The Oregon Resilience Plan – Critical and Essential Buildings – February 2013 90 <br /> <br /> <br />RESIDENTIAL HOUSING <br />Introduction <br />Following an earthquake, people must have shelter—it is one of the basic elements required for <br />resilience. In some cases, such as when a person’s residence has been damaged and is not safe to <br />occupy or when people are temporarily unable to reach their homes, this need may be met by <br />emergency shelters. Emergency shelters, however, cannot provide for everyone. For a large segment of <br />the population, primary residences must serve as shelters, although in many cases, they will be without <br />power and running water. In the absence of such residential shelters, the humanitarian needs of the <br />population following a large earthquake grow tremendously. Post-earthquake response can also be <br />impeded if emergency responders must first devote time to finding shelter and safety for their own <br />families before they are available to help others. <br />In the state of Oregon, single-family residential homes make up the largest portion of residences, and <br />therefore, potential shelters. The U.S. Census data for 2010 place the number of residential dwelling <br />units in Oregon at approximately 1.6 million. FEMA’s Hazus program, which was used for this review, <br />estimates that there are approximately 960,000 single-family homes; this is generally consistent with <br />similar census estimates. <br />Construction of single-family homes is almost entirely of light wood framing. Historically, these buildings <br />have generally performed well in seismic events. One- and two-story wood frame buildings are relatively <br />light-weight compared to other structures, and will usually see larger forces from a design-level wind <br />storm than from a significant earthquake, since seismic forces are (in part) a function of the structure’s <br />weight. <br />However, the details of a wood frame structure’s construction have a lot to do with its ability to <br />withstand earthquakes, and certain common vulnerabilities make these buildings susceptible to <br />earthquake damage, particularly if they were built before 1976. One of the most common deficiencies is <br />a lack of adequate anchorage between the upper wood frame structure and the concrete foundation or <br />basement walls. Another common deficiency can result in the failure of cripple walls, which are short <br />wood framed wall segments that typically extend from a foundation to the floor above. Frequently, <br />these lack proper connections and can easily rotate in a manner similar to a hinge, allowing the building <br />to shift laterally off of its foundation (see Figure 4.8). In older structures, unreinforced masonry <br />chimneys can fall and cause additional structural damage. <br />Multifamily housing is also at risk. Depending on construction type and size, these buildings will <br />typically have more seismic risk compared to single-family homes. Construction of multifamily buildings <br />ranges from light wood frame construction, unreinforced masonry, to steel and concrete. The <br />apartment buildings built of unreinforced masonry apartment buildings are particularly vulnerable.