Basic Terminology of Seismic Retrofit

This first illustration shows the basic structures under a house before it has been retrofitted.

1-Crawl space - The appropriately named space between the ground under the house and the floor you walk on. How much space you have to crawl in depends on how tall your cripple wall is. (see #5) The retrofit contractor does almost all of his work in this crawl space area.  To see a photo of a crawl space area, click here.

2-Floor - This is the area you walk on.

3-Foundation - This is the cement structure that supports the house and gives it stability. The primary goal of seismic retrofit is to attach the house to the foundation so it doesn´t fall off the foundation during an earthquake.

4-Mudsill - This is a piece of wood that rests on top of the concrete foundation. The entire house rests on it. When an earthquake hits, the mudsill can be jolted off the foundation causing the house to collapse.

5-Cripple wall - Not all houses have a cripple wall. If you go up a set of stairs to enter your home, you probably have one. Un-reinforced cripple walls are the weakest part of a house because they readily collapse in an earthquake. In the photograph at the top of this webpage, the cripple wall of the house collapsed. The entire house had to be jacked up, the cripple wall rebuilt, and the house placed back on top of it. To see a photo of a cripple wall, click here.

6-Floor framing - also called floor joists. In an earthquake, the floor framing can be jerked off the cripple wall, causing the floor of the house to collapse.

7-Top plate - This is made of 2x4 lumber lying on top of the cripple wall. The floor framing is nailed into this.

The next illustration shows the same structures as above but after the house has been retrofitted.

Bolts - These attach the mudsill to the foundation. There are several types, but basically they are long threaded steel rods that pass through the mudsill and deep into the foundation. Bolt strength is greatly affected by the type of washer that is used.

Mudsill plates - These are specially designed square washers that increase the strength of a bolt by 60%. Newer homes can be significantly strengthened by merely changing the traditional washers to mudsill plates.

Shear wall - This protects and strengthens the cripple wall. Basically, it is a sheet of plywood nailed onto the wood framing of the cripple wall to prevent it from collapsing in an earthquake. Specifics like the type of nail used, nail spacing, type of plywood used, the spacing of the framing and many other factors effect the strength of a shearwall.

Shear transfer ties - These are specially designed pieces of steel hardware that securely attach the floor framing to the cripple wall.

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Basic Concepts of How Seismic Retrofit Works    

To keep a house from falling off its foundation in an earthquake, seismic retrofit strengthens three different areas of the house. These areas are all located in the crawl space as defined in the two terminology illustrations above. If any one of these three areas is not adequately retrofitted, the house will be susceptible to damage in an earthquake.

These three areas are:

1) The bracing of the cripple walls with plywood.
2) The bolting of the braced cripple walls to the foundation.
3) The attachment of the floor of the house to the braced cripple walls.

 The following are simple illustrations to clarify these areas of retrofit.

Bracing the Cripple Walls with Plywood

Figure 1: Damage to a house due to lack of cripple wall bracing

Figure 1 shows what can happen to a house if it is not properly retrofitted with plywood on the cripple walls. Most people are concerned about bolting their house but history has shown that unbraced cripple walls are the first area to fail in earthquakes, often leaving the residents homeless. If a house is bolted to the foundation, but the cripple walls are not braced, the cripple walls can still easily collapse. Bolting a house to the foundation without bracing the cripple walls offers very little protection.
 

Bolting the Braced Cripple Walls to the Foundation

Figure 2: Damage to a house due to lack of foundation bolts.

The base of the house at the mudsill should be bolted to the foundation for obvious reasons. Without foundation bolts, a house can slide off of the foundation even if its cripple walls are braced with plywood shear panels.  Notice in Fig. 2 that plywood is only on the corners of the cripple wall.  It is not necessary to put plywood on the entire cripple wall.
 

Attaching the Floor of the House to the Braced Cripple Walls

Figure 3: Damage to a house due to no connection of floor to cripple wall.

As figure 3 shows, the floor of a house can slide off the top of the cripple wall. In Fig. 3 the cripple wall is braced with plywood to prevent collapse and it is also bolted to the foundation.  However, since the floor of the house is not attached to the top of the cripple wall, the house can slide off the cripple wall during an earthquake.

For more detailed and in-depth information about seismic retrofit, see the Retrofit Design section of our website.

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