Probabilistic MCE 1

[lana02]

Aawk this new code! CBC2007/ASCE 7-05

The IBC/CBC Section 1802.2.7 has an exception that allows a design in SDC D without a soils report if the peak ground acceleration of Sds/2.5 is used and Sds is determined per ASCE 7 Section 21.2.1. This section refers to a probabilistic MCE (2% in 50 yrs). Looking on the USGS Seismic Hazard Map web site,
(

http://gldims.cr.usgs.gov/nshmp2008/viewer.htm

)one can obtain probabilistic peak, .2sec and 1 sec ground accelerations.

For my particular site at 39 degN & 121 degW I get:
Peak Sa= 17.3%, Ss=42.7%, S1= 16.3% and applying the formula:
Cs=Sds/(R/I)=0.427/(2.5x6.5/1)=0.026 is obtained.

A base shear of only 2.6%W! This does not make sense to me. Using Sds/2.5 reduces the base shear below that of the standard procedure outlined in ASCE 7 Sec. 11.4. Also, shouldn't a peak acceleration be in fact a peak and not less than Ss.
Can anyone enlighten me?

 

[msucog]

you mean "seismic site class" instead of SDC (seismic design catagory)?

Sms=SsxFa
Sm1=S1xFv
Sds=Sms x 2/3
S1=Sm1 x 2/3

also, PGA is not necessarily the "max value"

there's other threads on the topic under "seismology engineering" probably "geotechnical engineering other topics". do a search...

 

[Galambos]

check your equation for Cs, again.

 

[lana02]

Thank you both for taking the time to address this tedious issue.

To msucog: Thank you for your direction - I guess I should have posted this in the geotech section.
I did mean SDC=D. Starting with an assumed Site Class D, I end up in SDC=D which per IBC/CBC 1802.2.7 requires a soils investigation except if PGA=Sds/2.5 is used.

To Galambos: Yes, my formula for Cs contains 2.5 in the denominator. This is precisely my point (or dilemma). How is PGA=Sds/2.5 to be applied? It seems convoluted to me that using the provisions to side step the soils report would result in a lower base shear.

There is nothing in the ACSE 7-05 commentary on this provision. I've searched Google books for comments and found nothing. Perhaps I can try another angle and contact someone in my SEAOC chapter who was on the ASCE seismic task committee.

-Alanna

 

[msucog]

http://www.eng-tips.com/viewthread.cfm?qid=212388&page=1

follow the links at the bottom (under my post). there's enough literature to keep you busy for a while and it is very informative.

good luck