Dissipative replaceable components and impact mass dampers towards more resilient earthquake-resistant steel structures

This thesis work deals with the passive control of the dynamic response of steel buildings against earthquakes. To provide easy repairability after an earthquake and mitigate internal actions different activities are pursued: i) hybrid simulations (HSs) of full-scale steel and steel-concrete composite frames endowed with dissipative replaceable components, and ii) development of an innovative system for seismic vibration mitigation based on impact mechanism. The aforementioned research issue is approached at two levels. The former level is based on the contribution to the European funded RFCS Dissipable project, that aims at promoting the employment of dissipative replaceable components into common design practice. In the context of performance-based earthquake engineering (PBEE), this is achieved by means of full-scale HSs on steel and steel-concrete composite frames equipped with specific seismic components that allow for the assessment of their dissipative behaviour and their replaceability easiness. In particular, innovative frames equipped with two different components are examined: namely, the dissipative replaceable beam splices (DRBeS) and the dissipative replaceable braced connections (DRBrC). The whole experimental activity is approached at the system level, which allowed for the HS of six-storey full-scale frames by physically realizing only their first floors, while numerically simulating the remainder of the structures. Physical and numerical substructures were coupled by means of electro-hydraulic actuators, that controlled the displacements of the physical substructure, while a real-time numerical algorithm generated the displacement command accounting for a time scaling factor. Within the PBEE approach and to assess the performance of the aforementioned dissipative components, HSs at damage limitation (DL), significant damage (SD) and near collapse (NC) limit states were carried out. Given the design limit states, the dissipative replaceable components successfully protected the parts of the frames that had to remain elastic according to the capacity design; to dissipate a large amount of energy, instead, the ductile components activated wide and stable hysteretic behaviours. The frames behaved as foreseen and the comparison with the predictions of the reference numerical models shows favourable comparisons. Moreover, the passive damaged components were replaced without any particular difficulty. The latter activity is also conceived in the context of passive structural control. Along this main vein and with the purpose of mitigating seismicinduced structural vibrations, an impact mass damper (IMD) was conceived and developed. The device is constituted by a mass installed on the top of a building, which is free to move along a linear path and impacts against two stoppers. The impact mechanism forces the controlled structure to be subjected to impulsive forces that, given their high frequency content, are able to transfer energy to the higher modes of vibration; these modes are mainly associated with lower participant masses and higher damping coefficients: in such instances, the design is challenging. More precisely, the foreseen damping system is controlled by three different parameters: the gap between the mass and the stoppers, the coefficient of restitution and the mass of the impacting body, respectively. To optimize such parameters due the stochastic seismic input, a relevant procedure is set by means of the design of experiments and the Kriging surrogate model. Relevant results show how the optimized parameters permit an effective vibration mitigation of the controlled system. Finally, a systematic comparison of the dynamic response of different steel frames, shows how short-period structures are more suitable to be seismically protected by specially-designed IMDs.