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About this Article
Written by: Andrew Jacobs
Written on: October 24th, 2004
Tags: civil engineering, building & architecture
Thumbnail by: Shustov/Wikimedia Commons
About the Author
Andrew Jacobs studied Building Science Civil Engineering at USC in the fall of 2004. His main academic interests include creative civil engineering design and architecture. He transferred to USC in fall 2002 from Westmont College in Santa Barbara to complete a dual degree program.
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Volume VI Issue I > Base Isolation
Base isolation has become a major feature of structural design in the past few decades. While a relatively simple concept, base isolation has a long and involved history of engineering influence and will take many more years to refine. Base isolators act on well-understood physical principles such as energy, oscillation, and damping. Rubber bearing base isolators, which are commonly used in modern buildings, owe much of their development to the work of James M. Kelly at the University of California at Berkeley, among others.

Introduction

Wikimedia Commons
Figure 1: The Los Angeles City Hall incorporates Base Isolation Technology.
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 [1]. These devices are currently protecting thousands of people worldwide (see Fig. 1), 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 [2]. Earthquakes shake the base of buildings laterally, applying what is called shear force to the foundation [3]. When such a large force occurs at the building's natural frequency, the building will fail and possibly collapse. Fig. 2 demonstrates the seismic loading response difference between stiffer and less rigid structures.
Illumin
Figure 2: Interaction animation of a seismic loading response comparison. (Flash)
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 [3]. 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 [1]. 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 [3]. Since base isolators must support the full weight of the building, they must be extremely strong [4]. 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 [3].