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In and Out of Tune with Earthquakes

May 08, 1988|RUTH RYON | Times Staff Writer

The Big One--L.A.'s predicted killer quake--still may be far enough in the future to take some design and construction steps to minimize damage, say architect T. Scott Mac Gillivray and structural engineer William J. Correia.

They talked in Mac Gillivray's Hancock Park office about harmonics and liquefaction.

"You don't hear these words often," Mac Gillivray said, "because they're fairly new concepts."

Liquefaction, when the shock wave from an earthquake liquefies the soil, is still not addressed in standard building codes, and harmonics first appeared in the codes after the Sylmar earthquake in 1971.

Harmonics? "Sure," Mac Gillivray explained. "A building is like a tuning fork. When you put two tuning forks next to each other and strike one, the other will start ringing. The same happens in an earthquake. If a building is harmonically in tune with the earthquake, the building will shake more."

The principle was the same in the 1940 collapse of the Tacoma, Wash., Narrows Bridge, he said, although an earthquake was not involved in that case.

"During a storm, the wind struck such a perfect chord with the bridge that the bridge was turned into an undulating sea serpent and broke up. The forces of the wind alone did not destroy the bridge.

"The compatible natural harmonics of the wind and bridge magnified the force of the wind to previously unpredicted levels."

Fortunately, he said, a building can be "tuned like a piano" so the building's resonant frequency is not a natural harmonic of the predicted earthquake wave.

"This isn't for a 1- or 2-story house," Correia, a structural engineer with offices in Huntington Beach, noted. Because houses are not as likely to suffer from the multiplying effects of harmonic action, they are not required by law to be tested for harmonics.

Most home builders wouldn't want to go to the expense and trouble, anyway, he said.

To test for harmonics, Mac Gillivray explained, "60- to 100- foot borings are done at the building site, and sonic waves are bounced off the earth layers to determine their depths and densities.

"Then the region around the site is scrutinized for probable earthquakes, type of wave action anticipated and the effect that distance from the building site will have. The results of this analysis set up a range of possible wave actions that can be compared to the building's size, configuration, mass, slenderness and rigidity."

The harmonics or frequency of the building can be changed, he said, by increasing the rigidity of the structure. "The building can be tuned by increasing the mass of the supports, much like a heavier piano wire will vibrate at a different pitch than a thin one of equal length and stiffness."

It's a complicated process--from determining the harmonics to fixing the building so that it is out of sync with a potential earthquake. And it's expensive. The soil report alone for the planned Los Angeles Mission, which Mac Gillivray designed to replace the mission's existing Skid Row facility, cost $16,000.

The cost is worth it, said Correia, "for major buildings with possible larger loss of life."

Mac Gillvray said he is designing the Ren-Mar studios in Hollywood to be more earthquake safe by installing what is known as seismic separation joints to connect the various sized buildings "so they can move back and forth without banging each other apart."

Like Oil Tankers

Without the separation joints, the buildings are like an oil tanker--"a big, long thing, subject to several waves, peaks and valleys, and the ship could break up," he explained.

Houses are less likely to fall down or apart, he said, but they could be subject to liquefaction, which he likened to a thick milkshake.

"Pretend the house is a spoon in the shake," he said. "The shock from an earthquake will make the spoon drop, but it won't sink completely. The fireplace might drop 6 inches, and the kitchen might drop 3 inches, so the house is crooked, and when that happens, you consider the building a tear-down."

Neither he nor Correia has heard of any cases of liquefaction in California, but they are possible "wherever there is sandy soil with a high water content," Correia said. There have been a number of problems with liquefaction in Japan and Chile, where there have been more earthquakes to study.

Learning from Quakes

With each earthquake comes new knowledge. After the 1933 Long Beach earthquake, Californians learned to anchor floors to walls "so neither will cave in," Mac Gillivray said. The state's first seismic building code section was written in 1934, and it was re- written in 1965.

The 1933 earthquake also pointed to the need to reinforce masonry buildings, while the 1971 quake showed what a mistake it is to build long-span concrete bridges and overpasses.

Anyone wondering about building a house on a sandy lot might seek a preliminary geological review if not a full soil report, Mac Gillivray suggested. A soils engineer or a geologist could give what Correia described as "a close answer without the cost of a report," for $100 to $400.

Checking for Landslides

This is also a good idea for checking on landslide possibilities, whether for a vacant lot or an existing house, he added. "Or you could get a soils engineer to bore down a bit if it's an iffy slide area, for $1,500 to $2,000." Another idea: Demand a soils report as a condition of sale.

But if the land under an existing house is subject to liquefaction, don't buy, he urged.

"We're talking about a science in transition," Mac Gillivray added, "but we're coming out of the Stone Age and maybe even the Bronze Age." He laughed.

"We're probably in the Steel Age."

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