Self-centering beam-column joints with variable stiffness for steel moment resisting frame

Abstract

Different from the self-centering joint with constant post-yield stiffness (CS joint), self-centering joint with various post-yield stiffness (VS joint) is proposed to control the seismic responses and improve the seismic resilience of frame structures under the extremely rarely occurring earthquake (EROE) level. The angle steel connectors and disc springs are adopted to provide the energy dissipation and self-centering capacities of the VS joint, respectively. First, theoretical study and pseudo-static tests are conducted, the results show that the VS joint has two stages of stiffness after the joint gap opens; the joint can deform and dissipate energy during the first stage with ordinary post-yield stiffness when the rotation angle of the joint is small; with the increase of the joint rotation angle, the weak stack of disc springs is flattened, causing the joint to enter the second stage with strengthened post-yield stiffness. It is found that the smaller the deformation capacity of the preloaded weak stack, the smaller the begin rotation angle of the second stage; the larger the stiffness of the strong stack, the larger the strengthened post-yield stiffness. Then, the updated displacement-based design method suitable for the frame structure with the VS joints (VSF structure) is also proposed. The non-linear dynamic time analysis results show that the plastic damage of the general moment resisting frame structure (GF structure) is severe and exceeds the extent of repairability under the EROE level; both the frame structure with the CS joints (CSF structure) and VSF structure have similar structural responses under the frequently occurring earthquake (FOE), the moderately occurring earthquake (MOE) and the rarely occurring earthquake (ROE) levels, and both can avoid plastic damage and have good self-centering capability even under the EROE level; due to the strengthened post-yield stiffness of the VS joints, the displacement responses of the VSF structure under the EROE level can be effectively controlled, while this is difficult for the CSF structure, whose responses exceed the performance target.