Sunday, September 25, 2011

Seismic design changes - 2012 IBC

DesignMaps Application and 2008 Design Data
USGS has released an updated DesignMaps application which is a great application.  However, before you start applying it to 2009 International Building Code please make sure to read their FAQ page which states:
This application should currently only be used to investigate which design values will likely be mandated in the future.

This recommendation by USGS is based on the hierarchy of the codes and standards, which is as follows:
2003 IBC2006 IBC2009 IBC2012 IBC
ASCE7-02ASCE7-05ASCE7-05ASCE7-10
USGS 2002USGS 2002USGS 2002USGS 2008

The DesignMaps application utilizes information from the 2008 USGS National Seismic Hazard Maps information.   For 2008 the hazard values in the Central and Eastern U.S. have been reduced by 10-25% in many cases, and most 1-second period ground motion values for the Western U.S. have also been reduced, in some cases by as much as 30%.

Example differences between 2002 and 2008 Design Data
Lets look at a comparison of several values based on the 2002 vs 2008 hazard values.  (Examples based  Site Class 'D' and Occupancy Category of 'II').
Atlanta, GA (Lat 33.7489954, Lng -84.3879824)
Data SourceSsS1SdsSd1Sds Design CatSd1 Design Cat
2002 Design Data0.2260.0850.2410.137BC
2008 Design Data0.1850.0900.1970.144BC

Sacramento, CA (Lat 38.5815719, Lng -121.4943996)
Data SourceSsS1SdsSd1Sds Design CatSd1 Design Cat
2002 Design Data0.5910.2440.5230.311DD
2008 Design Data0.6750.2940.5670.355DD

Casper, WY (Lat 42.866632, Lng -106.313081)
Data SourceSsS1SdsSd1Sds Design CatSd1 Design Cat
2002 Design Data0.3700.0770.3710.123CB
2008 Design Data0.2770.0740.2910.118BB
As seen in the last example for Casper, Wyoming, seismic bracing for fire sprinkler piping is not required based on which design criteria is applied.  The Sds value is 21% less in the 2008 values.  So whether you utilize the 2002 or 2008 hazard data can make a significant difference on your project.

Anvil Fire's Seismic Force Calculation App provides values based on the 2002 hazard values published by USGS and is basically identical to the Java Ground Motion Parameter Calculator provided by USGS.  Consult with your AHJ, but at this time we recommend that you follow USGS's recommendation to only use the DesignMaps application to investigate the requirements of the future 2012 International Building Code.

For more detailed information on how to determine the seismic design forces for fire suppression systems, please read our blog post on Seismic Design For Fire Sprinkler Systems.

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