damguy
Geotechnical
- Oct 2, 2003
- 8
I am a geotech trying to understand the requirements for site-specific analysis summarized in the 2003 edition of NEHRP and on the IBC 2000. The description provided in the NEHRP Chapter 3 Commentary gives a clear approach to obtain a site-specific, "ground" design spectrum. However, the description provided in Section 3.4 of NEHRP is not that clear, but confusing. I would appreciate if someone who has used this procedure before could answer a few questions I have:
1) The MCE rock spectrum used to select the input acceleration time histories for the analyses should be that spectrum with a 2% probability of exceedance in 50 years according to Section 3.4. The commentary indicates that the MCE rock spectrum can be, for example, that from a Site Class B rock if a site-specific analysis was not carried out. Does this mean that if I don't have a 2% in 50 year spectrum I can use the Site Class B rock spectrum?
2) The deterministic MCE ground motion is used instead of the 2% in 50 years MCE spectrum in regions of known active faults when this is lower than the probabilistic. Does this mean that the spectrum I use to select my input time history accelerations should now be the deterministic for this special case?
3) As described in the commentary, after I obtain a surface design spectrum from an average ratio of response spectra curve and the rock spectrum, do I compare this surface spectrum to the requirements of Section 3.4? i.e. should my surface design spectrum be less than 2/3 of the MCE rock spectrum? Or I shouldn't even be bothering to compute my surface design spectrum as described in the commentary, but use 2/3 of the probabilistic/deterministic MCE?
Any help will be appreciated.
1) The MCE rock spectrum used to select the input acceleration time histories for the analyses should be that spectrum with a 2% probability of exceedance in 50 years according to Section 3.4. The commentary indicates that the MCE rock spectrum can be, for example, that from a Site Class B rock if a site-specific analysis was not carried out. Does this mean that if I don't have a 2% in 50 year spectrum I can use the Site Class B rock spectrum?
2) The deterministic MCE ground motion is used instead of the 2% in 50 years MCE spectrum in regions of known active faults when this is lower than the probabilistic. Does this mean that the spectrum I use to select my input time history accelerations should now be the deterministic for this special case?
3) As described in the commentary, after I obtain a surface design spectrum from an average ratio of response spectra curve and the rock spectrum, do I compare this surface spectrum to the requirements of Section 3.4? i.e. should my surface design spectrum be less than 2/3 of the MCE rock spectrum? Or I shouldn't even be bothering to compute my surface design spectrum as described in the commentary, but use 2/3 of the probabilistic/deterministic MCE?
Any help will be appreciated.
