The primary purpose of seismic retrofitting is to keep your home or business safe from being displaced off of its foundation by an earthquake. Western Washington is on a dormant fault line, which home construction experts previously didn’t consider when building the frames and foundations of our homes. Today’s experts understanding of earthquakes have improved, giving us the opportunity and knowledge to retrofit any home to prevent failure from future seismic activity.
How Homes and Businesses Suffer Seismic Failure
Homes and businesses typically fail because of damage to the wooden frames. The three main types of failures are called sliding, racking, and overturning. These occur at the foundation or the cripple wall (the wall section between the foundation and first floor joists). The cripple wall is commonly considered the weakest part of older homes.
Sliding occurs when the home slides off of the foundation or cripple wall. Racking occurs when the cripple wall buckles and collapses. Finally, overturning happens when the house topples from the foundation or cripple wall.
Cripple Wall Seismic Retrofitting Needs and Solutions
A cripple wall is the short wood frame wall in the crawl space between your home and the foundation. The height of the cripple wall can be a few inches to several feet. This area of any unprotected home (or business) is the number one cause for seismic failure. If the cripple wall collapses, typically the house will fall to the ground and foundation.
In some cases a home may not have a cripple wall. In this scenario, the home is somewhat less likely to be displaced from its foundation, but the risk is still considerable. The first floor frame and foundation are connected by “toenails,” which can often result in sliding. While the house may only slide a few inches, portions of the floor may sag and utility connections may be severed.
When a home has a cripple wall there are two typical methods of seismic retrofitting: bracing and foundation holdown brackets. The first step is bracing, or stiffening the cripple wall to keep it from collapsing. This process is similar to building strengthened walls and flooring around the cripple wall’s joists, thus turning the weakest point of a home into a shear wall, or a wall capable of withstanding shear forces (earthquake forces).
Sometimes a home will require more than bracing, needing foundation holdown brackets. These are right-angle brackets and bolts that connect the cripple wall to the foundation, preventing a shear wall from lifting or rolling. Simpson Holdowns are installed at the ends of shear walls, held down by epoxy-anchored bolts.
If a home does not have a cripple wall, Simpson anchors make all the difference in the world. These homes require special inspection because of the different options for the different home situations.