Effects of pinching in the hysteresis loop of rack connections

The non-linear behaviour of beam-column joints plays a fundamental role in the seismic response of frame structures. Experimental tests on rack connections have shown that their cyclic behaviour is characterized by the “pinching” phenomenon, that is a progressive deterioration of the rotational stiffness. This phenomenon is expected to influence the overall performance of pallet rack structures during an earthquake. In this paper, the authors propose a new finite element (FE) numerical model (Pinching Model) to investigate the pinched cyclic behaviour of rack connections. The model is easily implemented in commercial software packages, commonly used for non-linear seismic analyses and it is validated by comparison with both experimental results of laboratory tests and data available in literature. For a preliminary understanding of pinching effects, a T shaped specimen, representing an exterior rack beam-column joint, is analysed as case-study. Two FE numerical models (Takeda Model and Pinching Model), differing in the modelling of the hysteresis loop, are used to perform a non-linear time-history (NLTH) analysis of the joint with the goal to highlight consequences of the pinching on its structural behaviour. As a general remark, pinching reduces the dissipated hysteretic energy, so the structural response of rack structures is different from frames whose joints have a stable behaviour. For this reason, a reliable seismic vulnerability analysis of rack systems should be performed by a NLTH analysis, taking into account the pinching in the moment-rotation curve of beam-column joints.