Seismic Strengthening

When an earthquake occurs, your home will have a better chance of surviving and may only sustain minimal damage. Also, retrofitting may lower your earthquake insurance premiums and deductible. In some cases, insurance companies will not insure a home that is not strengthened. Increasing your home’s earthquake fitness enhances its value and improves its salability.

Why Earthquakes Damage Post and Pier Houses

Two major weaknesses occur in the load path of houses supported by a post and pier foundation. One is the absence of sheathed walls below the first story exterior walls. The other is the absence of an adequate foundation under those perimeter walls.

Base Shear and Cripple Walls

A cripple wall is a wall that is less than full story height. The cripple wall usually occurs between the first floor and the foundation, and is generally the most vulnerable part of older buildings. These cripple walls are weak because they are typically sheathed with only stucco or horizontal wood siding on the exterior side of the wall. These sheathing materials are weak wall-bracing methods for seismic loads.

Because seismic forces in the building accumulate all the way down to the ground, they are greatest at the base of the building. The seismic force at the base of the building is called the “base shear”. Earthquakes often damage buildings at this level. For buildings with cripple walls, this means that weakest part of the building must resist the greatest force. This is why retrofit standards require strengthening of the cripple walls.

Wood Destroying Organisms

Another important consideration before doing any earthquake or seismic retrofitting of any wood framed building is to examine all the existing wood members to be used in the load path and determine if they need to be replaced because of fungi infections, commonly called dry rot, that destroy the wood fibers.

The Actual Seismic Inspection

Typically the seismic inspection will take 2 hours, and then a one or two page report will be generated on the findings.