Monotonic and cyclic inelastic tensile response of European preloadable gr10.9 bolt assemblies

High strength preloadable bolts are key components influencing strength, stiffness and ductility of beam-to-column joints. In Europe the HV and HR systems are the most commonly used types of pre-loadable bolt assemblies. EN 1993-1-8 provides design rules for bolted connections, without distinguishing the type of high strength preloadable bolt system. However, HV and HR are characterized by both different force-displacement responses and failure modes, which may significantly influence the performance of bolted joints under monotonic and cyclic actions, especially for seismic applications. The rational selection of the most suitable bolt type per specific structural demand may avoid premature reduction of joint strength for higher levels of joint rotation, or provide further rotational capacity. In order to investigate these features, experimental monotonic and both variable and constant amplitude cyclic inelastic tests were carried out on both HV and HR bolt assemblies, varying the shank diameters. The monotonic tests enabled to identify the failure modes and the relevant force-displacement monotonic response and ductility per type of bolt assembly. The variable amplitude cyclic tests allowed quantifying the strength degradation induced by cyclic actions, while the constant amplitude low-cycle fatigue tests enabled to investigate the fatigue capacity at different threshold of plastic strain and to determine the strain-number of cycles (i.e. e-N) curves. The influence of the type of bolt assembly on the response of T-stub connections is also discussed on the basis of analytical models.