Earthquake retrofitting your home.
Since the most recent earthquake in Napa on August 24th I have had a lot of request for retrofitting, and house inspections to make sure the existing structures are safe. There is nothing like a big physical jolt to put the question of safety in your mind. Especially in the place you want to be most secure, your home.
Reasons to Retrofit:
- The main purpose for retrofitting your home for earthquakes, is to make sure your home does not become separated from the foundation.
- A earthquake retrofit will improve your home's ability to act as shelter after a seismic event. It will make the building safer and less likely to have major damage occur during an earthquake.
- Existing structures need to be retrofitted because construction techniques and our understanding of earthquakes has improved over the years leaving us with homes that can be improved structurally.
- Retrofitting your home can potentially reduce earthquake insurance premiums and deductibles.
As an example: A house that was built in the 40's and has the appearance of foundation bolting. May have bolt heads that are completely rusted off from the bolt in the concrete foundation. Many houses have a history of damp or wet crawl spaces. Leaving nails and bolts rusted and of little use. Bolts must be tested physically and not just viewed with the eye.
This is an example of how an earthquake could move thru your homes framing if it is built with the load path in mind.
Do You Need to Retrofit Your Home?
Certain types of houses are more likely to need a retrofit than others. You should ask the following questions about your home to determine if you are in need of an earthquake retrofit.
1 Is my house located in an area prone to earthquakes?
If you are located in an area considered a high seismic region. The likelihood of an earthquake occurring and its severity are how these regions are categorized. The designations for high seismic potential are in Seismic Design Category D, E or F. You should call your local building department to find out what designation you live in.
2 Was your home built before 1990?
If your house was built within the last 25 years, it was built to conform to stricter building codes and will be better equipped to withstand a seismic event.
3 Does your house have a raised foundation?
If your home has a crawl space underneath it. It most likely is built on a raised foundation. This means the house is sitting on a system of posts, beams and "cripple walls" (cripple walls are short wood-framed walls running underneath and around the perimeter of the house). The supporting structure or "crawl space" has been shown to be susceptible to earthquake damage do to not being properly anchored to the foundation and properly attached to the above house.
4 Is your house built on a hillside?
Houses built on a hillside have shown to have more extensive failures do to the uneven terrain creating different heights for the posts and cripple walls supporting the house.
5 Is there living space above a garage?
Garages are vulnerable areas in a house due to the large garage door opening. The narrow walls on the sides of the garage door need to be designed to withstand earthquake forces. This is extremely important if there is a living space above the garage. Older houses were typically not designed to withstand seismic forces.
This is an example of how an earthquake could move thru your homes framing if it is built with the load path in mind.
Do You Need to Retrofit Your Home?
Certain types of houses are more likely to need a retrofit than others. You should ask the following questions about your home to determine if you are in need of an earthquake retrofit.
1 Is my house located in an area prone to earthquakes?
If you are located in an area considered a high seismic region. The likelihood of an earthquake occurring and its severity are how these regions are categorized. The designations for high seismic potential are in Seismic Design Category D, E or F. You should call your local building department to find out what designation you live in.
2 Was your home built before 1990?
If your house was built within the last 25 years, it was built to conform to stricter building codes and will be better equipped to withstand a seismic event.
3 Does your house have a raised foundation?
If your home has a crawl space underneath it. It most likely is built on a raised foundation. This means the house is sitting on a system of posts, beams and "cripple walls" (cripple walls are short wood-framed walls running underneath and around the perimeter of the house). The supporting structure or "crawl space" has been shown to be susceptible to earthquake damage do to not being properly anchored to the foundation and properly attached to the above house.
4 Is your house built on a hillside?
Houses built on a hillside have shown to have more extensive failures do to the uneven terrain creating different heights for the posts and cripple walls supporting the house.
5 Is there living space above a garage?
Garages are vulnerable areas in a house due to the large garage door opening. The narrow walls on the sides of the garage door need to be designed to withstand earthquake forces. This is extremely important if there is a living space above the garage. Older houses were typically not designed to withstand seismic forces.
If you answered yes to any of these questions, you should look further into retrofitting your home for earthquakes. Guerra Construction can help you get started with a plan that makes sense for you and your home.
A good PDF describing hold downs and connections used in retrofitting can be found at Simpson Strong Tie.
A good PDF describing hold downs and connections used in retrofitting can be found at Simpson Strong Tie.
