Foundation, Structural Framing

If you've been itching to spend some of that equity your home has earned in recent years, consider using it to shore up your home against the seismic forces of an earthquake.

Two recent moderate quakes don't specifically forecast future seismic activity, but the two wake up calls -- literally -- should stir some home owners to at least give their homes a shake down to determine if an earthquake retrofit is warranted.

Upgrades your home may need can not only help protect you and your family from injury or worse, retrofits also can help make your home more saleable.

When many households were just about to rise at 6:50 a.m. on April 20, a magnitude 5.1 shaker hit 15 miles southwest of Plattsburgh, NY.. Proving California isn't the only quake-prone state, the Plattsburgh pounder was felt from Cleveland, OH to Maine and Baltimore, MD.

A few weeks later, when households on the other coast were just about ready to nap for the night at 10 p.m., a magnitude 5.2 quake rumbled through a San Andreas fault zone three miles southwest of Gilroy, CA. With the epicenter about 30 miles south of Silicon Valley, quake shaking extended to San Francisco, 100 miles north of the epicenter and beyond.

Assessing risk

Earthquake faults are abundant primarily in the West, the Northeast and in the Midwest, but homes in a given location in those regions may or may not need additional shoring up. Determining if you should purchase a quake-related home improvement requires an examination of several factors. In most cases, a conventional home inspector can't do the job. A soils engineer, geologist, structural specialist or all of the above may be necessary to give your home a real once over.

Seismic proximity. Generally, the nearer you are to quake faults, the greater likelihood of damage to your home. You can determine how near your home is to a seismic hazard with U.S. Geological Survey maps, but analyzing mapping data is matter often left to a geologist.

Seismic activity. If your home sits on an inactive fault, there's less chance it'll get shaken off its foundation than if it sits on an active fault. Even moderately active faults stress structures more frequently and pose a greater threat than less active or inactive faults. Determining the true activity of any fault, however, is an inexact science.

Soils. Soils engineers say if your home is constructed on bed rock the land form will absorb seismic energy much better than landfill or silty, sandy and loamy soils The later can liquefy and amplify seismic energy. Flat land is also safer than sloped land, which also can be more prone to land slides.

Construction. The age of your home is a factor too. Newer homes are generally safer than older homes because of newer building codes' seismic considerations. A general home inspector can tell you if he home is bolted to the foundation and otherwise retrofitted or constructed with ample strengthening techniques.

"Soft-story" construction -- first floor garages or other open spaces -- provide little support for enclosed structures above them. Homes with "cripple" or "pony" walls between the foundation and first floor, reduce the strength of a home.

Newer homes with slab foundations are also slightly safer than perimeter spread footing foundations. Both are safer than hillside homes or homes built on stilts or piers.

The inherent rigidity -- the inability to "give" and absorb energy -- in unreinforced masonry poses the greatest structural threat.

Even moderate quakes like the recent ones in New York and California can topple chimneys and reduce masonry fireplaces to rubble.

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