How is your house constructed?

1. Crawlspace/Basement - 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.
2. 1st 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 or pony 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 domino over and collapse in an earthquake.
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 same structure after retrofitting.

• 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.
• Bearing Plates/Mudsill Plates - These are specially designed large 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 shear wall.
• Shear Ties or A35s - These are specially designed pieces of steel hardware that securely attach the floor framing to the cripple wall. Retrofitting is technical work. Done correctly, it increases the chances of your home surviving an earthquake. However, an improperly or incompletely retrofitted home might actually suffer more damage during a quake than if nothing had been done at all. Almost any contractor can perform a seismic retrofit, but why would you trust your home to anybody but a specialist?