Anchor bolts and shear walls are the way to beef up older homes in earthquake-prone areas.
Synopsis:What to do with an older house built in an earthquake-prone area that lacks the metal connectors and structural stiffeners required by today’s codes? The author, an engineering specialist in the Bay Area, explains how to tackle the problem.
Seismologists and geologists tell us that parts of the West Coast and the rest of the United States mainland are on two different, shifting tectonic plates. Geologically speaking, the communities on the western plate are seceding from the rest of the Union at a rate of approximately 2 in. per year. It doesn’t take an earthquake to see the results of this movement. Even in quiet times, you can see the effects on the land in this area. It cracks and subsides, slides and creeps, occasionally winding up on the neighbor’s side of the property line.
During a sizable earthquake, such as the 6.7 shock that hit Coalinga, Calif., in 1983, things happen in a hurry — especially to older homes. Foundations fail; gas lines break, causing fires; framing connections prove inadequate; and unreinforced masonry crumbles.
I work for a company in the San Francisco Bay Area that specializes in upgrading older homes to meet current standards of structural integrity. We work closely with soils engineers, structural engineers and civil engineers to develop strategies for reinforcing complicated structures. We also do work on simpler buildings, which can be strengthened following some basic guidelines. This article talks about those guidelines, and the tools and materials that we use to reinforce older wood-frame houses.
Most of the houses that we upgrade are built upon short stud walls, called cripple walls, which in turn bear on concrete foundations. In my 10 years of experience in this business, I have seen enough old buildings to draw some conclusions about what makes them fail in an…