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IBC and IRC seismic zone ?


Oct 17, 2009
Big Sky Country
Why does the IRC have multiple "D" seismic zones and the IBC only has one "D" zone. Can anyone explain the significant differences.

I would like to simplify the design requirements between the 2 codes but not sure how to do that,
We are an IBC "D" zone and an IRC "D1" zone
You can theoretically prescribe a Seismic Design Category (SDC) for IRC residential; however, for IBC buildings, the SDC should be case specific and will typically be classified under the geotechnical report. See IBC Section 1613.

Also, note that the IBC considers both the SDS and SD1 whereas the IRC only typically considers the SDS.
Just an anecdote, but I was with an AHJ where under the IRC, we were SDC C.

However, nearly every project built to the IBC was classified as with an SDC of D. This was due to the SD1 being greater than 0.2 (see Table 1613.2.5(2)). A few projects, through the geo report, were able to get out of Site Class D (not a seismic classification, but a classification of soil/rock below the site).
The question is asked is why are the seismic zones different in the IBC and the IRC? Both codes recognize that some locations have greater exposure to earthquake shaking but where do we draw the lines. The answer likely has more to do with the fact that different people were on the two committees drafting the proposals. It is more important that the codes be complied with than we try to rationalize the differences.