Introduction

There are currently hundreds of buildings worldwide that are held up by rubber—that is, they are not rigidly fixed to the foundations on which they rest. It may come as a surprise that these rubber foundation elements can actually help minimize earthquake damage to buildings, considering the tremendous forces these buildings must endure in a major quake. For nearly four decades, seismic analysis engineers have been perfecting unusual and complex systems called base isolators to protect buildings from earthquakes (Skinner et al 10). These devices are currently protecting thousands of people worldwide, but many in the general public know little about them and the history of earthquake protection design. To catch a glimpse into the world of modern seismology and the work of seismic engineers like James M. Kelly, we will focus our attention on the engineering behind a specific kind of base isolator: the rubber bearing.

Seismic Intuition

Most of us think that a stronger, more rigid attachment of a building to its foundation will result in less damage in an earthquake. Our intuition tells us: "strengthen to resist damage." The problem with this idea is that if the foundation is rigidly attached to the superstructure, all of the force of the earthquake will be transferred directly and without a change in frequency to the rest of the building (Fan et al 329). Earthquakes shake the base of buildings laterally, applying what is called shear force to the foundation (NISEE). When such a large force occurs at the building's natural frequency, the building will fail and possibly collapse.

Engineers know the importance of keeping the superstructure stable while the foundation is being shaken by an earthquake. However, designing a system that puts this concept into practice presents significant challenges. How can a superstructure not rigidly attached to the foundation be guaranteed to stay in place during an earthquake? The first attempts at solving this structural difficulty were made around the turn of the 20th century, but proposed designs did not become practical to build until a few decades ago (Kelly et al. 1987). In 1967, three engineers working at the Physics and Engineering Laboratory of the Department of Scientific and Industrial Research (PEL, DSIR) in New Zealand began significant research on and development of seismic isolation devices (Skinner et al 10). R. Ivan Skinner and his associates, along with many other engineers doing independent work in other countries, have produced a wealth of information about base isolators and seismic control. Since their initial efforts, base isolators have become common knowledge to civil and structural engineers.

The Ultimate Shock Absorber

Rubber bands and the shock absorbers in cars have a lot in common. They both allow stretching, but as they stretch they increasingly resist further stretching. When the stretching force is released, they return to their natural length. Engineers wanted to use a similar system to protect buildings from earthquakes. But unlike shock absorbers, which provide vertical isolation from road bumps, the vibration control needed for buildings is lateral, because the most destructive seismic motions for buildings are lateral (NISEE). Since base isolators must support the full weight of the building, they must be extremely strong (Sackman et al 1). This level of strength can be achieved using a laminated rubber bearing (LRB). This rubber and steel device is able to resist the weight of a structure while simultaneously allowing the foundation to move with the ground during an earthquake and isolating the superstructure from the harsh acceleration, frequency and energy of the earthquake (NISEE).