Seismic Retrofitting

Seismic Retrofitting Basics

The horizontal (or lateral) motion in earthquakes create the forces that can damage wood framed homes. Homes tend to fail in one of three ways – all of which are between the foundation and the first-floor joists.

  1. The sill plate can slide off of the foundation.
  2. The cripple wall (or pony wall) can collapse (or rack).
  3. The floor joists can slide off the mudsill or cripple wall.

There are a few variations in these themes, but most of our work involves installing structural elements proven to prevent these three types of failure. These standards and methods, developed in 1999 by Seattle Project Impact, are consistent with other building department permit requirements. Insurance providers and structural engineers generally ask that the same methods be used.

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Basic Concepts: How a Seismic Retrofit Works;
To keep a house from falling off its foundation in an earthquake, seismic retrofit strengthens three different areas of the house. These areas are all located in the basement and/or crawl space. If any one of these three areas are not adequately retrofitted, the house will be susceptible to damage in an earthquake.
These three areas are:
  1. Bracing the cripple walls with plywood.
  2. Bolting the braced cripple walls to the foundation.
  3. Attaching the floor of the house to the braced cripple walls.