Answer Geek: Earthquake-Safe Homes
-- Q U E S T I O N: I survived the Seattle earthquake! Now there's a lot of talk about earthquake retrofitting. I know it essentially has to do with making buildings more earthquake-proof, but if I want to retrofit my home, what is involved?
— Ian C.
A N S W E R: After spending the last month looking at the mysteries of the power grid and the technologies behind various sorts of alternative energy sources, it's a bit of a relief to take a break and talk about another topic. Never fear, though; I still have a couple of good power-related questions up my sleeve, and we'll get to some of them in the coming weeks.
Well, Ian, congratulations for coming through the Feb. 28 earthquake intact. Fortunately, and for reasons that are still something of a mystery, everyone survived here. It seems to have been a big surprise to everyone who studies earthquakes that the 6.8-magnitude event did not wreak more destruction than it did.
Enough to Budge a House
Your basic assumption about retrofitting is correct: It is an attempt to re-engineer an existing structure to help it withstand the rocking of a powerful quake. How it is accomplished depends on the type of structure being retrofitted. But since you asked, we'll confine ourselves to houses, which can take a fearful beating.
During the 1994 Northridge earthquake in California, for example, nearly 50,000 homes suffered significant damage. That quake measured 6.7. Damage from the recent quake in Washington state was much lighter, but it's pretty easy to spot toppled chimneys and other evidence that an earthquake can do some serious damage to a house.
The problem: If the ground shakes with enough force and violence, the back-and-forth motion just might nudge a house off its foundation, causing major damage or even complete collapse. This is mostly a problem for older homes where the superstructure of the house isn't connected securely enough to the foundation.
The typical solution: Bolt that baby to the foundation. Normally this is done by drilling a hole through what is called the sill plate — the board that sits directly on the foundation — into the concrete foundation. The generally accepted standard is that the bolt must extend at least four inches into the foundation. Two kinds of bolts are used: expansion bolts, which expand inside the hole, and something called chemical anchors, which use adhesive materials like epoxy or polyurethane to get a good hold.
Pony Walls Are Most Fragile
A second structural problem that makes many homes vulnerable to earthquake damage is the pony wall, the short stretch of wall that rises from the foundation to meet the bottom of the first floor. Typically constructed from vertical boards with exterior sheeting in the form of horizontal wooden siding, pony walls in older houses don't usually have enough lateral bracing to survive a good shake.
To deal with that, the recommendation is that structural panels — plywood or particle board at least 15/32nds of an inch thick — be nailed to the studs supporting the pony wall, forming a complete sheer wall between the foundation and the main floor.
As I mentioned before, this is mostly an issue for older homes. In many areas, recent building codes include more stringent requirements that are designed to ensure that your house won't shake off the foundations in a big quake. In Seattle, for example, houses built after 1975 are required to have bolts, although strengthened pony walls did not become a requirement until as late as 1985.
How effective is earthquake retrofitting? One study estimates that if an earthquake measuring 7.3 strikes San Francisco, 14 percent to 20 percent of homes that are 60 years or older will suffer enough damage to be uninhabitable. Add bolts and structural panels and those percentages drop to 2 percent or less.
Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.