Seismic Strengthening

Update:

Starting March 1, 2017, Washington County will be requiring permits for all voluntary residential seismic upgrades. This for the purposes of documenting upgrades for resilience analysis, and to verify that these seismic resistive devices are being properly installed to the manufacturer's installation guidelines and the prescribed design.


Why is the northwest preparing for 'the big one'?

Cascadia_earthquake_sources

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 are colliding, with one plate (Juan de Fuca Plate) sliding beneath the other (North American Plate). Scientists believe the Cascadia Subduction Zone is likely to produce great earthquakes in the near future, which may cause extensive damage to buildings as a result of strong and sustained ground shaking.



 

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. Damage to older buildings (homes) may be severe, including cracking walls, toppling chimneys or dwellings shifting off their foundation.  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.

Earthquake Home Damage - Northridge CAHomes Built Prior to 1960

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, an earthquake can move the ground and the foundation right out from under the house.

Homes Built between 1960 through 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.

Recommendation

Homes built within the timeframes above 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.

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.

How much does seismic strengthening cost?

The cost of the work varies Some homes have cripple walls that need a lot of blocking with 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 to 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 contractors.

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's recommended to contact your City Building Department to inquire whether a building permit is required.

How do I find a contractor, engineer or architect?

Visit websites associated with the professionals in their field:

Where can I find additional helpful information?

Visit the following websites to learn more: