Earthquakes, Part II

Q. I was wondering if you know about the possibility of an earthquake on Long Island or what kinds of preventive things I could do to my house. After watching what's happening in Haiti, I'm worried about what would happen here.

A. Historically, higher-intensity earthquakes are rare on Long Island. The results could cause the ground to behave like water, a phenomenon known as "liquefaction." As with hurricanes and floods, buildings are required, by code, to have reinforcement to withstand earthquake forces.

Although it's rare that Long Island experiences tremors that are felt, earth movement regularly occurs and must be addressed. The average home has little defense against serious movement, except the redundancy of wall studs and ceiling joists, acting together, yet acting independently of one another.

Think of a centipede. If it lost a few legs, it would still be able to get around. A building with many separate supports also has the ability to transfer loads from one "leg" to another. With their many legs, most buildings have the redundancy to survive loads by bowing, twisting and heaving, as needed. The World Trade Center had limited redundancy because there were few internal supports, only an outside structure.

The elevator shafts were, for the most part, not supporting the structure.
Your home must withstand several types of pressures, from internal and external positive and negative pressures caused by wind to uplift caused by expansion of the ground and heat buildup of the building's parts that are exposed to sun and weather. Forces such as sheer, which sounds exactly like what it is -- the action of sheering apart, like the effect of a big scissors cutting through. This effect is usually caused by wind, but earthquakes cause wracking that will also sheer one wall surface past another, leading to collapse.