EXPERIMENTAL RESPONSE OF FRICTION JOINTS WITH DIFFERENT BOLT ASSEMBLIES AND DISK SPRINGS LAYOUT

As recently shown in many research studies, Friction Joints (FJs) can be considered an optimal solution for Moment Resisting Frames in seismic areas. Indeed, differently from the traditional beam-to-column joints, they can dissipate large amounts of energy providing only low or negligible structural damage. The slip resistance of FJs, whether they are realised with Symmetrical or Asymmetrical Friction Connections (SFCs or AFCs), strongly depends on the values of the initial friction coefficient and bolt preloading, as well as on their evolution during a cyclic loading history. Recent works have pointed out how the bolt preloading techniques, creep, vibration, and wearing of the friction shims in the post-sliding phase, can lead to a loss of the initial bolt preload and, consequently, to a reduction of the joint flexural resistance. A possible solution to limit the loss of initial preload is represented by installing Belleville Disk Springs (BDSs) in the bolt assembly. Indeed, these conical washers can reduce the stiffness of the bolt assembly compensating for short-and long-term effects, as well as for the possible damage occurring in the friction interface in the post-sliding phase. Within this framework, to evaluate the efficiency of BDSs in the reduction of the loss of preload, an experimental campaign dealing with the hysteretic behaviour of four beam-to-column joints equipped with different BDSs layouts has been performed at the STRENGTH Laboratory of the University of Salerno. The monitored parameters are the evolution of the preloading force and the slip resistance. Finally, the joints’ performances have been compared in terms of stability of the hysteretic response.