Experimental and numerical analysis of steel frames equipped with repairable dissipative seismic components

The conventional design of buildings in seismic zones entrusts energy dissipation to the structural elements. The capacity design, adopted in the main national and international design standards, ensures that the formation of plastic hinges occurs at specific points of the structure to facilitate a ductile collapse mechanism. Although this strategy allows for designing structures capable of dissipating energy under seismic loading, they do not guarantee ease of repair after an earthquake, resulting in a long downtime/business interruption of the structure. Moreover, buildings designed according to these approaches may undergo significant damage, whose repair work is often not feasible or too expensive. Therefore, reducing damage to structural and non-structural elements after a disaster is fundamental for costs and functionality. The work presented in this thesis was developed in the framework of the project DISSIPABLE, funded by the European Research Fund of Coal and Steel (RFCS). The project was funded to perform large demonstration tests on steel frames equipped with easily repairable seismic dissipative devices, aiming to demonstrate their effectiveness in mitigating seismic hazard and their ease of substitution/repair. The tested frames were equipped with three innovative components, namely the dissipative replaceable link frame (DRLF), the dissipative replaceable beam splices (DRBeS) and the dissipative replaceable braced connections (DRBrC). In order to fully characterize the seismic behaviour, the tests were conducted at three limit states of increasing intensity, i.e. damage limitation (DL), significant damage (SD) and near collapse (NC). Hybrid simulation (HS) and the substructuring technique were exploited, allowing for reduced experimental costs by testing only part of a full structure yet providing meaningful and accurate results. Six-storey full-scale frames were investigated by physically realizing only their first floors and numerically simulating the remainder of the structure, ensuring compatibility between the parts by controlling the displacements and implementing a real-time numerical algorithm, namely Generalised α (G-α) algorithm. Prior to performing the experimental tests, the stability and accuracy analyses of the G-α algorithm were carried out, also considering possible differences between the estimated and the effective stiffness of the physical subdomain. The study proved that the algorithm is stable and first-order accurate considering the discrepancies in the stiffness matrix estimation. The laboratory test results highlighted that the dissipative replaceable components successfully protected the irreplaceable parts of the frames, which remained elastic at the design limit state, i.e. SD limit state. The devices dissipated a large amount of energy through wide and stable hysteretic behaviours at both SD and NC limit states. Finally, the damaged components were replaced without any difficulty. Furthermore, the comparison with the predictions of the reference numerical models shows favourable outcomes. Moreover, the novel algorithmic correction implemented in the G-α algorithm in hybrid simulation was validated. Via an extensive investigation, stability and accuracy were studied for the G-α algorithm along with the proposed correction, considering the inherent realistic laboratory sources of error, e.g. delay and noise in the signal. The analyses confirmed that the algorithm is stable and first-order accurate. Finally, following the results of the tests, high-fidelity models of the structures were developed and calibrated on the results of the experimental campaign. This provided a deeper insight into the seismic behaviour of the structures and allowed for the derivation of reliable experimentally calibrated fragility curves by means of incremental dynamic analyses (IDAs). In particular, frames equipped with the seismic dissipative components developed in DISSIPABLE were compared with a state-of-the-art reference model. It turned out that the frames equipped with the seismic dissipative components, at the same probability of failure, can be repaired more quickly, and they are more cost-effective.