Seismic Site Response Analysis for 2007 CBC

[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

 

[dgillette]

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