Seismic behavior of looseness-induced inclined mortise and tenon joints in ancient timber structures: Experimental tests, multi-scale finite element model, and behavior degradation

Abstract

The inclination damage of mortise and tenon (M-T) joints, caused by the looseness between the mortise and tenon, has an detrimental impact on the overall safety of structures. To investigate the effects of varying inclination rotation on the seismic behavior of ancient M-T joints, four 1/3.2-scaled straight M-T joints with different inclination rotation, one intact joint for comparison and three looseness-induced inclined joints, were cyclically tested. Typical failure modes of the damaged joints were identified. The influence of the tilt deterioration on the joint's hysteretic and rotational behavior was evaluated. The test results showed that with an increase in the inclination rotation, the size of the hysteretic loops of the inclined joints was increasingly smaller, the negative initial slippage and pinching effects were more apparent, the ultimate moment-resisting, initial elastic stiffness, and ductility of the specimens degraded significantly. Furthermore, the multi-scale finite element models of the looseness-induced inclined M-T joints were established using the ABAQUS. Based on the validated numerical model, the parameter analysis was performed. The influence of the frictional coefficient of wood, inclined rotation, and horizontal gap between the tenon and mortise on the joint's hysteretic behavior and energy dissipation mechanism was quantified. It is found that with a friction coefficient of 0.6, the positive and negative ultimate moments-resisting of the joint improved 65.99 % and 35.89 %, respectively; the joint's slip moment and energy dissipation increased by 12.18 and 3.78 times, respectively. With a 0.043 rad inclined rotation, the positive and negative ultimate moments of the joint decreased 23.32 % and 30.46 %, respectively; the joint's positive and negative initial elastic stiffnesses and cumulative energy dissipation reduced 49.12 %, 28.96 %, and 56.33 %, respectively. When the inclined rotation remained at 0.022 rad, the positive and negative ultimate moments of the joint including a 6 mm horizontal gap reduced 26.63 % and 34.20 %, respectively; the joint's positive and negative initial elastic stiffnesses, and accumulative energy dissipation decreased 42.42 %, 59.43 %, and 79.50 %, respectively.