Overturning calculation for sonotube

[LearnerN]

Obviously a sonotube is not meant to be used for much overturning capacity. But when doing a calc for its overturning capacity, do you typically maybe treat the sonotube area and base dimensions as a square circumscribed with a circle? Doesn't make too much difference, but I though I'd ask what others do. Thanks.

 

[Ron]

Yes. I use the projected area of a sonotube or drilled pier when computing overturning resistance. Be sure to use the proper coefficient of lateral earth pressure.

 

[Once20036]

IBC provides some guidance on overturning of drilled foundations - check IBC `09 section 1807.3

 

[Jerehmy]

https://www.dropbox.com/s/xrfvckoxldf1iye/Nomograph.pdf?dl=0

enjoy

 

[LearnerN]

How is the pivot point determined for the sonotube pier foundation?

 

[Ron]

Depth of fixity is determined from the moment on the pier. Once the depth to fixity is determined, the embedment depth is used to compute the OT resistance, which generally needs to by 150% of the applied moment.

 

[LearnerN]

Ron, how is the depth of fixity determined from the moment on the pier? My understanding is that the passive-minus-active earth pressure result will counteract the effect of the moment on the pier to the point where they balance each other out as sum-of-the-moments is concerned. So I assume this sum-of-the-moments balance point is where the "depth of fixity" is located? And then on the other side of the pier, will it simply be passive-minus-active earth pressure result all the way down to the bottom of the pier?

 

[dhengr]

LearnerN:
Unless you tell us otherwise, I suspect your sonotube pier is not deep enough (long enough embedment) to really develop a point of fixity, like might exist on a long drilled pier or pile. Your pier will more than likely just tend to pivot about its base plane (top of footing), and you will be dealing with passive and active soil pressures like you might have with a retaining wall, foundation wall or some such. There are a number of papers on pier foundations for signs, flag poles, canti. columns and the like, and their resistance to moments applied at grade, but I can’t point you to one off the top of my head. I think the IBC has a procedure, a number of highway depts. will have some methods, etc.

 

[Jerehmy]

The nomograph I posted is from equations generated by the outdoor advertising association of america. Results from their equations match up quite nicely with the IBC's.