You might not think you live in earthquake country, but at least 39 states are considered at moderate to very high earthquake risk. As you evaluate your home's ability to withstand an earthquake and prepare for a retrofit, knowing these simple steps will help to ensure your home is structurally sound and earthquake resistant.
Knowing how seismic forces affect your home will help you make your home safer and more secure. There are two ways an earthquake can affect your home:
- Lateral (or shear) forces
Horizontal forces that result in back and forth (side to side) movement, also known as racking. Lateral forces can shake the house and weaken its frame and cause it to slide off the foundation.
- Uplift forces
vertical forces that result in up and down movement. Uplift forces can cause the house to overturn and lift off the foundation.
A continuous load path that is. A continuous load path is a method of construction that ties your house together from the roof to the foundation using a system of framing materials, metal connectors, fasteners (like nails and screws) and reinforced walls called shearwalls. This system connects and strengthens the structural frame of the house. If your home is built with a continuous load path, it will be better equipped to resist the forces of an earthquake by transferring these forces from the frame of the house to the foundation.
Certain geographic areas are more prone to earthquakes than others; your house may be in a seismic zone and you may not know it. The best way to find out if you live in a seismic area is to call your local building department and ask if you live in a region of moderate or high seismic risk. You also can visit the U.S. Geological Survey website and review its seismic hazard maps at http://earthquake.usgs.gov/research/hazmaps/.
Many existing homes were constructed prior to new earthquake-resistant building code requirements. If your home was built prior to 1985, it's most likely in need of a seismic retrofit. A retrofit adds bracing and reinforcement to strengthen the critical connections within a home; bolting the home to its foundation is a key step. A home that has been retrofitted is able to resist greater earthquake forces and has a lower risk of being damaged.
Before starting a seismic retrofit project you'll need to inspect and evaluate the structural integrity of your home. Hiring a professional will help ensure the retrofit is done right. There are seismic improvements that you can do on your own to strengthen your home, however, some cases may require an engineer and contractor. Knowing what is needed from the beginning will set you on the right course to a safer, stronger home. Remember when hiring a retrofit specialist, be sure they are licensed and have a good reputation.