Sign Footing Design (high moment footing design). 1

[dannyypk]

I am designing a concrete footing for a 26'tall sign. The overturning moment is about 80.97 k-ft.The ultimate momeny Mu = 1.5 * 80.97 k-ft = 121.5k-ft
I have a question about the design of that footing. The client wants to embedd the HSS tube steel directly burial into the concrete. I would have to design the footing to resist the overturning moment and the thickness to resist the wind base shear.
The footing now is mainly designed for resisting the overturning moment and the vertical load is so small. I would like to see if anyone can help me with designing a footing with high moment and the small vertical load.

q = 1500 psf (assumed) = Pu/A + Mu/S

The above equuation seems not working in the above situation.

 

[UcfSE]

If you embed the pole or design a relatively deep (thick) footing, you will be able to use the passive resistance of the soil to help resist the overturning. If you choose to do so, don't forget that the active pressure will be acting on the opposite side.

Either way, you'll either have a wide footing or a thick one, or a bit of both. There's only so much you can do when you don't have much vertical load and a high moment.

 

[aggman]

Your formula is only valid if the full base area of the footing is in contact with the soil. If your resultant is beyond the middle 1/3 = L/6 then you have to use

qmax = 2*P/(B*x) where x = 3*(L/2 - e) --> (For one way bending)

(x) is the distance from the edge of the footing to the point of zero pressure.

(e) = M/P = distance of resultant load from the point of application.

Reference Peck, Hanson, & Thornburn or any other foundation handbook on the design of eccentricly loaded footings.

It's fairly common to use a drilled pier design as UcfSE points out.

 

[jheidt2543]

dannyypk

You might find the paper "Analysis and Design of Laterally Loaded Piles and Caissons in a Layered Soil System", by T.R. Naik and A.H. Peyrot. It was published in the 1970's in an ASCE journal. The paper is based on a modificaiton of Broms' method and it has a very practical approach with a good example problem and a pretty extensive list of references.

 

[cap4000]

For that large moment "aggman" has the right formula. With a convential spread footing it will be a very large rectangular shape and very thick to resist that moment and a heavily reinforced bottom mat. With very little column load, the dead weight of the concrete footing with backfill on top of it is the only stabilizing forces. If you give your approx.size I can review it for you.

 

[valleyboy]

Is it feasible to guy the top of the sign to reduce the overturning moments?

I looked at something similar a little while back, which was a foundation to some artwork 15m high. I opted there to use ground anchors acting in tension to achieve overall stability of the base instead of providing a mass base.

VB