Circular Footing with eccentric load and "negative" soil pressure (Flagpole)
Circular Footing with eccentric load and "negative" soil pressure (Flagpole)
(OP)
Hi all:
I'm a bit confused about the approach for flagpole foundation designs. I designed a pole foundation based on IBC's straightforward approach in Section 1807 (Unconstrained round piers subject to lateral load). It was relatively narrow and deep (~2.25' wide and ~7.5' deep.) The contractor then prepared on-site an excavation for a premade "kit" which is wider and shallower with flared ends, which subsequently failed inspection. This off-the-shelf solution fails the IBC equation badly at the given dimensions (and doing some research it looks like flagpole foundations are often this odd shallow, nearly square "plug".) Are any of you familiar with how these foundations are justified? Since the steel for the kit is currently in the ground, the contractor wants to keep the shallow depth and widen the foundation to our satisfaction. Doing this with the pole-foundation equation gives huge diameters, and the only other method I'm familiar with is treating it as a bottom-supported spread footing with an eccentric load. I'm not aware of any design approach that combines ground-support with lateral moment resistance.
A constructably-reasonable circular spread footing results in there being negative soil pressure at the heel. So the actual pressure distribution would be circular with a "chord" missing in plan view. I don't have the time to relearn the calculus needed to do derive that from scratch. Are any of you familiar with a design approach or approximation that would give the actual toe pressure under those conditions?
I'm a bit confused about the approach for flagpole foundation designs. I designed a pole foundation based on IBC's straightforward approach in Section 1807 (Unconstrained round piers subject to lateral load). It was relatively narrow and deep (~2.25' wide and ~7.5' deep.) The contractor then prepared on-site an excavation for a premade "kit" which is wider and shallower with flared ends, which subsequently failed inspection. This off-the-shelf solution fails the IBC equation badly at the given dimensions (and doing some research it looks like flagpole foundations are often this odd shallow, nearly square "plug".) Are any of you familiar with how these foundations are justified? Since the steel for the kit is currently in the ground, the contractor wants to keep the shallow depth and widen the foundation to our satisfaction. Doing this with the pole-foundation equation gives huge diameters, and the only other method I'm familiar with is treating it as a bottom-supported spread footing with an eccentric load. I'm not aware of any design approach that combines ground-support with lateral moment resistance.
A constructably-reasonable circular spread footing results in there being negative soil pressure at the heel. So the actual pressure distribution would be circular with a "chord" missing in plan view. I don't have the time to relearn the calculus needed to do derive that from scratch. Are any of you familiar with a design approach or approximation that would give the actual toe pressure under those conditions?






RE: Circular Footing with eccentric load and "negative" soil pressure (Flagpole)
Negative pressure on the heel isn't a deal breaker for me - but obviously it's not possible. I'd use a different pressure distribution - triangular - instead of trapezoidal. You still have 2 unknowns - the pressure at the toe and the width over which the pressure acts. Pressure at the heel is zero.
I look to keep bearing pressure over ~2/3 the width of the footing.
Where I think your question gets a little trickier is flexural reinforcing of the footing - given that the wind can blow from any direction.
RE: Circular Footing with eccentric load and "negative" soil pressure (Flagpole)
The bearing pressure due to an eccentric load is equal to (2*P)/(3*a*B), where a is the distance from the leading edge of the footing to the load, and B is the width of the footing.
DaveAtkins
RE: Circular Footing with eccentric load and "negative" soil pressure (Flagpole)
RE: Circular Footing with eccentric load and "negative" soil pressure (Flagpole)
The simplest approach to that problem if you're starting from scratch:
Assume linear bearing distribution y=Ax+B.
Assume a point of loss of contact, from which you can find A/B by setting y = 0.
Use Simpson's rule in a spreadsheet to integrate bearing over the area and compare to total weight. That'll let you find A and B.
Use Simpson's rule to integrate bearing x moment arm over the area to find the resulting moment. Compare that to the actual moment. And adjust point of uplift and repeat until you hit the solution.
RE: Circular Footing with eccentric load and "negative" soil pressure (Flagpole)
DaveAtkins