Seismic design base shear to ASCE7-16

[EngrRC]

I am designing a tall building to ASCE7-16. I have designed the superstructure using response spectrum demands (scaled to match the equivalent static force base shear). However, I am designing the foundation based on overturning moments from Equivalent Static as mandated by the approving authority. However, in my understanding, I should be allowed to use the response spectrum foundation loads (roughly 1/5 of the overturning moment). This leads to massive difference in pile loads.

What are your thoughts on this?

 

[driftLimiter]

If AHJ says you need to use ELF for foundations I don't see how you can get around that. However you could consider use of ASCE 7-22 12.13.4 if permitted. The intent of this section is to address the issue that you present. But If you are saying that RSA gives you 1/5th the overturning of the ELF, something feels off there thats a massive difference, it suggests that most of the mass participation is wrapped up in higher order modes.

If you have already verified that the base shear from RSA matches the ELF, have you investigated why the 1/5th overturning demand? Check mode shapes and mass participation etc. ??

 

[Obadah]

I'm going through a similar situation here, I have scaled my RSA base shear up to match the ES base shear.
For a cantilevered shear wall, if I try to get overturning moment by integrating shear diagrams:
I get almost 100% accuracy comparing ETABS ES moment with ES moments I calculate based on integrating ES shears diagrams.
But I only get around 55% RSA moment from ETABS vs what I would get if I try to integrate RSA shear for the same wall.
You could say " this is expected because dynamic is different, blah blah", but I need a bit more than that so I have confidence in these results!

BTW, the modal participation ratio is 98%

 

[HTURKAK]

. However, in my understanding, I should be allowed to use the response spectrum foundation loads (roughly 1/5 of the overturning moment).

This is alot of difference. You did not supply alot of info . to see the case . But i think we have the right to guess.
My guess is ,this tall building has alot of rigid basement storeys which can activate at higher mods and you did not consider this at ELF and RSA analysis.

 

[duckhawk]

Sounds like the approving authority wants the foundations to not govern the design.