Over the years we have learned the effect seismic activity has on a building and how to minimize the impact of a quake.
Earthquakes can cause a building to slide, shift off of its foundation and overturn. They can also buckle cripple walls, causing them to collapse.
House bolting is also called foundation bolting, anchor bolting, or seismic retrofitting. The purpose of house bolting is to make the structure safer and prevent damage during an earthquake. The bolts strengthen the wood frame by connecting it securely to the foundation. The two types of bolts commonly used in house bolting are epoxy foundation bolts and expansion foundation bolts.
If you live in an older home, an epoxy-set foundation bolt will be your best option. Older homes tend to have weaker concrete. Epoxy bolts are longer than expansion foundation bolts, which means they must be inserted deeper into the concrete. The deeper insertion improves the bolt’s performance.
Epoxy bolts are also a good choice in locations where the motion of a quake can lift up the structure. Because of the length of the bold, the chances of the wood frame pulling away from the concrete foundation is reduced.
Expansion foundation bolts are best used with foundation concrete that is in good shape, such as concrete in a new home or a new foundation in an older home. This is because the bolt requires “strong concrete” to be effective. The bolts can be used as long as they don’t crack the concrete.
Anchor bolts need to be installed with square plate washers. The washers create a sturdier and more secure connection to the foundation. Before square plate washers came into use, the bolts were secured with round washers. However, round washers did not have enough holding strength.
Sometimes other types of anchors may be necessary, depending on whether or not the home has cripple walls. Homes can also have a combination of cripple walls and places where the floor rests on the foundation.
To find out if your house needs bolting, go down into your home’s crawl space. The crawl space is the area between the foundation and the first floor. Find the sill plate, which is the board located on top of the foundation. Most but not all sill plates are made of lumber. The board runs horizontal to the vertical wall.
Inspect the sill plate for the head of anchor bolts. In most cases they will appear every four to six feet.
Many homes built during and after the late 1930’s will have some form of house bolting (Los Angeles and other earthquake-prone cities especially). Any work that gets done to strengthen the structure of your home needs to meet current standards. A foundation repair expert or foundation contractor should be consulted to determine if any existing bolting needs to be replaced, and the extent of any new bolting that may be necessary.
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