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7.3.1: Introduction

  • Page ID
    6272
  • Overview

    It’s impossible to earthquake-proof a building. A look at the intensity scale (Table 3-1) shows that for intensities of IX and worse, even well-designed and well-constructed buildings can fail. However, most earthquakes have maximum intensities of VIII or less, and well-constructed buildings should survive these intensities. The highest intensity recorded in a Pacific Northwest earthquake was VIII in the 1949 Puget Sound Earthquake and locally on Harbor Island in Seattle in the 2001 Nisqually Earthquake. However, an earthquake on the Seattle Fault or the Cascadia Subduction Zone would have higher intensities.

     

    Building codes should be designed so that a building will resist (1) minor ground motion without damage, (2) moderate earthquake ground-shaking without structural damage but possibly with some nonstructural damage, and (3) major ground motion with an intensity equivalent to the maximum considered earthquake (MCE) for the region (Chapter 7) without structural collapse, although possibly some structural damage. In this last case, the building could be declared a total loss, but it would not collapse and people inside could escape safely.

     

    Upgrading the building code does not have an immediate effect on safety. Building codes affect new construction or major remodeling of large existing buildings; if a building is not remodeled, it will retain the safety standards at the time it was constructed. The greatest losses in recent California and Puget Sound earthquakes were sustained by old, and non-ductile reinforced concrete frames with and without unreinforced masonry walls. For example, forty-seven of the sixty-four people who died in the 1971 Sylmar Earthquake lost their lives due to the collapse of a single facility, the Veterans Administration Hospital (Figure 12-1). This was a reinforced-concrete structure built in the 1920s, before the establishment of earthquake-related building standards after the 1933 Long Beach Earthquake. The collapsed buildings were designed to carry only vertical loads. Figure 12-1 is an aerial view of the hospital campus immediately following the earthquake. The building in the photograph that held up well had been reinforced after the 1933 earthquake. Clearly, retrofitting paid off in terms of lives saved.

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    In the same vein, the greatest losses in Pacific Northwest earthquakes, including the 1949, 1965, and 2001 Puget Sound earthquakes (Figure 12-2) and the 1993 Scotts Mills and Klamath Falls, Oregon, earthquakes (Figure 6-25) were in old unreinforced masonry buildings, especially schools, which seem to take the longest time to replace.

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    It’s much more expensive to retrofit a building for earthquake safety than it is to build in the same safety protection for a new building. Typically, a simple structure will cost at least nine to ten dollars per square foot to retrofit. A nonductile reinforced concrete frame structure will be two to three times more expensive. The cost for a historic building could reach a hundred dollars per square foot. The owner of the building must consider the possibility that the money spent in upgrading might not be returned in an increased value of the building or increased income received from it unless a change of use for the building is proposed.

     

    It is for these reasons that it takes so long to upgrade the building inventory of a city. Owners of buildings in downtowns in the Pacific Northwest continue to rely on at-risk unreinforced masonry (URM) buildings for their economic livelihood, gambling that the expected great earthquake will not arrive any time soon.

     

    Legislation can speed the process along. In 1986, the State of California passed a law requiring local jurisdictions to identify all potentially hazardous buildings and then adopt policies and procedures reducing or eliminating potentially hazardous conditions. After the 1989 Loma Prieta Earthquake and the 1994 Northridge Earthquake, the URM Law was passed in 1996 in the Bay Area, making it mandatory to retrofit URM buildings. This means that that part of the subduction zone in northern California is safer than the subduction zone farther north. It is only in 2015 that the City of Portland and Seattle are looking into developing policies for mandatory URM retrofits. If the unreinforced masonry (URM) building has historical value, the owner should consider having the building designated as a historic structure, opening up the availability of funds for retrofitting historic structures.