The one feature all of these guidelines is that they do not apply to any house that has cripple walls over 48″ tall. Last I looked at the laws of physics do not change at 49″ but then again, I did not go to engineering school.
These guidelines were written by engineers and my suspicion is that they wanted all the jobs where the cripple walls are over 4 feet tall for themselves. Under 4 feet and you might need to crawl under the house and get your clothes dirty.
You can crouch or even stand for cripple walls over 48″. Why not take these easy ones and leave the dirty low-crawlers to the contractors?
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
Retrofit Guidelines for the City of Portland
Simpson StrongTie Homeowner Guideline
Special Design Provisions for Wind and Seismic
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
Section A301.3 Alternative design procedures:
“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.”
California Historic 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”
