Antifriction and Multi-Step Base Isolation

Educational level: this is a research resource.

Antifriction and Multi-Step Base Isolation (AF&MS BI), also called Shock Evader, is relatively recent type of seismic vibration control. In spite of the fact that the first attempts to isolate buildings from potentially shaky ground were made thousands years ago, the modern concept of seismic isolation (flexible mounting + damping) is foreign for earthquake engineering: it has not been inherited, it has been borrowed from mechanical engineering.

Building model on Shock Evaders at FEMA conference, Alexandria.

Though the concept is working perfectly in all sorts of vehicles, in seismic isolation everything is not so smooth because the conditions in both cases are quite different.

Shock Evader in action at a museum shake-table, Los Angeles.

In a car, for instance, the working stresses in auto parts are far below their ultimate bearing capacity. Therefore, some overloads associated with heavy damping are of no practical importance here. Another matter is a building structure: during a strong earthquake, it is intended to perform at the near-to-collapse level and, therefore, any extras can become crucial for its safety.

However, there is an alternative to the contradictory damping mechanism of those base isolators. It can be found in the utmost lessening the damping and substituting its positive, mitigating quality with any sort of tuning-out mechanism which satisfies the following requirements:

• Let the earth move its way.
• Prevent resonant amplifications.
• Restore the structure in its pre-earthquake position on the foundation.

It is not the building, it is the earth that should be vibrating if the building is supported on the ideal isolation system. Any attempt to reduce a relative displacement of the superstructure with respect to the base will inevitably result in additional transmission of earthquake energy into the building.

This new concept has been embodied in Shock Evader or, which is the same, in the Antifriction and Multi-Step Base Isolation (AF&MS BI) that incorporated the merits of the traditional flexible mounting but without its drawback - a compulsory damping mechanism [1].