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Modern Engineering for Earthquake Safety

by Gloria Lenhart

A conversation with Robert Reitherman, Executive Director of the Consortium of Universities for Research in Earthquake Engineering (CUREE), a non-profit organization devoted to the advancement of earthquake engineering research, education, and implementation. For information on CUREE and their research go to: www.curee.org.

What was learned from the 1906 disaster that helped engineers make buildings safer? The 1906 San Francisco Earthquake occurred a little too early in history for major engineering lessons to be learned. Today we know that unreinforced masonry buildings -- buildings with brick walls that have no steel reinforcing bars in them -- are seismically hazardous. But that method of construction was still widely used after 1906. In fact, in the 1980s San Francisco compiled an inventory of its unreinforced masonry buildings and found that over 90% of its 2,000 unreinforced masonry buildings were built after 1906, not before.

Is a building that has been built up to the requirements of a recent edition of the building code earthquake-proof? The aim of the building code is to provide safe performance but not earthquake-proof performance. In a big earthquake, portions of a modern seismically-designed building may well be damaged.

What methods are used today to make modern buildings safer during quakes? Ductility, the ability to bend and deform without breaking, is a key aspect of current seismic design. Certain locations in the structure are designed to become bent or cracked to a controlled extent to protect other more fragile areas of the structure. A building designed to be ductile won’t collapse, but it could sustain damage that’s not repairable.

Another strategy is to use “damping” or energy dissipation systems. Seismic dampers act like shock absorbers, reducing the swaying motion of the building. But perhaps the most effective method for providing earthquake protection is what we call base or seismic isolation.

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Base isolators like this one allow a building to move independently from ground movement.
Photo courtesy Forell/Elsesser Engineers Inc.

What does seismic or base isolation involve? Seismic isolation is a method of decoupling a building from its foundation so that it can move somewhat independently from the ground motion. The shaking of the building will be only roughly one-third as intense as it would be without the isolators. This minimizes or can even eliminate earthquake damage to the structure. The weight of the building or bridge rests on special isolation bearings, rather than rigidly connecting the above- ground structure to the concrete foundation that is embedded in the ground.

Is seismic isolation a new technology? It was first developed and used in New Zealand in the 1970s and 1980s. The method is frequently used in Japan, but the U.S. has been slow to adopt it. There are only about two dozen buildings in the entire Bay Area that incorporate this technology.

If seismic isolation is the best method for minimizing or eliminating earthquake damage, why don’t more Bay Area buildings use it? Cost. It costs more to design and construct the seismically isolated building. Typically the only buildings using seismic isolation are essential facilities, like hospitals, or buildings that merit special property protection, such as museums or historic structures.

What structures in San Francisco are using seismic isolation? Prominent examples include the new de Young Museum in Golden Gate Park and two in the civic center, City Hall and the Asian Art Museum. The Golden Gate Bridge approach spans (but not the main suspension bridge) have been retrofitted with isolators. The suspension bridge itself has been retrofitted with other techniques. A display explaining the use of the isolators, with a cutaway view of one, can be found at the San Francisco end of the bridge, just west of the highway. They work by using a sandwich of layers of steel and a special rubber that deforms back and forth sideways in a "soft and squishy" manner.

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Base isolators underneath San Francisco’s Asian Art Museum.
Photo courtesy Forell/Elsesser Engineers Inc.

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This display explains that the ramps leading on and off the Golden Gate Bridge have been seismically engineered using base isolation.
Photo courtesy Golden Gate Bridge, Highway and Transportation District.






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Drawing from R. M. Stephen, J. P. Hollings, and J. G. Bouwkamp (1974), UC Berkeley report EERC 73-17. Used with permission.

What about the Pyramid Building, formerly called the Transamerica Building? It isn't seismically isolated. It is not mounted on ball bearings, as many believe. Instead, it is rigidly connected to a reinforced concrete mat or slab 9 feet thick located three basement levels under ground. It is anchored rigidly in the ground like a fence post. I don't know where the urban legend started that it is isolated, but it is a myth that is as durable as it is false. Another myth you'll hear a lot is that Frank Lloyd Wright's Imperial Hotel in Tokyo was seismically isolated.













Earthquake Myths

We asked Robert Reiterman to help us explore some common myths about earthquakes and seismic safety.

San Francisco sits on an active earthquake fault. False.

The San Andreas Fault, which was the source of the both the 1906 earthquake and the 1989 Loma Prieta Earthquake, does not pass through any portion of the City and County of San Francisco. The fault passes offshore to the west.

Distance from the epicenter determines how hard the ground shakes. False.

The epicenter is the point on Earth's surface below which the fault first started to rupture. In a large magnitude earthquake, the fault breaks along a considerable distance and vibrations are sent out from that rupturing length. In the 1906 earthquake, a segment of the San Andreas Fault 300 miles long ruptured, and strong ground motion occurred all along that length.

Seismologists measure the size of earthquakes on the Richter scale. False.

Most magnitude figures used today are measured on magnitude scales other than the one Charles Richter developed in 1935, for example on the surface wave magnitude scale or one called the moment magnitude scale. Most of the time when you read "Richter magnitude" in a newspaper or other non-technical source it is inaccurate.

The 1989 Loma Prieta Earthquake caused very severe ground shaking in San Francisco. False.

The peak ground acceleration, the peak "lurch" or "jolt" during the 1989 earthquake was about 1/4 g or less in San Francisco. Many earthquakes have generated several times more severe shaking, up to and over 1 g.

The Loma Prieta Earthquake was named for the fault that caused it. False.

Loma Prieta is a mountaintop close to the epicenter of the 1989 earthquake, which was caused by a slip on the San Andreas Fault. Loma Prieta is Spanish for dark hill. Seismologists name earthquakes for locations near their epicenters. For example, an earthquake offshore of northern California in 1980 ended up with the somewhat confusing name Offshore Trinidad Earthquake, because the little town of Trinidad, California was nearby.

Filled land is unstable during an earthquake. Not always.

It is generally true that harder, older geologic material makes better building sites than softer, younger material. The building code classifies soil from A (rock harder than is typically found in San Francisco or the Bay Area) through F (the softest, wettest soil). But in recent decades, geotechnical (soil) engineers have learned how soil can be trucked in and compacted to form solid manmade ground. It is also possible to use piles, which are like long concrete posts, embedded deep in the ground to provide adequate support on soft soils.

Animal behavior can foreshadow an earthquake. False.

Studies have shown that there is no connection between earthquakes and animal behavior.

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