Recent shake-table tests on a wood-framed house with a newly developed low-cost structural system for seismic resistance confirmed researchers' predictions of drastically reduced earthquake damage. The goal of the study, considered a big step toward more quake-resilient single-family housing, is to "dramatically improve" the seismic performance of light-frame residential construction while keeping a lid on costs.
The system relies on base isolation under a strong and stiff unibody frame. "Seismic isolation has been cost- prohibitive for houses," says Gregory Deierlein, a professor of civil engineering at Stanford University and one of three principal investigators in the three-year research program. "The key is to make it inexpensive," says Deierlein, whose co-principal investigators are Stanford engineering professor Eduardo Miranda and California State University, Sacramento, professor Benjamin Fell.
Three series of tests validated the team's design methodology, which combines low-cost isolation to separate the structure from the ground with a stronger and stiffer superstructure.
"We are taking a separate route, which is contrary to conventional practice of providing ductility," says Deierlein. "Houses don't need that much ductility, provided they are stiffer and stronger."
The new approach exceeds building codes, which address only life safety. After the 1994 Northridge quake, insurance payouts for damaged homes hit about $20 billion, according to the researchers. In addition, more than 60,000 residents were displaced.
The study "represents an important step by demonstrating what is possible," says Rene Vignos, a principal with seismic structural engineer Forell/Elsesser Engineers Inc., which is an industry partner on the project and an adviser to the researchers. The work is significant because it is focused on ways of modifying current house design and construction techniques to reduce damage, instead of inventing a brand-new construction method that would have slower acceptance, he adds.
The $400,000 testing program—called Seismically Isolated Unibody Residential Buildings for Enhanced Life-Cycle Performance and conducted at the shake-table laboratory at the University of California, San Diego's Network for Earthquake Engineering Simulation—is part of a $1.3-million National Science Foundation project.
The first two series of tests shook a full-scale isolated structure, first with sliding dish-shaped isolators, then with sliding flat, hockey-puck-like isolators. Dish isolators recenter the structure; flat isolators don't. The final series of control tests, completed last month, shook the same house sitting on a fixed base, instead of a system of base isolators.
In all three cases, the tests "substantiated our predictions," says Deierlein.