PUSH-AND-RELEASE TESTS OF HYBRID ISOLATION SYSTEMS: EXPERIMENTAL RESULTS AND MODEL CALIBRATION

This study illustrates the comparison between the experimental testing of a two-storey braced-frame structure seismically protected by means of and isolation system and the simulation of such tests. More in detail, the steel structure rests on a reinforced concrete slab isolated from the foundations through a hybrid system consisting of 28 High-damping Rubber Bearings (HDRBs) and 36 low-friction sliders (LFSs). During the building design the removable push-and-release device, the reaction wall, and the pushing point were also designed to minimize the cost of the tests and to allow repeated testing throughout the service life of the building. The global dynamic response was tested up to displacement amplitudes similar to those induced by extreme seismic events. In particular, maximum horizontal displacements of 285 mm and 227 mm in the quasi-static and dynamic tests are reached respectively. Consequently, the test can be de-facto considered as an in-situ verification of the seismic behaviour of the isolation system. However, in the case of elastomeric bearings this testing procedure is not properly equivalent to an earthquake excitation, mainly due to the slow deformation velocities in the loading path and the related not negligible viscous effects of HDRBs. Therefore, this work aims at understanding and evaluating the observed experimental behaviour by assessing and calibrating a model able to simulate all the viscous effects and the energy dissipated by the sliders. This way, the viscous displacement accumulated during the loading phase, causing the system to oscillate around a non-zero displacement during the release phase, can be simulated.