A Construction Technique for Earthquake Resistant Buildings

Enlarge Image Image Gallery Sill-anchoring technique for constructing earthquake resistant buildings. ILLUSTRATION: MOTHER EARTH NEWS STAFF

Although it's true that some parts of the country, such as California, are more prone to earthquakes than are others, it's also true that virtually no place in the United States is entirely safe from the threat of quake damage. All or parts of 39 of our states lie in zones classified by scientists as being at risk from seismic shocks of moderate (magnitudes of 6 to 7 on the Richter scale) or major (magnitudes greater than 7) strength. Thus, more than 70 million people in this country are exposed to significant quake-related hazards. And no state is completely free from the possibility of at least "minor" damage from these natural upheavals.

In our part of western Washington, in fact, ground tremors are fairly common . . . and the potential for a substantial quake is very real. So when I built a 24' X 40' barn and two 25' X 28' workshops about ten years ago, I incorporated a relatively inexpensive and very simple building technique that has since protected those structures through more than a decade of recurrent ground tremors ... including the considerable shaking we experienced when Mount St. Helens—just 90 miles away as the crow flies—erupted in 1980.

Earthquake Resistant Buildings

Although earthquakes can produce more than one kind of shock wave (some travel vertically, some horizontally, and others in circular patterns), it is horizontal acceleration (the side-to-side movement of the earth) that causes the greatest amount of severe damage. The concrete foundation of a building tends to move with these vibrations during a tremor, and if the above-ground portion of the structure is not firmly secured to the base, the framework can break away . . . resulting, of course, in partial—or total—collapse.

According to the Uniform Building Code (UBC), which has been adopted widely by inspection authorities throughout the country as a standard for all new construction, wood-frame structures with concrete or reinforced masonry foundations must be affixed to their bases by a specific method: First, anchor bolts of at least 1/2" in diameter (most builders use hardware that's 5/8" in diameter by 10" long) are embedded vertically 7" or more into the foundation—all along the perimeter, at intervals of no more than 6'—with a portion of each bolt projecting above the mortar or concrete. Then the building's sill plates (the bottommost, horizontal wooden members of a frame structure) are drilled so they'll slip down over the protruding anchor bolts, and once the plates are set in place—flush against the foundation—nuts are tightened down onto the wood.