I am trying to determine the seismic acceleration factors (in G’s). These factors will be inputted into CAESAR II under the uniform load. I have reviewed the California Building code 2010 and am getting results that are much larger than I have ever seen. Can some explain how they come up with this factor?

Obviously, I have no idea on what you seen previously, so it is hard to answer you...I just try to give you few ideas.

- There is an update of the codes/documents based on 2008 USGS hazard data versus codes/documents which makes use of 2002 USGS hazard data.
I'm not sure this is the root of your concern because the differences are not so big.

USGS was not explaining the differences; they rather say "High values on these maps do not necessarily indicate high ground motion values, but simply an increase in ground motion values from the 2002 update".

Moreover, I think 2010 CALIFORNIA BUILDING CODE is based on IBC-09 and ASCE 7-05, even some reference to 2008 USGS maps can be found there; anyway I'm not sure about this point.

I think the best for you is to establish which set of USGS map you need to consider- if I understand well, 2008 USGS is linked to IBC 2012 and ASCE 7 2010; 2002 USGS is the basis of IBC 2009 and ASCE 7 2005.

- There is an USGS software on USGS site that gives you spectral accelerations on locations in USA and allows you to switch between USGS maps versions.
See http://geohazards.usgs.gov/designmaps/us/application.php

The report generated by that application constructs both MCE Spectrum and Design Spectrum.
Maybe your concern is related to values of MCE Response Spectrum that are 1.5 larger than the "design" values?

See attached two reports generated by USGS application for the same location.

Just hope this help.

My best regards.

Two links to nice presentation outlines/ a 2007 USGS workshop
Ground Motions for Design
http://earthquake.usgs.gov/hazards/about..._Design(v8).pdf
and
Seismic Risk/Loss Analysis
http://earthquake.usgs.gov/hazards/about...nalysis(v3).pdf

Best regards

It is told that the FEMA recommendation to use the 2475-year earthquake was made to "provide for a uniform margin against collapse" throughout the United States. In that decision were involved also experts from California that felt 475 years return period (or 10% probability in 50 years) gives good results for California but not for the rest of the country, whereas other experts had objections on using 2475-year earthquake (2% in 50 years) for Eastern of USA.

The ratio of PGAa at 0.2 sec for (2% in 50 years/ "ASCE 7"/ 2475-years return) to (10% in 50 years/ "UBC"/ 475 years) is about 2 - but this is just a rule of thumb, however used worldwide. Now USGS software has a section of "Worldwide Seismic Design Maps" where they say for most locations:
"These values are rough approximations based on the probabilistic 10%-in-50-year peak ground accelerations (PGA's) from GSHAP. The GSHAP values are multiplied by 2 to approximate 2%-in-50-year PGA values, and then multiplied by 2.5 and 1.0, respectively, to estimate SS and S1."

The point is that it is quite difficult to have an answer to your question- why particular values of a Code are so large.
Maybe you can share with us more details on your inquiry.

Best regards.