Comparison of different models for friction pendulum isolators in structures subjected to horizontal and vertical ground motions

This research focuses on the different modelling approaches for the simulation of the seismic response of structures with friction pendulum isolation systems. The behaviour of such systems is strongly affected by several parameters, as for example the friction coefficient and the axial load. The latter has a particular importance in presence of seismic actions characterized by the simultaneous presence of high horizontal and vertical accelerations. To study these aspects, several nonlinear dynamic analyses have been performed considering a two degree of freedom model isolated at the base and subjected to recorded horizontal and vertical ground motions. The response of the selected isolation systems has been studied through different models, starting by the more simple ones based on a constant friction coefficient to the more complex ones based on a friction coefficient varying with the sliding velocity and axial force. A set of ground motions with near field records characterized by different values of the ratio between peak vertical and horizontal accelerations has been considered. The results have allowed to compare the different models and to study the effect of the vertical seismic component on the response of the isolators.