In the remaining year of funding, the team expects to develop specific design recommendations, validated by the recent tests combined with reliability analyses, for the enhanced superstructure under both isolated and non-isolated conditions.

Related Link: NEESR: Seismically Isolated Unibody Residential Buildings for Enhanced Life-Cycle Performance

Research on the non-isolated house will include study of the extent to which the foundation system's interaction with the soil affects and likely improves performance, says Deierlein. For this, the team will conduct nonlinear computer analyses.

"We believe we will show that some flexibility and slight uplift between the structure and foundation will reduce the shaking severity during earthquakes," says Deierlein.

Each isolated test series included 15 shakes that mimicked the force of the code's maximum considered earthquake (MCE) or above. The house was essentially undamaged, says Deierlein.

The more economical isolators are made from less costly materials, including galvanized plates and high-density plastic. The materials are appropriate for lightweight framing because vertical bearing pressures are lower than on heavy frames.

The test house is framed using 2x4s and joists. Instead of the more common shear walls made of plywood or oriented strand board, the engineers created stiff and strong walls by attaching gypsum wallboard to the frame using structural adhesives and screws. All walls have vertical tie-downs to resist overturning forces.

Deierlein calls the results of the tests on the fixed-base structure "exciting." The house performed well under the MCE-level shaking. It survived the intense shaking with small lateral drift and very limited, reparable damage to the wallboard, he says.

The wallboard system is about two to three times stronger and stiffer than a house designed using conventional shear walls and standard wall finishes, say the researchers. It has both slightly higher collapse safety and would result in much less damage than conventional systems under moderate-to-large quakes.

In all, the isolation system is very effective at preventing damage under extreme ground motions that can occur in near-fault regions, conclude the researchers. The non-isolated system can work almost as well, with little damage in moderate seismic regions.

Under very extreme shaking, which is about three times more severe than the MCE, the fixed-base structure experienced severe damage that brought it almost to collapse.

"The benefits of the isolated system would be in regions where ground shaking is likely to be severe enough to exceed the strength of the fixed-base system," says Deierlein.

Retrofitting houses for base isolation might be expensive, says Deierlein. For new construction, however, he estimates the isolation might add about $15,000, or less than 4%, to the cost of a $400,000 townhouse.

Deierlein says the new approach will likely find its way into the codes in five years or so. Meanwhile, the team plans to "build experience and confidence" in the system by seeking design and construction firms to implement it.