Short- and long- term loss of preloading in slotted bolted connections

One recent solution to reduce the building repairing costs, in case of exceptional or destructive seismic events, is represented by the Sliding Hinge Joint (SHJ), initially proposed by the research group of the University of Auckland in 2005. This connection typology, using slotted bolted connections in beam-to-column joints of Moment Resisting Steel Frames (MRFs), ensures a large energy dissipation capacity with negligible damage. The slip resistance of the SHJ is usually provided by Asymmetrical or Symmetrical Friction Connections (AFCs or SFCs), whose response depends on the friction coefficient of the shims and on the bolt preloading. Since the bolt forces directly affect the resistance, the control of preloading during the life-time of the connection represents a key issue for the proper functioning of the device. Within this framework, considering the lack of previous experimental works specifically devoted to the long-term response of SHJs, this paper presents the results of an experimental activity on subassemblies of friction dampers, assessing the loss of preload under service loading conditions, examining the possibility to limit the loss through different conical washer layouts (Belleville disk springs). Short-, mid- and long-term tests have been carried out in a joint experimental program carried out at the Universities of Liege and Salerno. The experimental results have been exploited to calibrate an analytical model able to predict the loss of preload over time, accounting for the influence of the external loads. The adoption of pre-set disk springs, reducing the stiffness of 1/10, shows a substantial reduction of the loss.