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There's more to structural design than just making sure that a building doesn't fall down under snow loads or working loads. It also has to stay up and hold together under horizontal loads on the sides and on the roof. These loads come from two sources: wind gusts and earthquakes. If you doubt this, think back a couple of months to the evening news on central Oklahoma, or worse yet, the pictures following Hurricane Andrew. On light wood-framed buildings of one to three stories, a little calculation shows that the wind effects are far greater than the seismic effects, at least up through seismic Zone 3. Most code officials assume that things are okay seismically if the designer