Characteristics of various elastomeric bearings in tension

It has been a long time since seismic isolation was invented with the aim of protecting lives and structures against earthquakes. Seismic isolation is achieved by installing special bearings, so-called isolators, between the foundations of buildings or the piers of bridges and the superstructure. These isolators support the structure in the vertical direction and at the same time allow horizontal movements during earthquakes so that the seismic force applied to the superstructure can be much reduced. In Japan elastomeric bearings are often used for this purpose and existing seismic-isolated structures have demonstrated their effectiveness in the past. A number of research projects concerning elastomeric bearings have been carried out and their performance with respect to compression and shear loads is well understood. However, not much attention has been paid to their characteristics with respect to tensile loads. In some design standards a certain degree of tension in rubber bearings is already allowed to occur during earthquakes. This is inevitable since the ability of rubber to sustain tension was discovered and in seismic isolation applications in high-rise buildings and bridges there is a possibility that isolators undergo tension due to the overturning moment. There are, however, still many uncertainties; for instance, the influence of damage to or cavities within the rubber caused by tension, the interaction between the shapes and dimensions of bearings and the tensile strength, and the influence of the out-of-plane deformation of flanges are not well understood. Currently, the Japanese design rules for both bridges and buildings regulate the allowable tensile stress without distinguishing the type of rubber, shapes or dimensions. This merely empirical value will need to be reconsidered.