Numerical assessment on the seismic response of a base-isolated building under Bi-directional motion

Experimental and analytical studies on Concave Sliding Surface devices have un- derlined that the hysteretic characteristics under bi-directional input motion are highly non- linear, in comparison with the uni-directional case. The force response of the device along both directions of motion strongly depend on the shape of the trajectory, because of the step- wise changing of direction of the frictional force, rather than in a uni-directional motion in which all forces have the same direction. Together with these aspects, also the variation of the frictional properties with respect to sliding velocity (Velocity Effect), vertical load (Verti- cal Load Effect) and heating phenomena originated at the sliding interfaces (Cycling Effect) increase the non-linearity of the behavior of such kind of isolators, leading to dispersions of the peak quantities when time history analyses are performed: such dispersions can not be neglected a priori, but they have to be accurately evaluated. In addition, experimental tests have shown widely sparse frictional properties of the sliding materials commonly used for CSS devices, even when the same conditions of loading are considered. Moreover, in real ap- plications, the spatial distribution of both the devices and the isolated building affect the overall response, especially when irregularities in the installation of the isolators are consid- ered, which results into uneven inclinations of the sliding surfaces with respect to the horizon- tal plane. In this endeavor a wide numerical campaign has been carried out on a case study, considering several layout conditions, the actual dispersion of the frictional characteristics of the device, and different models of the friction coefficient for all the isolators. The results of all the numerical simulations, performed by means of time history analyses, have allowed to underline some important aspects which must be accounted for when designing a structural system, isolated using a grid of CSS devices, under bi-directional seismic excitations