Impact characterization of innovative seismically designed connections for robustness application

Modern design approach and standards are nowadays requesting the design of resilient structures with the corollary that ensuring the safeguard of human life is not anymore sufficient. Indeed, a resilient design includes the conception and the realization of a structure able to sustain accidental or exceptional events with limited and easy-to-repair damages. The objective of this thesis is the investigation, under static and impact loading, of an innovative beam-to-column connection called “FREEDAM” (an acronym for FREE from DAMage connections), which have been conceived and developed in the homonym RFCS European project (Research Fund for Coal and Steel). This innovative joint solution, designed to act as dissipative zones in moment resisting frames (MRFs), is exploiting the concept of supplementary energy dissipation (also known as passive control) integrated into the seismically designed structure using a friction damper. The main peculiarity relies on the mechanism of energy dissipation based on the activation of this friction damper, whose high friction resistance is obtained with the use of specific coating materials and controlling the bolt preloading. The loss of preload in time can substantially affect the performance of the device along with its life since the bolt can experience a reduction up to 20-30% of its initial preload, which may cause premature activation of the friction damper. However, if the bolt preloading is adequately mastered, these connections can withstand severe and frequent seismic events without suffering of almost any damage, which is cost-effective if compared with traditional connections where large plastic deformation are expected to occur under such loading conditions. Moreover, this solution can also be effectively exploited to improve the robustness of steel buildings through the increase of the local joint ductility but the activation of such joints in a robustness context requires to master the behaviour of the joints subjected to different scenarios and, in particular, under dynamic loadings. However, the behaviour of joints (and in particular of friction joints) when subjected to dynamic actions has not yet been deeply investigated and is still requiring significant research efforts. Within this framework, the present thesis first investigates the evolution of the preload in the bolts of the damper as a key parameter to ensure the efficiency of the friction damper to develop and validate an analytical model able to predict the loss of initial preload during the lifetime of the structure. Then, the second part of the thesis is dedicated to the study of the behaviour of the FREEDAM joint subjected to impact to investigate the effect of dynamic loadings on the joint response and to propose a methodology to account for dynamic effects in simplified equivalent static analyses using the concept of amplification factor DIF (Dynamic Increase Factor). FREEDA