The spectra would then have to be multiplied by I/R = 1.25/3 = 0.417.
The ASCE clearly mentions the fact that the spectra needs to be divided by R/I and so this is not something users are likely to miss. The fact that the spectra may or may not be in terms of the g values is something which may be easier to miss. For the response spectrum generation as per the IBC 2006, the spectral accelerations are computed in m/s2 or in/s2 or ft/s2 and are not in terms of g and hence once does not need to scale it further using the g.
The ASCE clearly mentions the fact that the spectra needs to be divided by R/I and so this is not something users are likely to miss. The fact that the spectra may or may not be in terms of the g values is something which may be easier to miss. For the response spectrum generation as per the IBC 2006, the spectral accelerations are computed in m/s2 or in/s2 or ft/s2 and are not in terms of g and hence once does not need to scale it further using the g.