Modelling of 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. In many recent seismic events, as for example L’Aquila (2009) and Emilia (2012) in Italy, the vertical seismic component has been characterized by very high values of acceleration and has played a crucial role in damaging the buildings. 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 isolation systems has been studied through different models, starting by the more simple ones based on constant friction coefficient to the more complex ones based on a friction coefficient varying as a function of sliding velocity and axial force. The analyses have been performed considering a set of ground motions with near field records that present different values of the ratio between the peak vertical and horizontal accelerations. The records have been also applied considering increasing values of intensity, in order to determine the collapse acceleration. 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.