Engineers build, test earthquake-resistant house

Second, the engineers developed what they call a “unibody” design, a term borrowed from the automobile industry, in which every element of the structure contributes to its strength. Instead of simply screwing drywall to the wood framing, as in typical construction, they used glue to affix extra-thick, 5/8-inch drywall more securely. On the outside, they used strong mesh and additional screws to attach the white stucco tightly. These elements made the house stiffer and stronger, leading to a significantly better seismic performance.

How do you test an earthquake-resistant house? It takes a big earthquake simulator called a shake table. Deierlein and colleagues constructed their 36-by-22-foot three-bedroom home atop the biggest such platform in the country, the Large High Performance Outdoor Shake Table at the University of California, San Diego. The facility uses computer-controlled hydraulic pistons to move the platform back and forth in a pattern selected by the engineers, so it can replicate specific earthquakes like Loma Prieta.

The table is part of the George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES), with sites across the United States funded by the National Science Foundation. The engineers tested partial versions of their design earlier at Stanford, California State University, Sacramento, and a NEES site at the University of California, Berkeley.

After a seven-week build, in September it was time to rumble the house. First, the engineers tested the isolators, the flat versions and the dish shape. The dishes are designed so that after the temblor ceases, the isolators’ pegs will settle back into the lowest point of the dish. That way, the house always winds up where it started. Although flat pads are easier to build, they also leave the house more vulnerable to migrating from its original location.

While it is difficult to put the simulations on the Richter scale, the engineers shook the table at three times the intensity of the ground shaking during Loma Prieta, which measured 6.9 magnitude. The house slid from left to right, but held together. “Under the isolators, the house basically saw no damage,” Deierlein said. Even in a strong quake like Northridge, a 6.7 on the Richter scale, isolators should protect a home, he said.

Next, the researchers bolted the house to the shake table, to test how well the unibody system held up without isolators. They had developed computer models to predict when the house would fall, but it outperformed their expectations.

“We are really seeing very little damage,” said Ezra Jampole, a doctoral candidate at Stanford whose T-shirt read, “I’m an earthquake engineer… If I run you run.” Under the triple-Loma Prieta conditions, a few cracks appeared in the stucco and drywall, and a swinging light in the garage shattered. The test window and steel door stayed put, as did the table and chair that furnished the test house.

Encouraged, the engineers cranked up the table to shake 50 percent faster, the maximum quake the table can simulate. That did it. The engineers whooped and clapped as the house sashayed from side to side. The window and door fell out and stucco sheared off. The house wound up listing to the side like the Tower of Pisa.

“It came really close to collapse,” Deierlein said. He said the engineers still have some work to do to figure out precisely how much shaking a unibody house can withstand before crumbling.

Want your own earthquake-resistant home? Though it should be possible to retrofit houses with these modifications, it would be simpler to incorporate them into a new construction, Deierlein said. He and his colleagues intentionally designed protective features that were not only effective, but also affordable. The unibody system, requiring some glue, mesh and screws, should add less than a few thousand dollars to the cost of building a building the size of the test house, and very little time to the construction process, Miranda said.

The release notes that Deierlein estimated that building a house on this type of seismic isolators would add about $10,000 to $15,000 to the total cost of a 1,500- to 2,000-square-foot house; and it would take contractors about four extra days to install them before building the home on top.

He said, however, that one-time cost is minimal compared to annual earthquake insurance with high deductibles. Californians paid an average premium of $676 in 2013, according to the California Department of Insurance, but the majority of homeowners don’t carry a policy at all.

Contractors could start incorporating these changes into new homes anytime, Deierlein said, though it will likely take a few pioneering engineers to add them to designs and work with building departments to incorporate them into existing building codes.

“We are always cautious never to talk about earthquake-proof,” he said, “but our resistance is getting better and better.”