Protecting your home: Build shear walls to strengthen your foundation
If you live in an older post-and-pier foundation, your house may be vulnerable to severe damage from a large earthquake. No matter how strong your house is, if your foundation is weak, a large earthquake may be able to lift the house off its piers, or rack it back and forth.
In this photo by the U.S. Geological Survey, the house was completely knocked off its piers in the 1983 earthquake, landing two feet away.
This photo (Jim Griggs, USGS)shows a house in Kalapana that has been sheared, or racked from side to side, during the June 27, 1989 earthquake.
In the 1999 Pahala earthquake, this house fell off its foundation and suffered irreparable damage.
To prevent the kind of damage pictured above, you can build a series of shear walls to strengthen your foundation. Below right, three photographs illustrate the basic steps in retrofitting an older house foundation; videos at the bottom of this page provide details. Visit our materials section for a set of drawings.
Step 1: Setting the forms.
Step 1. Lattice is removed from the area to be retrofitted, and forms are set between existing piers. Note that the soil has been removed to a depth of one foot (less if you hit solid rock) and rebar installed prior to the pour; anchor bolts (not shown) will be set in the concrete.
Step 2: Concrete is poured, sill and cripple studs set.
Step 2. After the cement has hardened, a sill is anchor-bolted into place, and then cripple studs are toe-nailed into the sill below and existing girder above. Your house now cannot jump up off its piers in an earthquake, for it is tied down to a heavy mass of concrete.
Step 3: Plywood is nailed across the sill and studs.
Step 3. The final step is to nail a sheet of ½" plywood across the posts and cripple studs. This will now prevent your house from shearing or racking during an earthquake, much as the backing of a bookcase prevents it from shifting sideways. A coat of paint will complete the job.
Shear walls don't need to be built around the entire perimeter of your house. It is recommended that you build six walls altogether: one wall on each end of your house (8' long) and two walls on each side of your house (6' long) for a total of six walls. A project like this takes a little time and money, but is nothing compared to the cost of rebuilding your house if it shears in an earthquake.
Photos below show highlights from the 8-minute video.
A dog house illustrates the instability of posts.
Architect John Parazette describes shear walls.
It's easy to install shear walls..