Joint Retrofitted with Shape Memory Alloy Wires

In traditional Chinese timber structures, mortise-tenon joints are susceptible to initial damage under long-term gravity loads or seismic actions, leading to increased residual deformation, stiffness degradation, and compromised seismic performance. To address this issue, this study proposes a retrofitting technique using superelastic shape memory alloy (SMA) wires to enhance the energy dissipation and self-centering capacities of predamaged joints. Quasi-static cyclic tests were conducted on full-scale straight mortise-tenon joints, including an unretrofitted joint (SM-T1), a predamaged joint retrofitted with SMA wires (SM-T2), and an initially intact joint retrofitted with SMA wires (SM-T3). The failure modes, hysteretic behavior, energy dissipation, and self-centering performance of the specimens were systematically evaluated. A finite-element model was developed and validated against experimental results. Seismic fragility analyses were further performed to quantify the enhancement at the frame level. Results demonstrated that the unretrofitted specimen (SM-T1) exhibited severely pinched hysteresis loops, limited energy dissipation, and significant residual deformations. In contrast, the SMA-retrofitted specimens (SM-T2 and SM-T3) showed full and stable hysteresis loops, with cumulative energy dissipation increased by up to 16.93% and 50.16%, respectively, and residual deformations substantially reduced. The fragility analysis confirmed that SMA reinforcement significantly lowers the exceedance probability of the retrofitted frames under multiple damage limits. This study validates the effectiveness of externally bonded SMA wires as a practical strategy for rehabilitating predamaged mortise-tenon joints and improving the seismic performance of ancient timber structures.