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‘Damped Sway’ Alleviates Damage

Hills Publications

Architect Randolph Langenbach, builder David Fowler and engineer Gene St. Onge recently revealed a low-cost foundation system that they say will greatly limit damage caused in a serious earthquake.

“The ‘damped sway foundation system’ uses steel screw piles and viscous fluid dampers to absorb a temblor’s destructive energy.”

The “Damped Sway Foundation System” uses steel screw piles and viscous fluid dampers to mitigate and absorb most of the destructive energy of an earthquake. This system is similar in many respects to the kind used in retrofitting large buildings but this is the first time it’s been given a residential application. As such it should add about $15,000 to the cost of a foundation and help a new home resist a 7.5 quake with minimal damage.

The site for the experimental project is on the property of preservationist Randolph Langenbach, on Chabot Road in the Oakland Hills. Langenbach’s house was destroyed in the fire of 1991.

“This kind of damper system allows the whole building to move four inches in any direction. At that point the dampers are engaged,” said St. Onge “The dampers work much like shock absorbers in a car, dissipating between 50 and 70 percent of the energy that would normally be transmitted up into the building”. The four-foot long dampers are connected to a steel frame, which sits on six steel anchor piles.

David Fowler of Pacific Housing Systems has successfully used such steel pier foundations in homes in Palo Alto and Danville. The steel piers are strong enough to handle the full load of the building, yet flexible enough to sway and engage the dampers. They are easily installed in all kinds of weather.

Though the Langenbach house has a simple square foundation, the system is not limited to such a shape. “Steel framing is stronger and allows more flexibility in design. It is also becoming more cost-competitive relative to timber framing,” said Fowler.

The designers haven’t yet finished testing the prototype: they plan to bring in the UC Berkeley Structural Laboratory’s “shaking table” to simulate the sorts of vibrations produced in an earthquake. Using the device, St. Onge and his collaborators will be able to more accurately gauge the system’s response and its effectiveness under dynamic conditions.

The designers think that this sort of damper system is a cost-effective and pragmatic choice for homeowners. “Your only options really are either make the whole building stronger, which only works with certain kinds of building materials, or use a different system – but there aren’t any, to my knowledge, that are as economical.” said St. Onge.

The three entrepreneurs also hope the insurance industry will sit up and take notice. The industry has seen a precipitous rise in costs and premiums in the wake of the Northridge earthquake. In the future a home with a damped sway foundation system may be a realistic prospect for earthquake insurance, thus potentially making such insurance once again affordable to homeowners.

– By Aidan Harty