Preliminary analysis and characterization of a base isolator made with steel cylinders rolling on rubber layers

The present research fits in the general thematic of the seismic isolation of structures with special attention to light structures. The isolation bearing considered in this study consists in cylinders free to roll, in two perpendicular levels, between two rubber layers. The dynamic behaviour of this device has been first studied from a theoretical point of view in order to analyze the effect of the rolling contact between rubber and steel and to find out the appropriate dimensions of the device to reduce the effect of an earthquake in a structure. The device reduces the seismic energy both by decoupling the motion of the structure from the base and for the visco-elastic behavior of the rubber (or neoprene). The behavior and the efficacy of this device is explained by relations obtained for the rolling friction coefficient vs. the rolling velocity and for the horizontal force vs. the vertical load acting on the device. In the second part of the paper a prototype has been realized and subjected to characterization tests. From a preliminary design it has been determined the appropriate dimensions of the device and the details to link it to the testing equipment. The results have been discussed to get the principal characteristics of the device good also to perform a numerical analysis of the device when installed in a building and to check the behavior of the whole structure. Considerations have been drawn from the tests results on the behavior and the efficacy of this new device. The device is good to isolate low-rise light-weight structures, equipment and machinery during horizontal ground motions; the device is capable of wide applications for the seismic protection of low to medium height buildings, with low values of the normal stress at the foot of the columns and a low period of vibration. From the theoretical analysis the two main advantages consist in the proportionality of the shear force with the corresponding mass (typical of friction isolators), which involves the absence of eccentricity between the centre of stiffness of the isolation system and the center of gravity of the masses of the superstructure, and the absence of a characteristic frequency, that avoids the risk of resonance with the ground motion. Future steps will be to perform shaking table tests on a steel frame protected by these devices to check the behavior of the rolling bearings when subjected to a real earthquake.