DESIGN AND EXPERIMENTAL CHARACTERIZATION OF PRESTRESSED LEAD DAMPERS

A modern approach to improve the seismic performance of both new and existing structures is to provide the structural frame with a set of dampers where the dissipation of most of the energy input is concentrated. One of the limits of this solution is that current dampers can get damaged after a strong earthquake, leaving the structure exposed to possible aftershocks. The aim of the study is to introduce a new damper suitable to resist to multiple earthquakes that can be used in bracing systems of frame structures. The damper provides energy dissipation by the friction force that is activated between a moving shaft and a lead core prestressed within a steel chamber. Thanks to the ability of lead to restore its properties by strain relief due to static recrystallization taking place immediately after deformation, repeated cycles of loading do not produce damages, and the device can quickly recover its original performance after an earthquake. In the first part of the contribution, the conceptual design of the new damper is presented. Finite element analyses are used to predict the basic behaviour of the device and to express the dependency of the force-displacement curve from the prestress of the lead core. In the second part of the study, two prototypes of the damper are experimentally characterized by means of cyclic and ramp tests performed at different speeds and stroke amplitudes. The dampers are shown to provide a stable and repeatable response over repeated cycles, an essentially rectangular hysteresis loop with an equivalent viscous damping ratio ξeff > 55%, a low sensitivity on the loading rate, and can withstand multiple cycles of deformation to the seismic design displacement without deterioration of performance, ensuring maintenance-free operation in presence of repeated ground shakes.