Experimental research on progressive collapse behavior of Chinese ancient timber buildings with different joint strengthening methods

Many Chinese ancient timber buildings present various damages due to material performance degradation, natural environmental influence, and human improper action during the long history. These existing damages make them vulnerable to different progressive collapse scenarios. Investigation of scientific strengthening approaches to prevent them from progressive collapse is urgent. This study compared the effect of different joint reinforcing approaches that are commonly employed for Chinese ancient timber buildings through experimental studies. The structural performance of frames reinforced by U nails, steel plate, or Carbon Fiber Reinforced Polymers (CFRP) sheet under Single-Column Removal (SCR) scenario was compared. Then, the deformation and failure modes of these reinforced joints were investigated. Finally, the moment and axial force in the beam before and after the reinforcement were concerned to understand the strengthening mechanism of the above reinforcement approaches. The results show that: the reinforcement can improve both the load bearing capacity and the ductility of the structure in the collapse. The improvement of the ultimate load is because the reinforcement can increase the ultimate moment at the joint. A better ductility of the collapse should be ascribed to a large axial force in the beams due to the reinforcement. Compared with the no-reinforced frame, improvement of the load bearing capacity due to the aforementioned three types of strengthening is ordered as follows: steel-plate reinforcement (359-639%) > CFRP reinforcement (297-430%) > Unail reinforcement (90-286%). The failure of the steel-plate reinforcement is comparatively brittle while the failure of the other two presents great ductility. Besides, according to the experimental results, several suggestions for anti-collapse strengthening in engineering practice with respect to each aforementioned reinforcement are proposed. This suggestion is also applicable to the engineering strengthening practice of ancient timber structures in Asia.