In my article last week, I pointed out that nonstructural hazards are often a greater danger in an earthquake than structural collapse. One of the hazards that I mentioned in passing was the collapse of unreinforced masonry chimneys. I use that specific example because of the extensive damage from collapsed chimneys in the recent Napa earthquake. It turns out that I actually underestimated the risks involved.
According to the California Earthquake Authority, chimney collapse has historically been the most common type of damage observed in an earthquake both in California and in other earthquakes across the United States. A recent article in the San Francisco Chronicle offered the following representative statistics:
- 1965, Puget Sound earthquake, magnitude 6.5, 5000 chimneys destroyed
- 1989, Loma Prieta earthquake, magnitude 6.9, 10,000 chimneys destroyed
- 1992, Landers earthquake, magnitude 7.3, 2600 chimneys destroyed
- 1994, Northridge earthquake, magnitude 6.7, 15,000 chimneys destroyed
- 2001, Nisqually earthquake, magnitude 6.8, 1700 chimneys destroyed
Chimney failure doesn’t just occur in large magnitude events such as the recent magnitude 6.0 earthquake in Napa; it is observed in even low magnitude events. In fact, unreinforced masonry chimneys are one of the first things to fail in an earthquake. Unfortunately, many homeowners are unaware of this hazard. Even those homeowners who have their chimneys regularly maintained and serviced may not be safe. Chimney sweeps and inspectors focus on fire hazards and damage to the chimney, such as cracks in the masonry, but they are not engineers and are often not aware of the hazard that chimneys can pose in an earthquake.
Mitigation of unreinforced masonry chimneys is neither cheap nor easy. The chimney can be braced but this does not completely eliminate the hazard. The safest solution is to replace the chimney with a metal flue which can then be surrounded by a wood and brick façade. If this sounds drastic, repairs to chimneys damaged in earthquakes run well over $5000, not counting the high risk to you and your family. Having your chimney inspected and prepared for an earthquake is money well spent.
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