Background:
After the 1989 Loma Prieta and the 1994 Northridge earthquakes, there was a push in California to get cripple wall homes retrofitted. There was a building code for new construction, but there wasn’t any guideline or standard for the retrofit of an existing home with a cripple wall vulnerability. At that time a homeowner could hire an engineer to design a retrofit specifically for their home, but there was no consensus document on the best way to mitigate the cripple wall vulnerability.
There were also a growing number of construction companies that marketed seismic retrofits but, once again, these companies were not providing retrofits designed to any statewide standard.
In the late 1990s studies were performed in Northern and Southern California that found around 80% of cripple wall retrofits were not done properly. To remedy this serious issue the LA building department and a committee in Northern California, the Association of Bay Area Governments (ABAG), created “plan sets” that would provide complete and effective standards for cripple wall retrofits (i.e. The LADBS Standard Plan #1 and Standard Plan Set A). The Association of Bay Area Governments (ABAG) website identifies the Northern California committee members as representatives from the East Bay, Peninsula, and Monterey Bay chapters of the International Code Council (ICC), along with representatives from the Association of Bay Area Governments (ABAG) Earthquake Program, the Structural Engineers Association of Northern California (SEAONC) Existing Buildings, Seismology and Structural Standards Committees, the California Building Officials (CALBO) Seismic Safety and Emergency Preparedness Committees, the Earthquake Engineering Research Institute (EERI) Northern California Chapter, and building contractors specializing in home retrofit. In order to promote affordable retrofits, Plan Set A and LADBS Standard Plan #1 were designed as prescriptive retrofit standards that could be used by owner-builders or contractors without an individual engineered retrofit plan.
In 2011 the State of California adopted Chapter A3 into the California Existing Building Code. Chapter A3 is a prescriptive seismic retrofit of the sill plate anchorage and cripple wall bracing vulnerability. Both Plan Set A and the LA Standard Plan Set #1 are considered to be construction documents that are in conformance with Chapter A3.
The Earthquake Brace + Bolt program requires that retrofits be in accordance with Chapter A3 in order to qualify for the financial incentive of up to $3,000. To simplify the permitting process the EBB program also allows the use of Plan Set A or the LA Standard Plan Set #1 as construction documents.
Plan Set A/LADBS Standard Plan #1 vs a “designed” retrofit:
In order to cover the variations in houses, seismic hazard, and provide a margin of safety, Plan Set A/LADBS Standard Plan #1 are slightly more conservative than Chapter A3. Consequently, it may be possible to have an engineered design that specifies fewer anchor bolts or foundation anchors and less plywood sheathing. However, the cost of the engineering design would likely exceed the cost savings.
Please note that an engineered design must be completed by a civil or structural engineer registered in California or a licensed architect. A contractor is not legally qualified to design a retrofit. Also note that a permit is required throughout California for the seismic retrofit work. The EBB program does accept engineered and permitted retrofits provided they meet these legal standards and other program requirements.