Experimental investigation of ultra-low-cycle fatigue behaviors of plate bearings in spatial grid structures

Ultra-low-cycle fatigue failure of plate bearings in steel grid structures is likely to occur due to an earthquake, which leads to a failure or even complete grid collapse. In this study, the effects of different variables on the hysteretic behavior, skeleton curves, stiffness degradation, and angles of inclination of the bearings and the anchor bolts were investigated. These variables included the vertical force, diameter of anchor bolts and extra baffles. For these purposes, ultra-low-cycle fatigue tests on six groups of plate bearings were carried out under constant vertical loading and horizontal reciprocating variable loading. In addition, the morphological analysis of fatigue fractures was performed to investigate the failure mechanism of anchor bolts in the bearings. It was found that the anchor bolt is the weakest part of the plate bearings during strong earthquakes. Cracks originate at the root of the anchor bolt and expand as the stress concentration increases and a typical ultra-low-cycle fatigue fracture micro-morphology appears. Also, it was also found that adding a baffle in an appropriate position on the bearings can improve the resistance ability against falling from the base supports providing that the baffle capacity is designed properly. Furthermore, a formula for the relationship between horizontal stiffness degradation and cumulative damage of a plate bearing is established. This formula can provide a reference for the seismic capacity evaluation of plate bearings used in spatial grid structures.