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

Abstract

Ultra-low-cycle fatigue failure of laminated rubber bearings in steel grid structures is likely to occur due to an earthquake, which leads to a failure or even complete collapse. The present paper investigates the effects of different variables on the hysteretic behavior, skeleton curves, and stiffness degradation of bearings and anchor bolts. The variables include the vertical force and diameter of anchor bolts. For these purposes, ultra-low-cycle fatigue tests on laminated rubber 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 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 increasing the vertical pressure can increase the peak value of the horizontal reaction force of the specimen and increase the energy consumption. It means that increasing the diameter of the anchor bolt can increase the stiffness of the bearing, increase the peak value of the horizontal reaction force, and slow down the stiffness degradation. Furthermore, a formula for the relationship between horizontal stiffness degradation and cumulative damage of a bearing is established. This formula can provide a reference for the seismic capacity evaluation of laminated rubber bearings used in spatial grid structures.