The History and Theory of Earthquake Base Isolation

The Earthquake Engineering Research Centre (EERC), now known as the Pacific Engineering Research Centre (PEER), of the University of California at Berkeley, was the first institution in the United States to conduct a study on the feasibility of using raw rubber bearings as base isolators to defend buildings from earthquakes. This was in 1976. The study undertaken was a combined effort between the EERC and the Malaysian Rubber Producers Research Association (MRPRA) from the United Kingdom. In the beginning, this study program was fully financed by the MRPRA and latter on by the National Science Foundation and the Electric Power Research Institute.

The concept of base isolation had been around for quite a while. Most professionals in the field of structural engineering believed that the idea of using rollers or sliders for base isolation was totally nonviable and not suitable. The study program started with a simple three story, twenty ton model. Handmade bearings, produced from very low cost rubber, were used at the commencement of the research.

Shaking table trials carried on the model established that the application of isolation bearings was able to decrease the acceleration by a factor up to 10 in equivalence with conservative designs. The research program proved that the model reacted as a stiff body and all the twist took place only in the isolation system. This early research directed the need towards a certain degree of damping in the projected system; due to the tiny extent of the model, the use of more practical combinations of rubber was ruled out.

In 1978, a five-storied, three bay, forty ton model was used to study base isolation when commercially produced bearings with increased damping were used. This study, conducted with the help of five tests, proved that the isolation attained through rubber bearings had the capacity of considerably decreasing the increase in speed felt by the structure itself. The very same study also demonstrated that the utilization of extra elements like lead plugs in the bearings or energy absorbing devices made of steel in the isolation system to enhance the quantity of damping lessened the capability of the isolation system in decreasing the acceleration felt by the internal equipment. This directed the study in finding a well-organized way to add in the damping inside the rubber bearing, and not as a disconnected element supplemented to the isolation system.

Actually the rubber bearings are effective isolators because they are comparatively effortless to manufacture, can survive the test of time, are fixed, and are outstandingly resistant to ecological degradation. Rubber bearings manufactured by binding sheets of vulcanized rubber to thin reinforcing plates made of steel tend to be supple horizontally, while being very stiff in the vertical direction. Thus during seismic action, the sheet of bearings secluded the structure from the horizontal components of ground motion, and the vertical factors transferred to the building remain intact.

Vertical acceleration does not affect most structures, and the bearings also have the capacity to isolate the building from undesirable high-rate vertical vibration formed by sources such as traffic and underground railways.