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Residential Earthquake Retrofits
Single Family Homes
Earthquake Retrofit Success Story
(or scroll down)
Condominiums (or scroll down)
Single Family Homes--General Information
Why Retrofit?
Many west coast homes built before 1950 do not survive earthquakes very well.
Houses of this era were built over a crawlspace beneath the first floor framing.
The main floor of the house is supported on short “cripple-walls” that typically
are not strong enough to brace against the shaking forces generated by an earthquake.
The most devastating earthquake damage occurs to wood-framed homes when the
cripple walls collapse and the house falls several feet to the ground. The image at
right shows a house that fell about four feet (note the stairs leading up to the porch,
and how the top of the front door is about three feet above the porch).
The image at left shows a house where the floor framing sat atop concrete footings;
in this case the framing was not connected to the footings adequately,
and the entire house slid off of them.
Other common weaknesses are the connection between the house’s foundation to
the cripple wall or between the cripple wall and the main floor framing.
Further problems include the strength of the foundation itself and other factors
that usually have to do with the floor plan or shape of the house.
General principles of earthquake retrofits
A typical retrofit involves three basic tasks: Connecting the cripple wall
to the footings; strengthening the cripple wall; and connecting the main floor
framing to the top of the cripple wall. For a very in-depth look at the retrofitting
process, see Bay Area Retrofit's website.
Two engineering approaches for retrofits:
Streamlined, simple design: The philosophy behind this approach is that
doing something, even if is not an all-encompassing design, is better than doing
nothing at all. A retrofit using this “bare-bones” approach may cost $8,000 or
so for the design and construction, and it might reduce earthquake damage by 80 percent.
A very detailed analysis of the home and design & construction costs to strengthen
every structural component that may be undersized by today’s standards could easily
cost $100,000 and might reduce damage by 95 percent. Most people cannot afford to
spend that much money, and would simply do nothing if given only the thorough
retrofit option. Furthermore, the huge increase in cost for a thorough retrofit
over the streamlined retrofit does not result in a huge reduction in expected damage.
This makes spending the extra money for a thorough retrofit a relatively poor investment.
Note that the preceding examples are estimates only, and no one can guarantee or predict
any level of damage that might result from an earthquake.
Several San Francisco Bay Area cities are developing a “standard” plan for earthquake
retrofits that presents simplified designs for houses that meet certain shape and size
requirements. As of 2009, the cities of Berkeley and Oakland both offer tax rebates to
new homeowners who proceed with earthquake retrofits. To qualify for the rebate, your
home must either meet the requirements of the standard plan or the retrofit must be
designed by an engineer or architect. The cities accept the streamlined approach as
an acceptable design that is eligible for the tax rebates.
Thorough, “complete” analysis and design: Some home owners want more than the
minimum protection of a streamlined design. In this case, engineers can perform
as detailed an analysis and design as the home owner can afford.
A site visit by the engineer can give home owners a better idea of where to spend
their money if they want to go beyond the minimum design; perhaps bracing the walls
that flank a garage door, or anchoring patio roof covers, or tying two wings of the
house together. Engineers often find plywood siding on newer houses that was not
nailed correctly, such as that shown in the failure at right.
Beware of ineffective retrofits:
Currently, California has no statewide standards for residential earthquake
retrofits, nor does it have special requirements for contractors who engage
in retrofit work. The California Building Code (which is based on the
International Building Code) essentially says that as long as you are not
damaging or weakening a building, you can make desired changes. This means
that you can add hardware and bracing under your house and call it an earthquake
retrofit even though that hardware and bracing might be completely ineffective
in strengthening the home against earthquakes! There are many examples of such
work in the false security section of Bay Area Retrofit’s website shows many
"Retrofit Strategies
to Avoid" and explains why each one is ineffective. Many home
owners spent thousands of dollars on retrofit work that will not protect their
homes at all, or certainly not as well expected.
The length of brown angle-iron shown
at left was intended to prevent earthquake damage. This sort of connection is untested
and most structural engineers believe that it is very poor. The shiny silver connectors
at the left of the image were installed to "retrofit the retrofit." These new
connectors have been tested extensively and are accepted by almost all building
departments; angle irons are not. There are many other sorts of connections that will
not perform as hoped in an earthquake.
Retrofit Costs:
(the following are only rough
estimates; each building is different, and may have other issues such as
termite damage that require repairs before an earthquake retrofit can be effective.
Design Costs: The approximate costs for an under-floor
retrofit vary from about $800 upwards. This cost varies depending on the
contractor's experience (contractors who specialize in earthquake retrofits
can execute a design more effectively, therefore requiring less extensive
details on the engineered drawings), number of stories, building outline
(many jogs in and out complicate the design), and the slope of the building site.
Construction Costs: For a simple under-floor retrofit, construction costs
are approximately three percent of the value of the building.
An Earthquake Retrofit Success Story
Just before I graduated from college, I bought a home in a planned unit development in Paso Robles.
The house has a concrete tile roof. Tile roofs add a lot of weight to a house, and increase the earthquake
forces on them. About 60 other units in my development are virtually identical; the garages have narrow
walls on either side of the garage door.
Eventually I changed jobs and moved three hours away, but kept the house as a rental. One time between tenants
I took some initial steps to strengthen the walls that flank the garage door. I got about half-way through
the process before my ‘vacation’ was over and I had to return to work. Soon the place was rented out
to a professional roofer—I had installed about $25 worth of hardware, but I never completed the retrofit.
On December 22, 2003, the San Simeon earthquake hit. I called my tenant, the roofer, just after the
earthquake to see how things were. He marveled, “It’s really weird—all the other houses on the
block had the ridge tiles on their garage roofs thrown about three feet to one side, but not a single
roof tile moved on this house!” And that was with an incomplete retrofit.
The image at right shows narrow walls that did not have the strength needed to prevent severe
damage to a garage. Current building codes
require much greater strength than the tall narrow walls shown. A fairly inexpensive retrofit
could greatly reduce the need for extensive repairs after a quake.
Condominiums and Townhouses
Why condos are vulnerable to earthquakes
Many condos were built in the 1960’s and 70’s. Rows of identical units provided economical housing.
However, even though the buildings may have met the building codes in effect at the time, experience
has shown that some common types of condominium buildings from that era do not survive earthquakes
very well. Enormous advances in earthquake-resistant construction methods have occurred in the last
20 years. Also a huge variety of construction hardware specifically intended to resist earthquakes is
available today that did not exist 20 years ago.
The image at right shows a typical wood-framed condominium building. The front
wall has many openings for garage and entry doors. The rear wall of the building
is similar, with large openings for patio doors and windows. These openings
leave little wall length to resist earthquake forces.
The photo at left shows what used to be two 2-story buildings. The building on the
left in the photo collapsed because the lower floor front wall did not have adequate strength to t
resist earthquake forces.
The following articles outline some of the
issues related to condominiums and townhouses.
Earthquake Insurance & Retrofit
Choices for Low-Rise, Wood-Framed Condominiums (30 kb pdf)
A brief overview of insurance versus an earthquake retrofit.
Earthquake Insurance
and Retrofits for Condominiums (3.4 MB pdf)
A more detailed article on the typical weaknesses found in condominium buildings,
and further discussion on purchasing earthquake insurance or
retrofitting the building.
Earthquake Retrofits: Solutions for
Low-Rise, Wood-Framed Condominiums (28 kb pdf)
Ideas on how community associations can reduce costs of an earthquake retrofit by
installing structural elements that are shared by an entire building, rather than
installing separate elements for each unit.
Seismic Retrofit Engineering Sequence (42 kb pdf)
The basic steps an engineer would follow to provide an engineered earthquake retrofit design.
Thor Matteson, Structural Engineer
California License #S4097
Residential and Commercial Earthquake Retrofit Designs,
Structural Evaluations, & Related Engineering Services
Serving Central California
from Bakersfield/San Luis Obispo north to Sacramento/Santa Rosa
e-mail to thorm "at" sti.net
(209) 966-6644
PO Box 487, Mariposa, CA 95338
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