Episode 1 - Can My Wood Home Survive An Earthquake?
Wood framed houses and earthquakes
Our hosts, Elide and Andrew, talk with Kelly Cobeen, a renowned expert in wood construction.
They explore cripple walls and common deficiencies with cripple wall bracing and the connection of the house to the foundation, which are some of the most common, and potentially dangerous, problems with this type of construction.
Cripple walls are short, wood framed walls in a crawl space that support the first floor of a house. They are very common in wood framed houses, especially older ones. Hints that your house has a crawl space with cripple walls are the presence of an elevated porch, or vents and an access hatch around the perimeter of the house (Figure 1).
If your house has unsheathed cripple walls (Figure 2) - meaning that the wood studs are not nailed to a layer of plywood - the house can be severely damaged during an earthquake. Typically, the house “falls off” the foundation because the cripple wall racks sideways and collapses because there is not enough stiffness and strength in the cripple walls (Figure 3).
To address this issue, plywood or OSB (oriented strand board) sheathing can be nailed to the cripple wall studs. The sheathing braces the wall and prevents it from racking. In older houses, the exterior cripple walls may be weakly braced from siding or board sheathing. While these sheathing types are better than nothing, the use of plywood or OSB sheathing is a great improvement. In conjunction with proper floor attachments and foundation bolting, discussed next, this relatively simple retrofit can save your entire house!
The second issue Kelly discusses is foundation bolting. In older houses, the base of the wall (the “sill plate”) was generally not positively attached to the foundation (refer back to Figure 2 to see that there is no bolting between the wood and concrete foundation, and compare that to Figure 5).
The retrofit for this issue consists of adding post-installed anchors between the sill plate and the foundation.
You can find out if your house has this issue by checking for the presence of steel anchors within the sill plate. Without anchors, the house could slide off the foundation during an earthquake as seen in Figure 6!
Kelly mentions that addressing these common issues is relatively easy and cheap. In California, there is also a grant to help offset the costs for homeowners called the earthquake brace and bolt program.
For all the current and aspiring homeowners out there, the Deconstructed team has prepared the following resources with more in-depth information about the topics we discussed in this episode:
3) FEMA and NEHRP’s “Simplified seismic assessment of detached, single-family, wood-frame dwellings”
4) FEMA and NEHRP’s “Seismic retrofit guidelines for detached, single-family,wood-frame dwellings”