The History Of Seismic Retrofit Guidelines In The United States
These guideline evolved from two separate branches. The first branch was started in 1994 with the publication of the The Seismic Retrofit Guidelines for Existing Building and it is the source of most of the guidelines. Its current iteration can be found in Appendix Chapter A3 of the International Existing Building Code. There was a break in continuity when the chairperson of that committee, Jim Russel, told me he wanted to start over from scratch because of its many deficiencies and asked that I be a member of another committee he chaired that wrote Standard Plan A which was published in circa 2006 and revised in 2008. Standard Plan A is the primary source for FEMA DR-4193-RA2 Earthquake Strengthening of Cripple Walls in Wood Frame Buildings which is remarkably similar to FEMA P-1100: Vulnerability – Based Seismic Assessment and Retrofit of One and Two-Story Dwellings.
Except for these two guidelines that are based on Standard Plan A, all the other guidelines are rooted in The Seismic Retrofit Guidelines for Existing Buildings.
Here are all the guidelines and related material I know about.
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 Strong-Tie Homeowner Guideline
FEMA P-50-1 Seismic Retrofit Guidelines for Detached, Single-Family, Wood-Frame Dwellings
FEMA P-1100 has been modified slightly and will soon be published as 2022 ICC 1300. This will be a national standard published by the International Code Council. This 290 page guideline is the best funded of all the retrofit guidelines and remarkably it is the most expensive. If you go to this webpage you will find a comparison between it and Standard Plan A.
I am often asked “How big of an earthquake will my house withstand?” Retrofit designs are based on anticipated ground acceleration (g), not the Richter or Modified Mercalli scales.
Seismic retrofit designs use the Base Shear formula, which measures the amount of earthquake-generated shear force measured in Gs’ that will accelerate under the house. This force 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.
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.”
