Rolling-Ball Rubber-Layer Isolation System – small deflection and vibrational behaviour

The small-deflections behaviour of the Rolling-Ball Rubber-Layer (RBRL) seismic isolation system is investigated through numerical analyses, performed on the test results of a previous shaking-table experimentation, and by monoaxial sinusoidal tests specifically carried out at TARRC. The efficacy of the RBRL system in seismic mitigation is shown even for seismic events that induce only small deflections across the isolators (i.e. < 5mm). This good performance is due to the indentations developed by the balls in the rubber track under static load, caused by relaxation phenomena of the rubber. This peculiarity makes the system much more effective at low excitations than an equivalent sliding isolation system. Other useful conclusions were obtained from tests about the influence of: load on each ball, type of rubber and dwell time under static load. Finally, the results show a good retention of performance of the system when retested 15 years after its manufacture. The system, suitable for isolating light structures, is relatively economical and is easy to tailor for the specific case, in terms of geometry and performance, a great range of equivalent natural frequencies and coefficients of damping being achievable.