TAHOE/TRUCKEE — During an earthquake, the strength of a house is put to the test. Ground forces try to tear it apart and in many cases, cause severe damage. If you line in an area prone to earthquakes, it’s important that the structural frame of your home is strong enough to absorb an earthquake’s energy.
Fortunately, advances in structural engineering, lessons learned form past earthquakes and research performed by the scientific community have paved the way for the development of new construction techniques that better equip houses to withstand an earthquake. Local and regional building codes now require all new houses to meet these higher construction and safety standards.
Since many existing houses were constructed prior to the development of these newer code requirements, a retrofit may be necessary to increase structural safety. A retrofit adds bracing and reinforcement to strengthen the critical connections within the structure. A house that has been retrofitted is able to resist much greater earthquake forces and has a lower risk of being damaged.
Upgrading a house to the latest building code standards, however, is not always practical. For older homes this type of upgrade would require the homeowner to hire a licensed structural engineer and a specialty contractor to preform a detailed structural evaluation. This evaluation would include the removal of many finish materials, such as sheetrock, stucco, siding, etc., to examine the construction of the house.
Fortunately, there are less intrusive ways to retrofit older homes and improve their structural safety and performance during an earthquake.
Before you begin a retrofit projects, it’s important that you are familiar with the basic principles of residential construction and how earthquakes impact your home.
Earthquake Basics: Lateral and Uplift Forces
During an earthquake, a house experiences two types of forces: lateral forces and uplift forces. Lateral (or shear) forces are 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 foundations. Uplift forces are vertical forces that result in up and down movement. Uplift forces can cause the house to overturn and lift off the foundation.
Earthquake Basics: Continuous Load Path
Building codes new require houses to meet higher structural design standards, including the use of a “continuous load path.” This method of construction involves creating a series of sold connections throughout the structure. These connections are created by using a system of wood framing, metal connectors, fasteners (like nails and screws) and shearwalls.
These connections are critical during an earthquake. A continuous load path redistributes external forces from an earthquake by transferring these forces from the frame of the house to the foundation. A home is more likely to withstand an earthquake and stay intact when each part of the house is connected together.
In general, houses built within the last 25 years should have been constructed with a continuous load path. To confirm this was done in your home, you can hire a professional contractor or qualified design professional to preform a structural evaluation of your house. When hiring a design professional, you want to make sure they are licensed to practice in your state and have a good reputation.
Do I Need to Retrofit My Home?
There are certain types of houses that are more likely to need a retrofit than others. A quick analysis of your home can help determine if it fits the criteria.
1. Am I living in an area prone to earthquakes?
2. Was my home built within the last 25 years?
3. Is my home built on a raised foundation?
4. Is my home built on a hillside?
5. Is there living space above my garage?
Conduct Your Own Survey
Once you have identified that there is a potential need to retrofit your home, you should then conduct your own inspection and evaluate its structural integrity. Hiring a professional to help with this evaluation is recommended, but not required.
Since retrofits are not mandatory, building departments typically do not require the use of an engineer, architect or a licensed contractor as long as the house falls within certain prescriptive parameters (Note: you should contact your building department to make sure there are no special requirements).
The building code does provide construction details fro retrofitting the supporting perimeter walls of the crawl space, and with the right tools, a motivated homeowner can do this themselves.
— Information for this article was taken from, Simpson Strong Tie’s “Seismic Retrofit Guide, Improve Your Home’s Earthquake Resistance.”