Seismic Site Response Analysis with 2007 CBC and ASCE-7

[moe333]

Hello all,

The new 2007 CBC which is based on the 2006 IBC provides requirements for performing seismic site response analysis. The 2007 CBC actually references ASCE 7 for the details of the requirements.

A big problem I see with the ASCE 7 recommendations is that they require you to develop the response spectrum and time-histories using the MCE (2475 year return period EQ), then to scale the response down by 2/3 to come up with the design response spectrum and time-histories.

The problem is the site response you obtain by using the MCE input ground motion will be very different from what I would obtain by using 2/3 of the MCE ground motion. In using the MCE input ground motion, most all saturated sandy sites will liquefy and will have a base-isoltaed type of response after the liquefaction occurs. This is because the MCE PGA is at least 0.6g at least in my area of Southern California. I am using a non-linear effective stress code for the analysis.

Scaling this response down by 2/3 will retain the characteristics of the base-isolated type of response.

I believe the input ground motions should be based on 2/3 of the MCE, this way the estimated site response would be more represetitive of the site. But the ASCE 7 appears very clear that the input motion should be based on the MCE.

Anyone have thoughts on this topic?

Thanks

 

[msucog]

check out the following link

http://www.fema.gov/plan/prevent/earthquake/professionals.shtm#2

in particular, look at 451 and 451B under "new buildings". maybe this will help.

 

[moe333]

msucog,

Thanks but those publications are for design of structural aspects and are also not specific to the requirements of the California Building Code (CBC) and the ASCE-7 publication that is referenced in the CBC. I know how to perform the analysis and have done it before, it is the new requirement that the base motion conform to the MCE motion that I do not agree with.

 

[msquared48]

Write a letter to the seismic committee of the IBC and ASCE outlining your concerns and see what they say. If you have not already, I would also check the ASCE commentary for the section you are concerned about prior to writing that letter.

Mike McCann
McCann Engineering

 

[msucog]

I'm not familiar with CBC but thought the information relative to ASCE7 might help. Sorry I can't be of more assistance.