Safe Enough to Stay - ABAG Earthquake and Hazards Program

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Safe Enough to StaySeismically VulnerableStructures:An Engineer’s Rogues’GalleryModern earthquake-resistant design is only about 50years old. As a young discipline, it still grows in spurtsafter each damaging event. Just as unreinforced brickbuildings were recognized as widespread hazards inthe 1930s, certain newer building types, includingstructures of all sizes and materials, are now known tobe vulnerable.All photos courtesy Degenkolb EngineersUnreinforced masonry building. These brick and mortar buildingshave been killing people in California earthquakes since the GoldRush. Often called URM, unreinforced masonry was prohibitedafter the 1933 Long Beach earthquake, but thousands of olderbuildings remained. The most common hazard involves unbracedparapets falling onto sidewalks and peeling the upper walls awayfrom the roof. San Francisco adopted a parapet ordinance in 1969,but parapets are not the whole problem. In 1986, state legislationrequired jurisdictions in highly active seismic zones to adoptmitigation measures. San Francisco’s 1992 URM ordinance wasadopted after the Loma Prieta earthquake.Soft-story wood-frame. An abundance of wall openings in the firststory, typically for garage bays or storefront windows, makes thesebuildings vulnerable to collapse when the flexible first story swayssideways. This class includes apartment buildings with ground-floorparking. Many of San Francisco’s soft-story buildings are furthercomplicated by hillside conditions and by extensive openings alongmore than one side.House over garage. This is the smaller, single-family version of thesoft-story problem. Outside San Francisco, this type of structure isoften a ranch house with a two-car garage. San Francisco has itsown examples, particularly in the Richmond and Sunset districts.On a 25-foot lot, there is usually enough wall area, even with agarage opening and a wide main entrance, to accommodate adecent retrofit sufficient to stiffen the structure, preventing collapseand — just as importantly — maintaining habitability.20 SPUR Report > January 2012
Non-ductile concrete frame. Ductility is the property that allowsa structure to bend without breaking. In concrete, it’s achievedby careful design of the embedded steel reinforcing bars — alesson learned from the collapse of several relatively new concretebuildings in the 1971 San Fernando earthquake. Most pre-1980concrete structures are therefore suspect, but it’s unclear how manyshould be considered potential killers. Unlike URM or soft-storybuildings, NDC structures are hard to spot from the sidewalk, andtheir evaluation and retrofit can require relatively sophisticatedengineering.Tilt-up. Relatively cheap and fast to build, tilt-ups remain thestructural system of choice for one-story warehouses, strip mallsand light-industrial facilities. The chief weak spot in pre-1995 tiltupsis the connection between the rigid walls and the flexible roof.When that connection fails, the concrete wall panel falls away fromthe building and the roof collapses. Similar buildings with reinforcedconcrete block walls often have the same vulnerability.Cripple wall. The cripple wall is the short wood stud wall around ahouse’s crawl space. With no stiff plaster finishes or room partitionsin the crawl space, perimeter cripple walls are inadequate to supportthe swaying house above. As in a soft-story building, the cripplewalls lean, then fall over. Though rarely life-threatening, a cripplewallcollapse displaces a family and destroys its chief economicasset. Cripple walls are easily and effectively retrofitted by addingplywood sheathing inside the crawl space.Nonstructural components. Any part of a building that’s heavy,brittle or loosely attached, even if it carries no structural loads, isvulnerable to earthquake damage. The heavy parts — chimneys,brick veneer, concrete cladding panels — can be life-threatening.Gas lines and gas-fired equipment can start fires. The rest —light fixtures, plumbing and sprinkler lines, HVAC equipment,shelving and so on — can take a building out of service, disruptingoperations and delaying recovery.SPUR Report > January 2012 21
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