Long-term viscous effects of in-field tests on HDRBs-based seismic isolation systems

Push and release tests on full-scale base-isolated buildings can be considered de facto as in situ verification tests of the actual behaviour of a base-isolation system. However, for isolation systems based on High Damping Rubber Bearings (HDRBs) the test response significantly depends on the pushing phase velocity. If this is quite low (due to the limited capacity of the hydraulic system), the free vibration response can remarkably differ from the expected one (considering nominal properties of HDRBs) due to the viscous behaviour of HDR compounds. Therefore, a model that adequately describes the viscous effects related to different strain velocities is crucial for correctly interpreting the experimental measurements obtained from the tests. This paper examines in detail the viscous effects observed in both laboratory tests conducted on a couple of HDRBs (type tests) and the experimental in-situ tests performed on a base-isolated structure equipped with an hybrid system comprising HDRBs and Low Friction Sliders (LFSs). A simple linear viscoelastic model of the HDRBs is developed based on type tests and then used to simulate the base-isolation system response. Results show that the model effectively predicts the experimental response (the pushing phase, the free vibration response and the final residual displacement) and that it can be used to verify that nominal properties of HDRBs (as well as friction of LFSs) are consistent with the design specifications, even if the recorded response differs from the expected one considering those parameters.