An SE consultant said he can support Site Class "C" for our project, but the geo-tech characterized it as Site Class "F". The B.O. defers to the geo-tech and kicks the design back to us for rewrite. As nearly as we can tell in IBC 2003, loads must be increased to "not less than 80% of Site Class "E"", which would be, variously, more *or less* than Site Class "C" on Fa for Ss>1.00 and Fv for S1>0.4. The SE says no worries, mate, use the original values for Fa, Fv, Sms, etc. and just change the notes to say Site Class "F". Are we being stroked? Does change from "C" to "F" have design impact, if the geotech already did the site-specific evaluation establishing Ss & S1?

The "new" thing that the IBC introduced (as compared with the UBC) is the strong impact that site class has on the seismic demand of a building.  Going from C to F can be huge in terms of seismic applied forces to a structure.

JAE... I think the current seismic thinking is along the lines of NEHRP... which embraces the site issues... on a bit of a learning expedition currently... been about 3 years since I've done any seismic stuff and the new NBCC follows the NEHRP.

Seems reasonable since the amplitude of a wave going from a dense medium to a less dense one increases dramatically.

Dik