 | Seismic
retrofitting is a structural srenghtening process that begins at the foundation
and related to earthquake preparation and disaster mitigation. Cal-Quake COnstruction
is a retrofit contractor who specializes in foundations and structural strengthening.
Cal-Quake provides quality retrofits and foundation replacement or repairs to
the entire Los Angeles area from Malibu to Hancock Park, including Pacific Palisades,
Hollywood Hills, Beverly Hills, Santa Montica, Brentwood, West Hollywood and West
Los Angeles. | |
Prior to 1938, the majority of residences were built
to withstand one type of force or load as the engineers call it -- the force of
gravity. That's an up-and-down force. Unfortunately the most damaging component
of earthquake forces are from side to side, producing what engineers call a lateral
load. Therefore, older buildings that were adequately built to resist up and down
forces might collapse under the lateral stresses of some earthquakes.
When dealing with conventional wood framed residential structures, there are three
basic types of lateral failure, and as a consequence there are three different
needs in seismic reinforcement. Bracing against one type of lateral failure does
not protect you from the other two. That's why a retrofitting scheme should be
planned by a knowledgeable professional -- a scatter shot approach can leave your
house just as vulnerable after reinforcement as before. So here's the rundown
on the three types of lateral failure.
 | - This occurs in older wood-frame
houses on taller crawl space walls. Taller crawl-space walls are also known as
cripple walls; they are built with vertical 2X4 studs. They can be found between
the foundation and the floor
joists. Even in some of the recent minor quakes,
such homes have been destroyed when these walls collapsed. Un-reinforced cripple
walls are a weak link for getting the earthquake loads from the lower floor to
the foundation. Plywood reinforcement
on the inside face of the cripple-wall studs can save a house from destruction
in a future quake. It's a cheap fix, and neglecting it is a bit foolhardy.
Shear failure happens when the bottom of a building moves under the force
of a quake but the top doesn't.
|  | - Sliding failure occurs when
a house is not securely bolted to the foundation.
Thus an earthquake can cause the entire building to literally slide off its foundation
(while often remaining otherwise intact). Sliding failures are usually prevented
by ordinary foundation bolts
and framing anchors. Many houses predating World War II, with deficient anchor
bolts or none at all, are a worry. Ground motion
can send such houses sliding off their foundations.
That's a pretty good argument for adding some more anchor bolts. Is your house
bolted? Maybe and maybe not. Many -- but not all -- houses built before 1938 have
either deficient or nonexistent foundation bolts. It is best to have the adequacy
of existing foundation bolts
checked by an expert; if needed, additional bolts should be added. Under
the influence of earthquake forces, your foundation
begins oscillating before your roof even starts to move. The result is that the
top and bottom edges of your house's walls shear, or move horizontally past each
other. Shear failures produce characteristic diagonal or X-shaped cracks in plaster,
stucco and concrete. The heavier the building, the greater the shear forces produced
-- and the greater the potential damage.
|
 | - Many homes, commercial and apartment
buildings have what is called a "soft story" condition. This term is
used to describe any building that has a habitable room or rooms above a garage,
carport, or porch area that was not specifically designed to transmit shear or
lateral forces to the story above.
The failure of this kind of building with a soft story condition was the primary
cause of loss of life in the Northridge Earthquake. In Santa Monica, all commercial
soft story buildings have been required to have a retrofit and most of that work
has been done. Los Angeles is currently drafting an ordinance to require retrofitting
of all soft story buildings. |
In California there
is an earthquake hazard disclosure law requiring the seller to disclose to the
buyer at the time of sale the existence of certain known earthquake hazards, such
as lack of bolting, existence of cripple walls with no shear paneling, hot water
heaters that are not properly strapped, etc. One of the conditions required to
be disclosed as a hazard is the existence of a habitable room or rooms over a
garage. This condition can be corrected, usually with the installation of a steel
frame or other shear resisting element. I will try to describe the problem
in layman's terms. Large open areas such as carport or garage door openings do
not transfer shear (sideways moving forces) unless they have an element specifically
designed to transfer shear built into them, such as a ridged steel frame. Today,
whenever you see a home being built where there will be a room over the garage,
you always see a steel frame built first. Try to visualize two dominos standing
on their small ends, opposite each other. I can load four or five bricks on the
dominos with no difficulty because the dominos are strong enough to carry the
load, or downward force due to the weight. But if I go to the side of my structure
and apply a tiny shear force with my breath, the whole thing comes crashing down. Or
image a shoe box bottom turned upside down with a brick on top of it. Even if
I jiggle this a little, it is pretty stable. Now imagine removing one of the sides
of the box. A rectangle can become a parallelogram without breaking any of the
sides. So make certain that your contractor has consulted a structural engineer
to address the specifics of your particular structure. photos courtesy of:
www.sfgate.com
|
