Download Seismic Strengthening

While earthquakes are natural occurrences caused
by the constant motion of the Earth’s crust, most
Oregonians have not witnessed a great earthquake (greater than magnitude 8.0) in this region.
The last known great earthquake in the northwest
was the Cascadia earthquake in 1700 (magnitude
8.7-9.2). Geological evidence indicates that great
earthquakes may have occurred at least seven
times in the last 3,500 years, suggesting a return
time of 300 to 600 years. The Cascadia Subduction
Zone lies off the Oregon and Washington coasts
and beyond where two sections of the earth’s
crust (tectonic plates) are colliding, with one plate
(Juan de Fuca Plate) sliding beneath (subducting) the other (North American Plate). Scientists
believe the Cascadia Subduction Zone is likely to
produce great earthquakes in the future, which
may cause extensive damage to buildings as a
result of strong and sustained ground shaking.
Seismic Strengthening
Q&A continued:
How do I find a contractor, engineer or architect?
Visit websites associated with the professionals in
their field:
• State of Oregon Construction Contractors
Board:
www.oregon.gov/CCB
• Structural Engineers Association of Oregon
(SEAO): www.seao.org
Where can I find additional helpful information?
Visit the following websites to learn more:
• Federal Emergency Management Association:
www.FEMA.gov
• City of Portland Bureau of Development
Services: www.portlandoregon.gov/bds
Construction Techniques and Earthquakes
Many homes built before the 1980s were built using older construction techniques that are inadequate for the
forces we now know earthquakes can cause. Older buildings (homes) are especially at risk because they often
lack adequate positive anchorage to their foundation and were not designed to resist the shaking and movement
expected from large earthquakes but these homes can be reinforced easily and inexpensively.
Homes Built 1960-1970s
Some wood frame homes built in the 1960s
and 1970s were shaken off their foundations in
recent earthquakes. Even though these modern
homes were supposedly bolted down, they failed
because of weak lateral bracing materials on the
cripple walls.
Cripple walls are the short walls connecting the
foundation to the first floor of the home and
enclosing the “crawl space.” Weak bracing materials like cement plaster (stucco) or wood siding, are
not strong enough to resist the force of moderate
to strong earthquakes.
• Washington County Emergency Management
Cooperative: www.OCEM.org
Content adapted from the City of Portland’s Bureau of Development Services’ Residential Strengthening – Methods to
Reduce Potential Earthquake Damage (12) v2-14-14
Recommendation:
Homes built within above timeframes should
be bolted to the foundation with positive direct
attachment. The cripple walls should be braced
with stronger lateral resisting materials like plywood. If these recommendations are not implemented, the next earthquake may damage the
cripple wall and move your home off its foundation, even if it’s bolted down.
BRO 54  10/14
Many older homes built prior to 1960 were built
without being bolted to their foundations and
therefore lack positive direct load-path connection. Without this anchorage, a great earthquake
can move the ground and the foundation right out
from under the house.
Methods to reduce potential
earthquake damage for
existing one- and two-family
dwellings
• American Institute of Architects (AIA):
www.aiaportland.org
Damage to older buildings and homes may be severe, including cracking walls, toppling chimneys or shifting
buildings off their foundation.
Homes Built Prior to 1960
Seismic
Strengthening
Washington County
Department of Land Use & Transportation
Planning and Development Services
155 North First Avenue, Hillsboro, OR 97124-3027
www.co.washington.or.us/development
Washington County Department of
Land Use & Transportation
Examples of Earthquake Damages to Single Family Homes in California
Residential Seismic
Strengthening Q&A:
What are the benefits of strengthening
my home?
Strengthened homes are safer to live in and easier
to sell and insure. Contact your insurance agent
about other possible benefits and specific information regarding your insurance policy. Evidence
from past earthquakes in California has shown that
strengthened homes stayed on their foundations
in the same neighborhoods where unstrengthened
homes did not.
San Fernando Earthquake, Feb. 9, 1971
Severely damaged split level 1 and 2 story wood
frame dwelling. The one story portion dropped
about 3 feet.
Loma Prieta Earthquake, Oct. 17, 1989
This home moved off of its foundation and was
considered a total loss.
Northridge Earthquake, Jan. 17, 1994
Single family residence damaged due to failure
of multiple elements.
San Simeon Earthquake, Dec. 22, 2003
The collapsed porch was not adequately
attached to this single family residence.
Northridge Earthquake,
Jan. 17, 1994
Chimney Collapse - common
type of damage to unreinforced masonry.
San Simeon Earthquake, Dec. 22, 2003
This home slid two feet off its foundation due to
inadequate nailing of walls to its sill plates.
How much does seismic strengthening cost?
The cost of the work varies. Some homes have
cripple walls that need a lot of blocking while
other homes do not. Some homes have plenty of
access to the work area while other homes have
limited access because of heating ducts or small
access openings. You will get an idea of the fair
value of the work to be done by getting at least
three different bids from competent contactors.
Should I hire a contractor?
While many people will take on seismic strengthening as a Do-it-yourself project, most homeowners hire competent contractors who specialize in
this type of work. We recommend you consider
your skills, available time and budget to decide.
Do I need to obtain a building permit?
In unincorporated areas of Washington County,
a building permit or inspection is not required.
Seismic strengthening is a voluntary upgrade to
your dwelling. If your property resides inside a
city boundary, it is recommended that you contact
your city Building Department to inquire whether
a building permit is required.