INVESTIGATION OF THE LATERAL RESPONSE OF FRICTION-BASED ISOLATORS UNDER MULTI-CYCLIC EXCITATIONS

The frictional response of Concave Surface Slider (CSS) devices has been more and more investigated both experimentally and numerically. These isolators have shown many advantages in comparison to the commonly used typologies of devices, such as lead rubber bearings or low and high damping rubber bearings: when implemented in structural systems, the eccentricity of the resultant base shear with respect to the center of mass is significantly reduced, since the lateral response of the devices is a direct function of the applied vertical force, i.e. the weight of the structure; furthermore new innovative sliding materials have been studied and implemented in real applications, in order to achieve high levels of energy dissipation, together with a high recentering capability, due to the geometry of the steel sliding surfaces. On the other hand, a number of issues about the behavior of friction-based isolators still have to be accurately analyzed. Among the others the distribution of the vertical load applied to the device is usually assumed constantly smeared on the sliding pad: however, recent research works have shown rather than constant distributions of contact pressure and this aspect is expected to cause variations in the commonly known dependency of the friction coefficient on the vertical load. Moreover, when a CSS device is subjected to long lasting dynamic excitations, the so called “cyclic effect” leads to a decay of the friction coefficient during time. Such a decay trend can be analyzed in terms of friction coefficient as a function of the cumulative dissipated energy, and can be fully described by an exponential equation, properly calibrated; the decay behavior is also supposed to be characterized by dependencies on both sliding velocity and contact pressure, i.e. vertical load. Then, a direct comparison between flat and concave sliding motions needs to be carried out, aiming at highlighting the differences in the frictional response of these typologies of movements. In the present work the dynamic behavior of a friction based device has been deeply examined, thanks to the outcomes of a wide experimental campaign carried out at the EUCENTRE TREES Lab in Pavia on full scale flat and curved sliders, equipped with innovative sliding materials. Precisely, the cyclic effect caused on the friction coefficient by long lasting bi-directional dynamic motions has been characterized, by assuming several combination of sliding velocity and vertical load values. Moreover, the comparison among different diameters for the sliding pads in flat motions has been studied, in order to underline any “size effect” on the frictional response. Then, the comparison between flat and curved sliding motions has been carried out. Finally, such results have also been considered analytically, aiming at evaluating the consequences of the aforementioned features on the response of structural system base-isolated with CSS devices.