House bolting, also known as earthquake retrofitting or seismic retrofitting, is the process of attaching a house to its concrete foundation using foundation plates and anchor bolts. The concept of how retrofitting works is simple. The basic idea is to keep the house from falling or moving off its foundation during an earthquake.
To keep a house from moving off its foundation in an earthquake, house bolting or seismic retrofitting strengthens three different areas of the house. These areas are all located in the crawl space, which is the space between the ground under the house and the floor you walk on. 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:
The bracing and reinforcement of the cripple walls with structural-grade plywood. (A cripple wall is a short wall built upon the foundation of a house that produces a high crawlspace. 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 quickly collapse in an earthquake.)
The bolting of the braced cripple walls to the foundation.
The attachment of the floor of the house to the braced cripple walls.
By doing these three actions, your susceptibility to earthquake damage is greatly lessened. It’s highly unlikely that your cripple wall will collapse bringing your house down with it, requiring expensive repairs.
California is "earthquake country." We had a 7.2 magnitude earthquake in northern Mexico in April, with lesser aftershocks hitting as far north as Los Angeles in the time since then. We also know that the next “big one” could happen at any time, and probably when we least expect it, leaving widespread destruction in its wake.
We who live in Los Angeles or other parts of California need to be aware that the up-and-down and side-to-side motion of an earthquake can shake an unbolted house off its foundation. If this happens to you, it could mean very expensive repairs to your home, as the house would have to be moved back onto the foundation and secured against future damage.
The best thing to do is get a foundation inspection, which will include looking at the house bolting and seeing if it is adequate and if any other reinforcements are needed in your home. The inspection is usually free and the cost of any house bolting and reinforcements will usually be a lot less than moving your house back onto its foundation if an earthquake causes it to slide off.