Building Codes, Base Shear, and your Seismic Retrofit

By Howard Cook, Founder and General Manager of Bay Area Retrofit

Building Codes and Seismic Retrofitting

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

The Bay Area’s regional Standard Plan A  The author was on the committee that wrote this.

Project Impact-The Retrofit Guidelines for the City of Seattle 

Appendix Chapter A3 of the International Existing Building Code

Los Angeles Standard Retrofit Plan 1

Chapter 92 of Los Angeles Building Code

The best recent Retrofit Guidelines were published by the Federal Emergency Management Agency after the Napa Earthquake.

Building Codes are based on a simple formula  called base shear formula which measures the amount of earthquake-generated shear force that will try to push the house off the foundation base.   This forces is determined by multiplying the expected ground acceleration  by the weight of the building.  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.  

Remember, “V” stands for ground acceleration measured in Gs.  W is the weight of the house.   Let’s assume the house weighs 100,000#.   If we say V = 0.2W it means 10,000 pounds of force will be striking the base of the house.

As demonstrated below, base shear varies very slightly from code to code.

 

Retrofit Design Provision for New Buildings and the California Building Code.

2013 California Building Code and Seismic Retrofitting

 

 

CURRENT CODE for NEW CONSTRUCTION

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

DBS

LA Building Code

 

 

 

 

 

 

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

SECTION 8-706 LATERAL LOAD REGULATIONS

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

 

Bay Area Retrofit

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Bay Area Retrofit
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Albany, CA 94706
With field offices in San Jose and Hayward.

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