The following illustration shows earthquake forces pushing on the cripple wall causing the house to collapse, and the placement of plywood shear panels that prevent collapse of the house.

As the illustration shows, the cripple wall is subjected to the same force that can push a poorly bolted house off its foundation.  If the cripple wall is not strong enough to resist that force, it falls over and the house collapses.  This happens because the siding used on older homes, whether wood or stucco, is relatively weak and provides little support or bracing to the cripple wall.  Structural grade plywood properly nailed on the cripple wall in the form of a shear wall is almost 20 times stronger than typical siding material...that is a 2000% (two thousand percent) increase in strength.

Code Requirements:
The two retrofit building codes, i.e. The Uniform Code of Building Conservation and The Los Angeles Building Code, specify the type of plywood to be used in building shearwalls, the types of nails to be used and spacing of the nails, methods of attaching shear panels to the mudsill, and methods of attaching them to the floor of the house. The American Plywood Association also has published guidelines that address proper shear wall construction. However, because no Bay Area cities have adopted a retrofit building code (see False Security) there are no code requirements in the Bay Area for retrofit shearwalls.

The next part of this Retrofit Design segment is Shear transfer ties: attachment of the floor framing to the mudsill or top of the cripple wall.

Back to Retrofit Design