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Supplementary energy dissipation has proved to be an effective way of protecting structures from the disastrous effects of earthquakes and has been used in the last decades both in new and in existing constructions. In this regard, various procedures for the design of the damping system for the seismic retrofit of buildings have been formulated over the years, mainly focused on reinforced concrete (RC) constructions, which represent the largest part of the existing stock in many seismic-prone countries. The study deals with the assessment of a displacement-based design procedure for proportioning the damping system recently proposed in the literature for RC framed buildings, with the goal of establishing a good practice for the application of the procedure to steel buildings as well. The method was applied to three case-study frames, regular in plan and in elevation, which were assumed as being representative of old structures designed without consideration of seismic requirements. The retrofit was performed by using chevron braces equipped with dampers with an elastic-perfectly plastic behavior. The method aimed at defining the properties of the dampers to achieve a target performance in terms of the maximum lateral deflection for a specific level of seismic intensity. The effectiveness and reliability of the proposed procedure was eventually assessed by evaluating the seismic performance of the upgraded steel structures in static and dynamic non-linear analyses.

The breakaway friction coefficient of curved surface sliders (CSSs) governs the transition between the sticking and the sliding behavior of the isolators, and hence affects the response of an isolated building during an earthquake. When the inertia forces induced by low-to-moderate intensity excitations are not able to overcome the breakaway frictional resistance of the CSS isolation system, the structure behaves as a fixed-base building, thus experiencing higher accelerations, interstorey drifts and internal forces than the isolated building. The majority of structural analysis programs disregard the static coefficient of friction, and implement the dynamic friction coefficient only throughout the response history analysis, which implies an increased displacement demand for the isolation system but may concurrently lead to an unsafe design for the superstructure. In this paper, the frictional resistance to sliding before the breakaway is simulated through a bidirectional plasticity domain, which has been coded in a finite element of the isolator formulated in OpenSees to incorporate the transition between the breakaway friction in the sticking phase and the velocity-dependent friction model in the subsequent sliding phase. Based on this formulation, the influence of the breakaway friction on the response of buildings isolated with CSSs is investigated numerically through an extensive parametric study comprising more than 9000 bidirectional nonlinear time history analyses (NLTHAs). The parameters cover a range of friction coefficients, superstructure properties and a large group of natural spectrum-compatible bidirectional ground motions having different intensity levels and frequency contents. The results from NLTHAs are processed statistically to elaborate regression formulae and design recommendations that can be useful to predict the trigger acceleration at which sliding motion starts, as well as to suggest how to achieve more accurate estimates of the seismic response when the breakaway friction is ignored in the structural analysis model.

Supplemental energy dissipation devices are employed both in new and retrofitted constructions in order to prevent structural damage, increase life-safety and achieve a desired level of performance. Among these devices, hysteretic dampers have been proven to be an appropriate and economically affordable solution to reduce the vulnerability of ordinary structures, such as residential, school and industrial buildings. The study presents an experimental and numerical investigation of a Prestressed Lead Extrusion Damper (P-LED), an emerging energy dissipation device which provides energy dissipation by means of the plastic extrusion of lead through an orifice created between a containing tube and a moving shaft and achieves high specific output force by preloading of the working material. The experimental investigation is performed following the provisions set in the European standard EN 15129 for Displacement Dependent Devices. A damper prototype is tested in cyclic tests at different displacement amplitudes and in a monotonic ramp up to the amplified design deformation. The damper shows a rigid-plastic behavior, without strength degradation regardless of the imposed deflection; the shape of the hysteresis loops is essentially rectangular, resulting in an effective damping of 0.55, very close to the maximum theoretical level; the device is able to sustain multiple cycles of motion at the basic design earthquake displacement, anticipating a maintenance-free operation even in presence of repeated ground shakes. A 3D finite element model of the P-LED is formulated in Abaqus and validated upon the results of the experimental tests. The model enlightens that the output force of the damper accounts for two contributions, namely the extrusion force of the lead and the friction force between the lead and the moving shaft. The model is then used in a parametric study to investigate the influence of the device dimensions, namely the diameters of the shaft, of the containing tube and of the bulge, and the length of the shaft, on the output force. The numerical data points are fitted by a simple model which can be used for design of the damper to a specific quasi-static force.