Roll-out instability of small size fiber-reinforced elastomeric isolators in unbonded applications

Unbonded fiber-reinforced elastomeric isolators, due to their light weight, low cost and easy installation, are viable devices for seismic mitigation purposes, both for housing and building applications in highly seismic areas, even in the developing world. Important aspects of these isolators are that they do not have thick base plates, they are not bonded to the top and bottom structure, and their reinforcements are flexible. These features allow fiber-reinforced isolators to experience roll-over deformation under lateral loads. Roll-over deformation is stable, if the stabilizing moment due to vertical forces is higher than the overturning moment due to horizontal forces; however, if this condition is not verified, the isolator experiences a “roll-out instability.” This paper proposes a model for prediction of roll-out instability based on the applied forces equilibrium condition and on the assumption that a triangular distribution for the compressive stresses acts on the isolator. The model is developed considering that the isolator contact area varies with the applied displacement, and consequently the compressive stresses vary. An expression for the isolator instantaneous stiffness is proposed. Results are presented for roll-out tests performed on fiber-reinforced isolators that have varied aspect ratios and are subjected to different compression levels. A comparison is made between the measured displacement at roll-out and the results obtained from the aforementioned model. The comparison proves that the model accurately and reliably predicts roll-out displacement of elastomeric isolators subjected to horizontal and vertical forces.