The History Of  Seismic Retrofit Guidelines In The United States


There are no actual seismic retrofit building codes.  There are only guidelines.  Here are a few.

Standard Plan A  

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 Plan Set

Simpson StrongTie Homeowner Guideline

FEMA DR-4193-RA2  Earthquake Strengthening of Cripple Walls in Wood Frame Buildings. 

FEMA P-50-1 Seismic Retrofi t Guidelines for Detached, Single-Family,Wood-Frame Dwellings

PEER Quantifying the Performance of
Retrofi t of Cripple Walls and Sill Anchorage in SingleFamily Wood-Frame Buildings”

FEMA P-547  Techniques For The Seismic Rehabilitation of Existing Buidlings

FEMA: Vulnerability – Based Seismic Assessment and Retrofit of One and Two-Story Dwellings

FEMA P-1100 2A Plan Set


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 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.

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.

2013 California Building Code and Seismic Retrofitting




Single Story V=0.17W

Two story V=0.19W

Three Story V= 0.20W




V = 0.186 W

Standard PLan A Retrofit Guidelines





City of Los Angeles Retrofit Guidelines

V = 0.143








Retrofit Design Provisions in the 2013 International Existing and Historic Building Codes


California 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


“The forces need not exceed 0.75 times the seismic forces prescribed by the regular code requirements”