Stanford, Calif. — Stanford engineers have built and tested an earthquake-resistant house that stayed staunchly upright even as it shook at three times the intensity of the destructive 1989 Loma Prieta temblor 25 years ago.
The engineers outfitted their scaled-down, boxy two-story house with sliding "isolators" so it skated along the trembling ground instead of collapsing. They also included extra-strength walls, to create a home that might replace the need for residential earthquake insurance, said project leader Gregory Deierlein, Stanford's John A. Blume Professor in the School of Engineering.
The modifications are inexpensive and could be incorporated into new homes as soon as designers and contractors decide to try them, according to the researchers.
"We want a house that is damage free after the big earthquake," said Eduardo Miranda, an associate professor of civil and environmental engineering. He co-led the project with Deierlein and Benjamin Fell, an associate professor in the Department of Civil Engineering at California State University, Sacramento.
Residential homes already do a good job of keeping the people inside safe when a temblor hits. But earthquakes typically do a lot of minor structural damage. For example, after the 1994 Northridge quake, the majority of the $25.6 billion in repair costs paid for fixes to 500,000 residential structures.
Most of those homes were not destroyed, but nonetheless thousands of families had to find a new place to live while their houses were repaired. Even if the walls stay up in a quake, wall finishes like drywall and stucco, along with architectural fixtures like cabinetry, are damaged because of the large sideways movements caused by earthquakes, Deierlein said.
The house that Stanford built had two major modifications to stave off earthquake damage. For one, it was not affixed into a foundation, but rested on a dozen steel-and-plastic sliders, each about 4.5 inches in diameter. Under those sliders were either plates or bowl-shaped dishes made of galvanized steel. These units are called seismic isolators.
"The idea of seismic isolation is to isolate the house from the vibration of the ground," Miranda said. "When the ground is moving, the house will just slide." Seismic isolators already protect large structures like San Francisco City Hall and structures at San Francisco International Airport, Deierlein said, but they are quite expensive. He and his team adapted the technology for residential use by incorporating inexpensive materials into their scaled-down isolators.
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.
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. However, he said, 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."
— By Amber Dance, a freelance science writer in the Los Angeles area.