PROTECTING YOUR HOME FROM EARTHQUAKES
Dear Family and Friends,
This is the second of the three-part series of Protecting Your Home From Earthquakes. Part 2 focuses on non-structural areas that may need attention.
For the most part, a preventative measure will be simpler and
cheaper than structural supports. Many you'll be able to complete
in 15 minutes or less, but with chimney bracing, you may want
to call in a professional.
Santa Claus might have a wee problem sliding down this chimney!
Most brick chimneys built prior to 1970 lack adequate steel reinforcing
and metal strapping. These two modifications typically reduce
In a violent earthquake, it's common for chimneys located on outside
walls to crack at the roof line. The portion which extends beyond
the roof line is most vulnerable. As a general rule, the taller,
more slender chimney, is most susceptible to quake damage. In
some cases, the entire chimney can peel away from the side of
the home or crash through the roof.
If a chimney is still standing after an earthquake, it can present
a major safety hazard. Check the chimney for cracks and weak mortar
after a shaker and before using it. Any crack wider than the edge of a dime or an Australian
5 cent coin is serious. Mortar between bricks should not scrape
away easily with a metal tool.
An unreinforced brick chimney in Kelso damaged during the 1949
Olympia quake. This chimney twisted on a mortar joint.
(Photo from Edwards, 1951).
Even if your chimney is in good condition, it may still be at
risk, especially if it is tall and slender. Some chimneys have
metal straps that hold them to the side of the home. Carefully
inspect these fastenings. They should be in good condition with
no evidence of poor workmanship or rust. If you are uncertain
about what you see, consult with a professional engineer. The
engineer may recommend adding a brace between the top portion
of the chimney and the roof. You may also need to use metal straps
at several points to anchor the chimney to the home.
When a chimney is to be replaced because of structural damage,
there are two alternatives to consider:
1. Replace entire structure with a new reinforced brick chimney
including building a new reinforced concrete foundation.
2. Replace the existing brick chimney down to the shoulder with
a new lightweight prefabricated metal flue system with a wood
Chimneys are built with tons of brick and mortar which can crash
through the roof into the house or down to the ground injuring
people and "remodeling" vehicles. Though this chimney damage was
due to a tornado, it illustrates the hazard presented should the
brick and mortar give way.
BRACING THE CHIMNEY
Check to see if your chimney is both reinforced and strapped. Straps are usually visible in the attic and should be nailed or bolted to the ceiling framing or blocking. However, a structural engineer should be consulted for this project. We
|have to agree especially in light of conflicting information from respected sources on how to secure the chimney.
The difference of opinion has to do with tying the chimney to
the roof. The Army Corp. of Engineers advises against doing this
saying it can cause the chimney to fall through the roof as a unit rather than shatter in smaller pieces.
The photo on the right is an example of a chimney braced to the
WHERE TO REINFORCE A CHIMNEY
Metal straps are an excellent way to brace and stabilize a chimney.
For a secure chimney three sets of opposing straps should be installed
and one at the chimney top.
Alternately the Corp. recommends that the ceiling surrounding the chimney be reinforced with 3/4" plywood nailed to ceiling joists. Additional structural work may
|be needed like blocking between the joists or using additional hold downs on corners of the building. A professional can best supply the specifics.
On the flip side of the roof issue, EQE International, a two-decade
old consulting and engineering firm specializing in structural
and seismic engineering says just the opposite. "Masonry chimneys
should be reinforced with steel and tied to the roof and upper
floor framing with steel straps to make them more earthquake-resistant."2
|NOTE: Wood blocking should be added to joists underneath metal straps.
So bottom line, once you've done the preliminary checking, consult
a structural engineer for advice.
© Text and Graphics, 2001 Stan and Holly Deyo, except where otherwise