How to calculate probabilistic MCE?

[EngineerofSteel]

The county engineer is asking for a complete soil investigation report per 2007 CBC (IBC 2006, pretty much), chapter 18, section 1802.2.7

The permit is ready to be issued. 1802.2.7 has this exception:

no study is required provided that peak ground acceleration is equal to SDs/2.5 is used. SDs must be determined according to ASCE7 21.2.1:


ASCE7-05, Section 21.2.1 Says "Probabilistic MCE. The probabilistic MCE spectral response accelerations shall be taken as the spectral response accelerations represented by a 5 percent damped acceleration response spectrum having a 2 percent probability of exceedance within a 50-yr. period."

This is confusing me. The USGS factors are given as a .2 second spectral acceleration. 21.2.1 asks for SDs using a 2 percent probability.

No idea how to get the SDs value. Any help?

(Note: this is a duplicate-- previously posted in foundation engineering forum, where it collected e-dust.)

 

[EngineerofSteel]

Here is what I have concluded:

I have not been able to find the basis for SDs. So, I am guessing that SDs IS the 2 percent probability statistic, and I do not need to convert this value.

I used the Fa value from the USGS Site Coefficient generator, and interpolated backward using Table 11.4-1 to get Ss. From here I used SMs = Fa * Ss and SDs=2/3*SMs.

Can anyone critique my assumption?

 

[EngineerofSteel]

Well....

The architect for whom this work was performed found an ICC report stating that this particular home MUST have a soil report... I am receiving this doc as a fax now...

But, if anyone can answer the question about the probability basis of SDs in ASCE7, I would like to know!

 

[EngineerofSteel]

The ICC paper referenced 1802.2-- which is not about Seismic Design Category D.

18.2.2.7 is the exception we tried to use.

The actual Cs for the site was .088. SDs/2.5 came to .229. In the N-S direction, this was doable. In the E-W direction (narrow dimension), this resulted in much higher concentrations and large hold-downs and increased footings. So, the owner will pay $1200 for a geo report.

 

[wannabeSE]

Dairy Designer,
You seem to be confusing a site specific ground motion study with a soils investigations. A soils investigation may or may not include a site specific ground motion study. Look at ASCE 7 Section 11.8.3 for requirements similar to the IBC but presented differently.