×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Seismic Site Response Analysis for 2007 CBC

Seismic Site Response Analysis for 2007 CBC

Seismic Site Response Analysis for 2007 CBC

(OP)
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

RE: Seismic Site Response Analysis for 2007 CBC

The response with 2/3 of MCE is more severe than 2/3 of the response with full MCE, right?  (due to base isolation and nonlinear response)  Might not be much difference in the surface response between input of 0.4 and input of 0.6 g at bedrock.  At 0.4 g, almost anything that can liquefy will liquefy, so the additional 0.2 g wouldn't make a lot of difference.

I'm not at all familiar with ASCE 7 (too lazy to look it up right now), but I think what you are saying makes sense (regardless of what's actually written).  Thinking of it in probabilistic terms, 2/3 of the base-isolated MCE response could be less severe and a lot more likely than the response to 2/3 of the MCE, so it would be unconservative to use 2/3 of the MCE response.

If the first sentence above is not correct, then never mind the rest.

Regards,
DRG

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources