There are no actual seismic retrofit building codes. There are only guidelines. Here are a few.
Appendix Chapter A3 of the International Existing Building Code
Los Angeles Department of Building and Safety Standard Plan Number 1
Project Impact – The Retrofit Guidelines for the City of Seattle
FEMA DR-4193-RA2 Earthquake Strengthening of Cripple Walls in Wood Frame Buildings.
I am often asked “How big of an earthquake will my house withstand?” Retrofit designs are based on anticipated ground acceleration, not the Richter or Modified Mercelli scales.
Seismic retrofit designs use the Base Shear formula, which measures the amount of earthquake-generated shear force measured in Gs that will try to push the house off its foundation base. This forces is determined by multiplying the expected ground acceleration (Gs) by the weight of the building. The product equals the amount of earthquake force that will strike the house, measured in pounds.
Earthquake don’t “strike houses.” Instead they accelerate under the building, and when this ground movement stops, the house will fly off the house due to momentum. To make this concept easier to understand we look at it as if the earthquake were striking the house.
For example, if the code expects a ground acceleration of 0.2 Gs, and the house weighs 100,000 pounds, the base shear will be 0.2 x 100,000# = 20,000# of shear force striking the base of the building. As a formula it is expressed as V (base shear) = 0.2GA x 100,000#= 20,000# base shear. This video on earthquake engineering and base shear explains it.
Astonishingly, there is no set amount for the anticipated ground acceleration. Los Angeles anticipates a different base shear than does the California Building Code. The International Existing Code recognizes yet a different ground acceleration. In light of this, it is always best to use a base shear that is tailored to your geographic region. In the Bay Area’s case, this would mean using the base shear found in Standard Plan A , though as you will see using 75% of Standard Plan A’s ground acceleration is quite rational.
Retrofit Design Provision for New Buildings and the California Building Code.
CURRENT CODE for NEW CONSTRUCTION
Single Story V=0.17W
Two story V=0.19W
Three Story V= 0.20W
City of Los Angeles Retrofit Guidelines
V = 0.143
Retrofit Design Provisions in the 2013 International Existing and Historic Building Codes
“The details and prescriptive provisions herein (the building code) are not intended to be the only acceptable strengthening methods permitted. Alternative details and methods may be used where approved by the code official
Designs shall be in accordance with all requirements of the building code, except that the base shear may be taken as 75 percent of those specified in the building code.”
SECTION 8-706 LATERAL LOAD REGULATIONS
“The forces need not exceed 0.75 times the seismic forces prescribed by the regular code requirements”