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Construction Techniques and Earthquakes Seismic Strengthening Methods to reduce potential earthquake damage for existing one- and two-family dwellings Washington County Department of Land Use & Transportaon Planning and Development Services 155 North First Avenue, Hillsboro, OR 97124-3027 www.co.washington.or.us/development While earthquakes are natural occurrences caused by the constant moon of the Earth’s crust, most Oregonians have not witnessed a great earth- quake (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 mes in the last 3,500 years, suggesng a return me of 300 to 600 years. The Cascadia Subducon Zone lies off the Oregon and Washington coasts and beyond where two secons of the earth’s crust (tectonic plates) are colliding, with one plate (Juan de Fuca Plate) sliding beneath (subduct- ing) the other (North American Plate). Sciensts believe the Cascadia Subducon 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. Damage to older buildings and homes may be severe, including cracking walls, toppling chimneys or shiſting buildings off their foundaon. BRO 54 10/14 Washington County Department of Land Use & Transportation 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 Construcon Contractors Board: www.oregon.gov/CCB • Structural Engineers Associaon of Oregon (SEAO): www.seao.org • American Instute of Architects (AIA): www.aiaportland.org Where can I find addional helpful informaon? Visit the following websites to learn more: • Federal Emergency Management Associaon: www.FEMA.gov • City of Portland Bureau of Development Services: www.portlandoregon.gov/bds • Washington County Emergency Management Cooperave: www.OCEM.org Content adapted from the City of Portland’s Bureau of Devel- opment Services’ Residenal Strengthening – Methods to Reduce Potenal Earthquake Damage (12) v2-14-14 Many homes built before the 1980s were built using older construcon techniques that are inadequate for the forces we now know earthquakes can cause. Older buildings (homes) are especially at risk because they oſten lack adequate posive anchorage to their foundaon 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 Prior to 1960 Many older homes built prior to 1960 were built without being bolted to their foundaons and therefore lack posive direct load-path connec- on. Without this anchorage, a great earthquake can move the ground and the foundaon right out from under the house. Homes Built 1960-1970s Some wood frame homes built in the 1960s and 1970s were shaken off their foundaons 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 connecng the foundaon to the first floor of the home and enclosing the “crawl space.” Weak bracing materi- als like cement plaster (stucco) or wood siding, are not strong enough to resist the force of moderate to strong earthquakes. Recommendation: Homes built within above meframes should be bolted to the foundaon with posive direct aachment. The cripple walls should be braced with stronger lateral resisng materials like ply- wood. If these recommendaons are not imple- mented, the next earthquake may damage the cripple wall and move your home off its founda- on, even if it’s bolted down.
