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How to Improve Your Home’s Earthquake Resistance

The following is an excerpt from When Technology Fails: A Manual for Self-Reliance, Sustainability, and Surviving the Long Emergency [1] by Matthew Stein [2]. It has been adapted for the Web.

Ask anyone who has been through a major earthquake, such as the Loma Prieta or Northridge quakes, and they will tell you that a serious quake can be a terrifying event. Often the main differences between structures that pulled through in relatively good shape and those that got the “bulldozer remodel” (dig a hole with a bulldozer, push the house into it, start over) were whether the building was of older construction and the contractor was sloppy with shear nailing of the building’s exterior plywood siding or there was insufficient use of “hurricane clips” and “Simpson Strong-Ties” to properly brace the frame.

Shelly and Phil Rodgers were in their home in California’s Santa Cruz Mountains when the Loma Prieta quake struck. The epicenter was about 7 miles from their home. The house shook violently and all of their cupboards opened, throwing every dish, jar, can, bookcase, television, and appliance to the floor. Phil said that the house floors undulated like a snake, appearing to change elevation by more than a foot in different parts of the house as the quake shook through. They were not able to leave the house until the earth stopped moving. Because their car keys and shoes were still inside, Phil had to brave the aftershocks and wade through broken glass to retrieve keys and shoes so they could attempt the drive to town to pick up their kids. He brought a chainsaw with him, which was needed to cut large limbs that had fallen across the road.

On their way to town, they passed the spot where a neighbor’s house should have been. It had been built on tall pylons overlooking the hillside. When the quake struck, it slid off the piers and down the canyon. The two occupants on the first floor managed to crawl out the door moments before it took off, but their son, who was sleeping on the second floor, went for the wildest ride of his life. He miraculously rode through it uninjured, as the first floor disintegrated and the roof split away and to the side.

Shelly and Phil’s home was of post-and-beam construction with massive timbers. Their contractor had paid careful attention to all the exterior plywood shear nailing and steel frame ties. Remarkably, not one window was broken and the house suffered no structural damage, even though many of their personal items were destroyed (they cleaned up their dishes and pottery with a shovel).

When an earthquake strikes, the most damaging motion is usually not the vertical but the horizontal component of the earth movement. The mass of the building tends to want to stay in one place as the earth moves side to side. The resulting sideways forces on the walls, which tend to push the walls into a diamond shape rather than a rectangle, are referred to as “shear forces.” As the mass and height of a building increase, the walls will be subject to greater shear forces in an earthquake.

Improving Earthquake Resistance

Recently, through the use of modern analytical tools and by studying what worked and did not work in actual earthquakes, seismic design has made a lot of progress. Although there is no way to “earthquake-proof” a structure, there are numerous ways to increase its earthquake resistance.

At some level of seismic severity, all structures will fail. However, good seismic design can dramatically increase the earthquake resistance of a structure. A few factors that affect the earthquake resistance of a building are described below.