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 paper presents the experimental characterization, the formulation of a numerical model, and the evaluation, by means of non-linear analyses, of a new friction damper conceived for the seismic upgrade of existing building frames. The damper dissipates seismic energy through the friction force triggered between a steel shaft and a lead core prestressed within a rigid steel chamber. The friction force is adjusted by controlling the prestress of the core, allowing the achievement of high forces with small dimensions, and reducing the architectural invasiveness of the device. The damper has no mechanical parts subjected to cyclic strain above their yield limit, thereby avoiding any risk of low-cycle fatigue. The constitutive behavior of the damper was assessed experimentally, demonstrating a rectangular hysteresis loop with an equivalent damping ratio of more than 55%, a stable behavior over repeated cycles, and a low dependency of the axial force on the rate of displacement. A numerical model of the damper was formulated in the OpenSees software by means of a rheological model comprising an in-parallel system of a non-linear spring element and a Maxwell element, and the model was calibrated on the experimental data. To assess the viability of the damper for the seismic rehabilitation of buildings, a numerical investigation was conducted by performing non-linear dynamic analyses on two case-study structures. The results highlight the benefits of the PS-LED in dissipating the largest part of seismic energy, limiting the lateral deformation of the frames, and controlling the increase in structural accelerations and internal forces at the same time.