Experimental and numerical studies on fire resistance of damaged traditional mortise-tenon jointed timber frames

The looseness and decay damage in mortise-tenon (M&T) joints are commonly seen in the traditional timber frame, severely threatening the safety and fire resistance of traditional timber structures. To study the influence of deterioration to M&T joint on the fire resistance of traditional timber frame, we fabricated four 1/1.14-scaled traditional M&T jointed timber frame specimens, two without damage, one considering the looseness to joint, and one considering the decay of joint. The static loading test for one intact timber frame was carried out at normal temperature. Based on this, fire resistance tests of the specimens with different types of damage and 30 % fire load ratio were conducted under ISO 834 fire exposure. The failure mode, section temperature distribution, and wood charring rate of the frames after being exposed to fire were obtained, and the influence mechanism of damage to M&T joint on the fire resistance was revealed. The results indicated that the joint damage would remarkably change the failure mode and reduce the fire endurance; The non-damaged specimen experienced the joint failure after being exposed to fire for 55.5 min, while the specimens with the looseness and decay of joint suffered the bending failure at the mid span of the beam after being exposed to fire for 28.3 and 33.2 min, respectively. Furthermore, the heat transfer and sequentially coupled thermomechanical analysis models of the traditional timber frames consider the joint's damage were developed and verified using the test results. The parametric analyses were also performed to analyze the effects of different fire load ratios, loose and decay degree of the joint on the fire resistance of the timber frame. It is found that the model predictions were in good agreement with the test results. When the fire exposure time was constant, with an increase in the fire load ratio and the joint's looseness degree, the bending stiffness of the beam and joint's rotational stiffness of the timber frame significantly decreased. With an increase in the fire load ratio and the degree of looseness and decay, and the fire endurance of the frame decreased. When the fire load ratio decreased from 30 % to 10 %, the fire endurance of the frame increased by 26 %; Conversely, when the fire load ratio increased to 60 %, the fire endurance reduced by 48 %. Compared to non- damaged condition, when the looseness degree of joint in the frame was 22 %, the fire endurance of the frame reduced by up to 70 %; With a decay degree of joint in the frame of 22 %, the fire endurance of the frame reduced by up to 60 %.