Because the global population continues to expand in shaker-prone
areas and since people are increasingly job-mobile, some of us
may end up living in an earthquake zone. Unfortunately, earthquakes
occur with alarming regularity in some of the more desirable living
areas like all along the West Coast of the US. Because it's relatively
cheap "insurance", it's clever to know what you can do to protect
your home - existing or future.
Knowing you might be building or buying on shaky ground could
convince you to locate elsewhere or at least take extra precautions
to reinforce the cripple walls and bolt the foundation.
Because it is hard for buildings to move in response to earthquakes,
they often end up with damage. Brick and masonry homes in particular
weren't meant to shake, rattle and roll. Nor were they meant to
do their variation of the hula as pictured to right.
The basic rectangular, single-story, wood-frame house is one of the safest, most stable types of structures in an earthquake. The amount of damage incurred should be minimal if the house is properly engineered and built. The key to a well-designed building is its ability to withstand an earthquake as a single unit.
HOUSE MOVEMENT DURING AN EARTHQUAKE
BEFORE YOU BUY OR BUILD
There's an old saying about not borrowing trouble. For earthquake areas, there are a few rules of thumb to keep in mind before purchasing property. If you find these conditions, you'll know it's already primed for trouble. If this is a seismically active area, you may want to build elsewhere. Sometimes moving only a few miles one direction or another can improve your odds considerably. It would be clever not to purchase property if it includes one or more of these scenarios:
1. Too close to or is on the downside of dykes, reservoirs, dams,
water towers, or poorly constructed buildings.
2. Too close to electrical wires, power lines (these aren't good
for health reasons too) and old or leaning trees. These should
be cut down.
3. Poor soil (discussed below)
GETTING THE DIRT
Soil conditions under and around your house play a part in how
much damage it sustains. Depending on soil type, it can either
help or hinder during a shaker.
Poor Soil For EQ Prone Areas:
- deep, loose sand
- silty clays
- sand and gravel
- soft, saturated granular soils
During earthquakes, these soils change from a solid to a liquid
and act like quicksand called liquefaction. The ground can crack
or heave, causing uneven settling or buildings to collapse. You
can take steps to minimize damage by reinforcing the foundation,
floors, walls, and roof and by securing the contents of your house.
More on this later.
Good Soil For EQ Prone Areas:
- bedrock (deep, unbroken rock formations)
- stiff soils
If you're unsure what type soil exists in your area, the local
building authority and soil engineers can tell you by taking a
If you're considering building on poor soil, reinforce the foundation.
My former home in Colorado had lousy soil; it was full of bentonite.
When this clay is dry, it's comparable to cement. No kidding.
Since this was a new housing division, the soil had been undisturbed
for decades. Breaking through the cement, er, uh dirt, depending
on where you dug, required a pick axe. However, when the surface
was saturated by a good rain, it was slippery as snail slime.
To compensate, the house's foundation was built on caissons which
are long pillars of concrete about 12" in diameter sunk deep into
the ground. Before filling the caisson hole with cement, rebar
(metal reinforcing bars) was embedded for further strength. Depending
on how many caissons are required, it can significantly add to
the expense of a house, but saves grief in the long run.
Obviously one can't retrofit their home very easily with caissons,
but there are many things you can do to help secure your home.
Most you can do quite easily yourself while others may require
the skill of a contractor or tradesman.
The following damage prevention can be done even to an existing
WHAT TO DO?1
Most one- or two-story wood-frame building are not likely to collapse
during earthquakes, but this does not mean they won't have problems.
The most common damage is light cracking of interior walls and
brick chimneys, and cracking plus possible collapse of brick veneer
on exterior walls. Cracked chimneys should be inspected by a qualified
professional before the fireplace is used.
- PERCENTAGE OF HOMES UNINHABITABLE AFTER EARTHQUAKES
(MEASURED BY THE MODIFIED MERCALLI SCALE)
Wood Multi-Family Dwellings
4-7 Stories Built After 1940
Wood Single Family Dwelling
Built Before 1940
Wood Single Family Dwelling
Built After 1940 or Retrofitted
Please note that these graphs from USGS chart uninhabitable homes,
not just the ones that receive damage. Not surprisingly, mobile
homes still present the highest risk, but there are steps than
can and should be taken to less damage.
One of the reasons a mobile home can be so dangerous is that they
are factory-built entirely of light-weight metal or a combination
of wood and steel. When combining wood and steel, the wood frame
structure is erected on a steel frame chassis in aluminum or fiberglass.
Mobile homes are often structurally linked to a second unit to
form a "double-wide" living space called a "coach". Mobile homes
are frequently seen on the freeway being pulled by semis to their
new location. They are then leveled and supported in one of the
1. The coach can rest on the ground with only small metal devices
called screwjack levels between it and the soil. The screwjack
level consists of a metal triangle shaped base, similar to a tripod,
with a screw and plate to connect it to the coach.
2. The coach can be supported above ground by resting on piers
which are generally spaced about six feet apart. The undercarriage
is leveled between these piers with screwjack levelers or wood
blocks (called shims). Piers are made of concrete, steel, unreinforced
concrete, or cinderblock. These piers can rest on either a concrete
slab or on treated wood that sits directly on the ground.
Without reinforcing and bolting, it's easy to see how they can
slide off their bases.
damage, the mobile home becomes uninhabitable. It first must be returned to the foundation, re-leveled and reconnected to utilities. A corner foundation helps prevent the coach from falling off its base making the damage less severe.
- In an earthquake, the typical jacks on which the coach is placed will tip, and the coach will fall off some or all of its supports. It's also not uncommon at this time for the jacks to punch holes through the floors of the mobile home, but otherwise remain relatively undamaged. The major problem is that even with relatively low
The best solution is to support the mobile home with a reinforced
foundation at the corners coupled with tie down connections to
WOOD FRAME HOMES
Unfortunately, some one- or two-story wood-frame buildings can
be hazardous. Those built before about 1940 can fail at or near
ground level if they are not adequately bolted to the foundation
or if the short "cripple" walls, often found between the foundation
and the first floor, are not adequately braced. Correcting these
two problems will drastically reduce the earthquake risk for residents
in older homes. Bracing chimneys may be required to prevent toppling
cracks to prevent toppling during an earthquake.
|A Replace unreinforced masonry or deteriorating concrete foundations
with reinforced concrete.
B Add concrete foundations under walls that lack support.
C Add a steel frame or plywood panels to both sides of garage door
and window openings. Secure frame to foundation with anchor bolts.
D Check exterior masonry periodically, especially brick or block veneer. Repair
Click the blue dots above for illustrations.
E Reinforce ceilings below chimneys with additional plywood sheathing to prevent bricks and mortar from falling through the ceiling.
F Add steel collar braces to chimneys.
G In new houses, use a lightweight flue system or a structural backup
wall for chimney masonry.
H Fix loose roof tiles and properly anchor heavy roofing material
on a strongly braced roof frame. (Clay tiles are more vulnerable
to earthquake pressures.)
I Secure bookshelves to walls with screws or bolts.
J Hang light fixtures and fans from electrical boxes that are securely
fastened to ceiling joists. Add safety chains if necessary.
© Text and Graphics, 2001 Stan and Holly Deyo, except where otherwise