House Passes Quake Test, but Furnishings Fly

SAN DIEGO — In the name of engineering, researchers did their destructive best Tuesday to simulate what happened in 1994 when the earth shook much of Southern California, killing 60 people and causing $40 billion in damage.

At the end of the unprecedented “shake test,” researchers confirmed their hypothesis that the modern home is a safe place, structurally, even in violent earthquakes--but that furnishings inside can be killers when they start tumbling and toppling.

“The minimal structural damage is the good news,” said Frieder Seible, professor of engineering at UC San Diego. “But things that aren’t tied down go flying everywhere.”

For 20 brutal seconds, a two-story wood-frame structure built in a special laboratory at the UC San Diego engineering department was jolted on a moving platform with the force of the 6.7 temblor that struck with its epicenter near Northridge.

With the exception of a few minor cracks in the exterior, the 600-square-foot structure, built to 1997 standards, was unscathed.

While researchers said the tests should provide a good night’s sleep for many residents worried about how their homes will withstand the next quake, they also noted that older homes, particularly those built before seismic standards were toughened, might not fare as well.

In addition, construction is not the only factor that determines earthquake damage. Where a house is built can be as important or more important than how it is built, experts note. Hillside houses were particularly hard hit in the Northridge quake, for example, and future research will consider such geographic factors.

If there was a surprise, it was that the test home’s stucco exterior seemed to absorb some of the mock quake’s energy and keep it from reaching the plywood and drywall within, helping to minimize damage.

“That’s good for most homeowners, since that’s what they have on the front of their homes,” said John Hall, an engineering professor from Caltech.

Inside the home, effects of the quake were more evident: A metal cabinet and an unstrapped water heater toppled to the floor. Framed pictures, books, trophies, two televisions and a stereo system crashed loudly. Flower pots were smashed.

Chandeliers swayed and jumped up and down. Dishes, wineglasses and assorted bric-a-brac were tossed around like ice cubes in a cocktail shaker. A stereo speaker crashed down onto a child-size bed, next to a teddy bear.

The test was conducted on a 4.8-ton platform known as a shake table, one of several at UC San Diego where bridge and freeway designs are also tested. A computer simulated the horizontal ground motions of the Northridge quake as recorded at Canoga Park, about 10 miles from the epicenter.

About 300 sensors and six cameras were placed inside the test structure to see how individual parts reacted to the stress of being shaken violently.

Although there have been other tests under the $7-million project funded by the federal and state government in the wake of the Northridge quake, this was the first with the test structure fully furnished and the stucco exterior applied.

Engineers have experimented extensively with high-rises, freeway pylons, bridges and other steel and concrete construction, said Robert Reitherman, executive director of the California Universities for Research in Earthquake Engineering, the academic coalition that is conducting the research.

But wood-frame structures, which are generally more flexible and survive earthquakes better than concrete or steel structures, have been largely ignored in studies of seismic damage--even though most structures in the United States are of wood construction, including 99% of California homes.

The tests are meant to assist in improving building codes and standards, making insurance ratings and loss estimates more accurate, and possibly assisting in the development of standards for retrofitting older structures.

Few California homes are as well built as the model tested Tuesday, Reitherman noted. Most homes built before World War II are not bolted to their foundations, and only the most recent construction has the kind of internal bolting of studs that can help homes withstand a strong quake.

Most of the advice dispensed about how to retrofit homes--from the efficacy of bolting to the right way to strap down a water heater--was based on anecdotal evidence collected by insurance adjusters and engineers who witnessed the aftermath of various quakes.

“It wasn’t until Northridge, when we saw residential [insurance] losses 10 times what was expected,” that engineers and insurers began to realize how little was known about earth movements’ effect on homes, said Mark Leonard of the California Earthquake Authority, the state-run earthquake insurance pool that also contributed to the study.

Among other issues raised in Tuesday’s test is the question of how many homeowners have carried through on plans to make sure items like water heaters and bookcases are anchored--the kind of plans made after an earthquake and often forgotten.

Seible joked that he plans to scour his own house to look for items not already bolted down or restrained. “I will make a weekend project out of it,” he said. Leonard said his agency will use the study’s data to help develop better standards for evaluating and fixing earthquake damage. For example, the agency wants to develop rules to help adjusters determine when a stucco crack is purely cosmetic and when it might indicate deeper structural damage.

The next phase of the research will be to simulate what happens when a Northridge-magnitude quake hits a three-story building in which the ground floor is a parking facility. In the Northridge quake, 16 people were killed in the collapse of the Northridge Meadows apartment complex that fits that description.