Seismic performance of cross hoop-head mortise and tenon joints under different degrees of looseness

Cross hoop-head mortise and tenon joints are commonly used in corner joints of traditional timber structures such as halls and ancestral temples. Due to prevalent shrinkage-induced looseness during long-term service, quasi-static tests were conducted on 4 specimens, including 1 intact joint and 3 joints with varying looseness degrees, to investigate the effects of looseness on seismic performance, along with parametric analyses of friction coefficient, elastic modulus and axial load on bending resistance. Key findings include: (1) The intact joint primarily fails through tenon root shear failure and longitudinal cracks in beams, whereas loose joints exhibit longitudinal cracking at mortise-tenon interlocking area and shear fractures in mortise. Cracks developed in the Pu-pai Fang as looseness increased. (2) Beam 1 exhibited superior load-bearing performance in reverse loading compared to forward loading, whereas Beam 2 demonstrated comparable performance under both loading directions. Beam 1 showed higher sensitivity to looseness, but Beam 2 achieved greater ductility. (3) At 12 % looseness, peak bending moments declined by 28.37 % for Beam 1 and 12.55 % for Beam 2, while the cumulative energy dissipation of the joint decreased by 32.25 %. (4) When the friction coefficient increased from 0.2 to 0.6, the forward and reverse peak bending moments rose by 5.20 % and 5.54 %, respectively. As the longitudinal and transverse elastic moduli increased from 0.75E to 1.25E, the peak bending moments increased by 16.75 % and 37.70 %, respectively. Axial load exhibited negligible impact on the seismic performance of the joints.