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Earthquake retrofit begins in crawl space

September 3rd, 2008 9:22 PM by Ron Mastrodonato

Earthquake retrofit begins in crawl space
Tips on properly fastening cripple walls to mudsill, floor joists

September 03, 2008

By Bill and Kevin Burnett
Inman News

Last week, a reader asked our opinion about insulating his crawl space and doing a little earthquake retrofitting under his home. We recommended that he install batt insulation between the floor joists, retrofit the cripple walls with plywood sheathing to make a shear wall, and put down a plastic vapor barrier over the dirt in the crawlspace.

Last week's column addressed insulating the floor. Today's column sets out the reasons to reinforce the cripple walls and the method for doing so.

We've mentioned that we experienced the Loma Prieta earthquake in 1989. Fortunately, our Alameda, Calif., homes suffered no damage. We'd just put a new foundation under Bill's house, and Kevin's "Old Vic" came equipped with a concrete foundation. But Loma Prieta made its impression and convinced Kevin that shear panels under his house were the way to go.

Cripple walls are short wood-framed walls extending from the mudsill resting on the top of the perimeter-raised concrete foundation. Floor joists rest on the top plates of the cripple walls, creating a crawl space. On a flat lot, cripple walls are usually 2 to 3 feet high. On a sloped lot, cripple walls are stepped down, following the slope of the lot. The top plate of the progressively higher walls forms a level platform upon which the floor joists rest.

Unreinforced cripple walls are a weak spot in a structure with a raised concrete foundation. They offer minimal resistance to the side-to-side forces of an earthquake and can buckle. Properly installed plywood sheathing on the cripple walls provides protection against these lateral forces. The side-to-side force is also called "racking" or "shear."

A word of caution is in order here: Properly installed shear panels on cripple walls help stabilize the structure during a seismic event, but they are only a partial fix. Floor levels above the crawl space remain unaddressed.

To explore a full earthquake retrofit, you should consult a structural engineer and/or a company that specializes in earthquake retrofitting. Also, before beginning this job, check with your local building department to see if permits and inspections are required.

Installing shear panels is more than just nailing some plywood to the studs. The goal is to make the foundation, the mudsill, the cripple wall and the floor joists into a monolithic unit by fastening all of the components together. This is accomplished with specialized metal fasteners and a skin of plywood or oriented strand board (OSB).

Just as the hip bone is connected to the thigh bone, etc., the mudsill must be bolted to the raised concrete foundation; cripple studs must be nailed to the mudsill; and floor joists must be secured to the top plate of the cripple wall. The whole is greater than the sum of its parts and provides significant structural defense against earthquakes.

Specialized fasteners are readily and economically available. Simpson Strong-Tie has been providing these connectors to the building industry for as long as we can remember. Simpson also provides a detailed Seismic Retrofit Guide at It's worth the look.

Your job is a bit easier because, as you report, the mudsill is already bolted to the foundation. Check the size, location and condition of the bolts. Current code requires foundation bolts to be 5/8 inches in diameter and located 12 inches from each corner of the house or break in the mudsill, and no more than 6 feet apart.

If you're short on bolts, you should add connectors between the foundation and the mudsill. Simpson makes a "Universal Foundation Plate" to connect the stem wall of the raised concrete foundation to the mudsill. You must rent a hammer drill equipped with a carbide drill bit to drill the holes in the concrete foundation to receive the anchor bolts.

Hopefully, the mudsill is the same width as the studs of the cripple wall. If it's not, no worries. It's just a bit more work. If, for example, a 2-by-4 cripple wall sits on a 2-by-6 mudsill, nail 2-by-4-inch blocking on top of the mudsill between the cripple studs. This allows the shear panel to be nailed on all four edges. Use four ten-penny (10d) nails to secure the blocking to the mudsill. You'll need to block only the stud bays that will be covered by sheathing.

The next step is to secure the floor joists to the top plate of the cripple wall. A 90-degree metal strap (A35 Shear Angle) is available, again through Simpson Strong-Tie, for this purpose. The brackets are nailed to the rim joist all around the perimeter of the house. It's pretty close quarters getting these brackets nailed in place, so we'd suggest you rent a compressor and a palm nailer to make short work of the job and to save your fingers.

With the mudsill secured to the foundation and the cripple wall secured to the rim joists, install the plywood sheathing. Use 1/2-inch plywood or save yourself a buck or two and use OSB.

For a one-story home, 50 percent of the length of each cripple wall should be sheathed. If the home is two stories, 75 percent of the length of each cripple wall should be sheathed. Gaps in the panel are allowed to accommodate pipes and heating ducts. The length of each section must be twice the height of the cripple wall. A little longer is OK; a little shorter is not.

In the case of 3-foot-high cripple walls, each section must be a minimum of 6 feet long. There must be a 6-foot section of sheathing at each corner. Runs of sheathing can be no more than 25 feet apart from center to center.

The edges of the shear panels should be nailed to the top plate of the cripple wall, the mudsill or block, and to the studs with 8d nails 4 inches on center. In addition, nail the panels to the studs in the field every 12 inches. Make sure the edge of each panel begins on a stud and ends on a stud so the panels are fully nailed.

Once the panels are nailed off, use a hole saw attached to a drill motor to drill a 3-inch diameter hole top and bottom through the sheathing at each stud bay for ventilation.

There you have it. It's a lot of work, but well worth the effort. And again, kudos to you for undertaking these projects.


Posted in:General
Posted by Ron Mastrodonato on September 3rd, 2008 9:22 PM


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