ASCE 7-05 foundation question 2

[SteelPE]

Hopefully this question hasn’t been asked before.

Section 12.13.4 of ASCE 7-05 is titled “Reduction of foundation Overturning” and basically allows for a 25% reduction of the overturning effects at the soil-foundation interface as long as the ELFA was used and the structure is not an inverted pendulum. Does this section mean we are allowed to reduce the seismic loads by 25% when applying the loads to the 0.6D + 0.7E load combinations in the calculation for overturning? Basically ending up with an effective 0.6D + .525E load combination (0.6D + 0.75*0.7E) for overturning?

 

[Lion06]

I believe that to be a true statement, but I also believe that it's for soil bearing pressures only - i.e. not for the strength design (rebar) of the footing.

 

[SteelPE]

Thank you for your reply.

I interpret this to apply to stability against overturning and bearing pressure. I agree that you would not include this reduction in the design of the footing itself (for moments and shears).

This also appears to be a requirement that appears in ASCE 7-05 (I can’t find it in ASCE 7-02).

 

[JoshPlumSE]

I have never seen that section of the code before and it is most certainly new.

This causes me to vent a bit here. Why the heck doesn't ASCE-7 provide at least some explanatory language in the commmentary for totally new code provisions?! ACI and AISC all do a way, way, way better job of helping engineers understand and track code changes.

What's funny is that the ASCE commentary then refers us to the NEHRP commentary and the 1999 SEAOC blue book. Both good references for sure. But, I am not confident that either of them will end up explaining where this code provision came from.

End of vent.... blood pressure returning to normal.

 

[WillisV]

That provision has been around for quite a while - just in various places - See IBC 2000 Section 1801.2.1.

As IBC is moving to push more technical content to the referenced standards provisions (such as this one) get pushed out to ASCE 7 etc.

 

[JoshPlumSE]

WillisV -

Thanks for pointing that out. I do sort of remember that provision of IBC. Though, for some reason my brain couldn't make the connection between using 75% of the demand and instead using a 25% reduction.... Maybe it has something to do with being a California engineer and taking such joy in actively ignoring the IBC for so much of the past 10 years.

To me, the ASCE section reads as if it applies to both soil bearing and overturning stability (but not sliding).

However, reading the IBC section makes it a bit more difficult to come to that same conclusion.

For now, I don't think I'd apply it for soil bearing. Especially since I'm probably already taking a 25% force reduction (or a 33% increase to my bearing pressure) based on the load combination.

 

[blackmaddog]

To add to Josh's venting above, why do they put such ambiguous provisions in there to start with. In this case, it is a liberal provision that if applied incorrectly will get people in trouble.

Does uplift count as an "overturning effect". If so, I guess you can make your footings 25% lighter (if uplift controls).

 

[phuduhudu]

I think the IBC is a bit clearer. Rather than calling it overturning effects it calls it soil-structure interaction effects due to seismic loads. So that would include all the forces arising from seismic loads which would make your effective combination of (0.6D + 0.75*0.7E) correct. In fact all earthquake combinations would have a 0.75 factor on them as far as the soil is concerned.

 

[WillisV]

This article provides some insight into the thought behind this provision:

http://www.gostructural.com/article.asp?id=1275