An Earthquake Brace + Bolt (EBB) retrofit includes both "bolt-only" and "brace and bolt" retrofits.
Bolt-only Retrofit
A bolt-only retrofit is needed if the wood frame of the house sits directly on the foundation (known as a stem wall) in the crawl space under the house. The foundation is bolted to the wood frame of the house.
Before:
This is a wood-framed house that sits directly on the foundation, so it requires a "bolt-only" retrofit.
After:
Note the metal foundation plates (bolting) around the perimeter of the foundation.
Brace and Bolt Retrofit
A brace and bolt retrofit is needed if there are short (4 feet or less) walls between the wooden frame of the house and the foundation (known as cripple walls) in the crawl space under the house.
Before:
This is a wood-framed house that sits directly on the foundation, so it requires a brace and bolt retrofit.
After:
Note that the cripple walls have been braced with plywood sheets, and the foundation has been bolted to the structure of the house.