A seismic retrofit can help protect homes during earthquakes
May 6, 2011
By Warren Kagarise
Strengthening a residence through a home earthquake retrofit is as simple as ABC: anchor, brace and connect.
Most homes built in the past 30 years or so do not need a retrofit to hold steady in earthquakes, but older homes may need some foundation tune-ups. If the foundation is not secured to the rest of the structure, major damage can result from the ground shaking.
The earthquake in Japan — plus major temblors in New Zealand, Chile and Haiti in the past year — has renewed the focus on seismic safety at home.
“When the earth starts shaking sideways, the foundation moves with the earth,” Sound Seismic co-owner Leif Jackson said. “This big, massive object is not going to immediately move with the foundation. It’s going to kind of lag behind, and it’s going to lag behind when that foundation oscillates back in the opposite direction. So, the house and the foundation get out of synch, and it can get jolted off of the foundation.”
Though most homeowners can take some small steps to reduce earthquake risks, older homes make for the likeliest candidates for a seismic retrofit, due to the adoption of modern building codes from the mid-1970s onward.
Jackson and brother Erik started the Seattle-based company in 1999, before the 2001 Nisqually earthquake reminded people about the risk in the region. Sound Seismic conducts retrofits throughout Western Washington each year, including a handful in Issaquah.
In a seismic retrofit, contractors brace the cripple wall, a short stud wall extending from the top of the foundation upward to support the floor. Because the cripple wall is constructed to support weight from above, the side-to-side movement in earthquakes can cause the wall to topple. Sound Seismic and other retrofit contractors brace the cripple wall.
Now, back to the ABCs.
Contractors attach the floor of a house to the braced cripple wall, use plywood to brace the cripple wall and then bolt, or connect, the braced cripple wall to the foundation.
“Certainly, homes built in the ’50s or earlier, they won’t have plywood, they won’t have anchor bolts, they won’t have anything, so they are absolutely candidates for a retrofit,” Jackson said.
Disaster serves as stark reminder
The scenes of destruction from Japan — and the inherent seismic risk in the Pacific Northwest — caused calls to Sound Seismic to increase tenfold since the March 11 disaster.
The most common question from potential customers is about cost. The price tag varies based on the home, although the project is less expensive to conduct in crawlspaces (about to $4,000 to $6,000) and unfinished basements (about to $5,000 to $8,000) than finished basements (about $8,000 to $15,000).
In Newcastle and unincorporated King County, earthquake retrofits require building permits.
Chimneys also merit special attention during a seismic retrofit. Many chimneys failed in the 2001 Nisqually earthquake, though numerous collapses left a mess rather than destruction.
“Most of them, fortunately, fell away from houses and landed in the yards instead of on and going through the roof,” Jackson said.
Homebuilders in the Pacific Northwest also use materials designed to ride out tremors.
“Our wood-framed houses are probably the No. 1 best-performing structure in earthquakes,” Jackson said. “The wood has a lot of give. It will bend before it breaks, and all those nails and all that wood fiber just gives it some flexibility. What that means is, your house is probably not going to come down in a heap.”
The seismic risk in East King County is real. Newcastle rests near the Seattle Fault, a shallow seismic zone stretched along the interstate from Puget Sound and east through lakes Washington and Sammamish.
Many seismic-retrofit practices and techniques come from earthquake-prone California.
“Everything we do — all of the methods that we use, and the science and engineering — we borrowed from building codes and requirements in California, because they’ve been kind enough to be guinea pigs for us,” Jackson said. “They’ve done the work and they’ve had it tested, and they’ve figured out how to improve it and had that tested. We just get to reap all of the rewards.”
Seismic safety tips
Inside a home, many injuries from earthquakes result from people running around as the ground shakes. They fall down, run into furniture, step on broken glass or take hits from falling objects. In and near older buildings especially, a much higher likelihood of broken windows, falling bricks and other dangerous debris exists.
The local American Red Cross chapter offers simple tips for residents to prepare for earthquakes:
- Bolt and brace water heaters and gas appliances to wall studs. Bolt bookcases, china cabinets and other tall pieces of furniture to wall studs. Brace overhead light fixtures.
- Hang heavy items, such as pictures and mirrors, away from beds, couches and anywhere people sleep or sit.
- Install strong latches or bolts on cabinets. Place large or heavy items in cabinets closest to the floor.
- Learn how to shut off the gas valves in your home, and keep a wrench handy for use.
- Learn about your area’s seismic building standards and land-use codes before you start construction.
What to know
Newcastle homeowners interested in a seismic retrofit can call the city Community Development department at 649-4444. Homeowners in unincorporated King County can call the Department of Development and Environmental Services at 206-296-6600.