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Preparing Your Home For An Earthquake

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Exterior Wall to Foundation Connection (anchor bolts)
Preparing Your Home For An Earthquake
© Simpson Manufacturing Co. 2011
Your home’s exterior walls need to be connected to your foundation walls. Seems logical, right? Well even so, this practice was not common in home construction until the 1940’s. Fortunately, it's now a code requirement that exterior walls are bolted to the foundation walls to resist lateral movement, especially like those found in an earthquake.

The horizontal 2x4 at the bottom of the exterior wall that rests on the foundation wall is called the sill plate (or mud sill in some parts of the country). The sill plate must be bolted to the foundation wall using galvanized anchor bolts so the wall does not move or slide. Spacing of anchor bolts in the sill plate can be no more than 6 feet on center and be within 12 inches of any sill plate joints. You can find the sill plate or mud sill in your crawlspace or basement at the top of the foundation wall. On top of the sill plate you should see the anchor bolt head protruding through a metal washer and nut.

Check with your building department if you do not see anchor bolts at appropriate spacing in your home to assist in deciding upon the proper action or local requirements before deciding how to proceed. There are different building code requirements on bolt diameter, length and spacing for new construction versus retrofit construction (UBC vs. UCBC). The length of the anchor bolt is also dependent upon the thickness of the sill plate, holding values required and the strength of the concrete foundation wall, but will generally range between 3.5” and 6.25” of embedment into the concrete. It is best to check your local building codes, building official or with an engineer or architect to determine the size of anchor needed.

If you have a crawlspace in your home you may have a framed wall above the foundation, and below the first floor, called a cripple wall. Retrofit anchoring of the exterior wall to the foundation is done one of two ways, either by anchor bolts (cripple walls) or retrofit foundation plates (no cripple walls).

If you elect to retrofit the anchor bolt installation on a cripple wall, you will need to drill through the sill plate and into the top of the poured concrete foundation wall. Foundation anchor bolts come in an expansion type for use in stronger solid concrete and in an epoxy setting type for use in older weaker concrete or brick. You can improve the holding power of epoxy setting type anchors by using longer anchors installed deeper into the foundation. You can improve the holding power of either type of anchor up to almost 60% by using large 3” square washers.

Another method of anchoring the sill plate to the foundation is by using approved screw anchors such as the Titan HD made by Simpson. For a good overview of sill plate to foundation fastening see Bolting: Attachment Of The Mudsill To The Foundation.

If your cripple wall needs blocking, make sure to install that first, before installing the anchor bolts and washers (see Cripple Wall Shear Reinforcement later in this tutorial).

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