How to calculate probabilistic MCE? 1

[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?

 

[msucog]

Consult a geotech knowledgeable on the topic.

The IBC maps provide Ss and S1 which will lead you to these values (Sds and Sd1) you are looking for after accounting for site soil conditions for the ~2500yr return period (2% in 50yrs & 5% damped). There are threads discussing how to determine Sds. I would suggest consulting with the project geotech. For a PSHA/DSHA site specific seismic assessment, utilize a knowledgeable geotech since it is not a "simple" task to learn. I would normally suggest performing the site specific and attempt to capture some of the seismic design parameter reductions allowed by code. I'm not familiar with the reduction potential (if any) that you might get in that part of the country.

Good Luck


Ryan Coggins, P.E., S.I.

http://www.linkedin.com/in/ryancoggins

 

[EngineerofSteel]

Thanks, msucog,

That is what I needed to know- that the maps in ASCE7 are for the 2% in 50 year probability. I could not find that information.

Do you know where this is?

 

[msucog]

It should say on the map itself (in the little box at corner of map) or in the documentation. Be aware that there are other return periods depending on what you're dealing with too (IBC, AASHTO, etc). But for structures in the U.S., under IBC 2006 2%/50yr is "typical". Also be aware that these maps are revised periodically depending on the Code year.

http://earthquake.usgs.gov/hazards/products/conterminous/2002/maps/

Ryan Coggins, P.E., S.I.

http://www.linkedin.com/in/ryancoggins