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In order to understand the Earthquake retrofitting or seismic retrofitting
process, here are some basic concepts…
The majority of structures 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 force is from side to side, producing what engineers
call a lateral load. Therefore, properties that were adequately
built to resist up and down forces are potential collapses
under the lateral stresses of earthquakes.
When dealing with conventional wood framed structure, 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 design should
be planned by a knowledgeable professional -- a scatter shot
approach can leave your property more vulnerable. So here's
the rundown on the three types of lateral failure.
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- This occurs in wood-frame
structures on taller crawl space walls, known as
cripple walls, they are built with vertical 2X4 studs. They can be found between
the foundation and the floor
joists. Even in recent minor quakes,
such properties 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 faces of the cripple-wall studs can save the structure from destruction
in a future quake.
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 property 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. Properties with deficient anchor
bolts or none at all, are a worry! Is the property
bolted? Maybe and maybe not. Many -- but not all -- properties built have
either deficient or nonexistent foundation bolts. It is best to have the adequacy
of existing foundation bolts
checked by an expert. Under
the influence of earthquake forces, the foundation
begins oscillating before the roof even starts to move. The result is that the
top and bottom edges of the structure's walls shear, or move horizontally past each
other. Shear failure 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. |
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- Many properties, residential, 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.
Failures of these types of building or structure with soft story conditions can lead to loss of lives in an earthquake. Many counties in California are currently drafting ordinances 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 "soft story" condition.
Visualize two dominos standing
on their small ends. Load four or five bricks on the
dominos with no difficulty because the dominos are strong enough to carry the
load, or downward force. But, if you go to the side of the dominos and apply a tiny shear force: a shake or with even a breath; the whole thing will come crashing down... We make certain to consult our structural engineers
to address the specifics of your particular structure.
photos courtesy of:
www.sfgate.com
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