III. Structural Problems and Strengthening Methods
C. Pier and Stilt Foundation

Many older houses are built on concrete piers or wooden stilts, without a continuous perimeter foundation. Because these footings are not connected to each other, they usually do not provide the strength the structure needs to resist shaking in an earthquake.

Pier foundations support houses on short columns attached to small concrete blocks. These structures need to be braced by a perimeter foundation and strong cripple walls. If access from the exterior allows, a new perimeter foundation can be constructed without shoring up the entire structure. The perimeter of the building can be lifted up slightly, the foundation built, and the house securely connected to the new foundation (see Figure). If access is limited, the house may need to be shored up for the foundation reconstruction.

Stilt supports generally are used in houses on steep hillsides, and involve tall 4 x 4's holding up the house on the downslope side. These stilts are generally very flexible, which makes the building vulnerable to earthquake damage. Stilts can be strengthened by adding diagonal braces or plywood walls between the stilts. Additional bracing should be added at the corners beneath the main structure.


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