Experimental assessment of the hysteretic response of resilient low-damage friction joints with different Belleville disk springs configurations

Recent research works have emphasized the optimal response of Friction Joints (FJs) in seismic regions, particularly for Moment Resisting Frames, due to their energy dissipation capabilities with minimal damage. One of the main limitations of FJs is related to the capacity to effectively control clamping forces and their variation during a cyclic loading history. One solution to mitigate the loss of preloading and the fluctuation of clamping forces is to incorporate Belleville Disk Springs (DSs) into the bolting assemblies. While the idea of including DSs into bolting assemblies is not new, as it was already proposed by Yang & Popov in the 90s, guidelines for DSs' selection are currently lacking and their effects on the response of FJs are not yet completely understood. Within this framework, in continuation of prior research efforts, new tests were conducted at the University of Salerno exploring the influence of different DSs layouts on the cyclic response of FJs. The study aims to assess DSs effectiveness in reducing bolt preloading losses and stabilizing the hysteretic loops improving the damping capacity of FJs. To this scope, two experimental campaigns were conducted: one based on component tests, and another based on full-scale tests on FJs. The parameters considered in the experimental work included: i) The configuration of the friction damper, considering both dampers parallel to the beam web (VFC-) and dampers parallel to the beam flanges (HFC-); ii) the type of DSs, conforming to European standards according to DIN 6796 or properly customized; iii) different layouts of DS stackings (cup-to-cup or crown-to-crown). The tests revealed the role of DSs in stabilizing the response of FJs, with preloading losses that are inherently influenced by the stiffness of the bolting assemblies. Tests showed that depending on the type of stacking, DSs may reduce wearing of the friction shims, improving clamping force distribution and stabilizing the friction mechanism. DSs can significantly mitigate preloading loss, potentially reducing it to 10% or less, compared to approximately 30% with flat washers. Building on the main outcomes of the two experimental campaigns, simple design tools have been developed for selecting the layout of DSs to be introduced in FJs with the aim to optimize the hysteretic behaviour of friction connections in terms of both equivalent damping and slip friction coefficient.