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Re: Isolated pole deep (flagpole) questions.

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I'm making a guess here, but it sounds like the intent is to allow an increase in resistance due to utilizing the full passive earth pressure. In order to fully realize the passive pressures, significant deflection is necessary.  The 1/2" motion is likely the deflection of the soil, and not (directly) related to the bending displacement of the pole, though there can certainly be some argument as to the merits of this point. The 1/2" is an allowable from a serviceability point of view, not strictly a structural one.

In my opinion, if the top is constrained, it would seem that the increase might be applicable if you were to allow the bottom of the pole (earth tip) to deflect by 1/2", thereby using full passive pressure at the bottom. However, you would not get the active condition at the top, so some middle ground should be chosen.

I suppose the best advise is that if the formulas don't work using the conservative values in the code, or you have to "guess" at the intent,  you should either do the longhand pressure diagrams and calculations yourself or pass it off to a soils engineer familiar with the design of embedded poles.


Puskas, David wrote:

The IBC (1805.7.2) provides two formulas in the design criteria to resist lateral forces (flagpole footing design). One formula is for non-constrained and the other for constrained. Also the code allows an increase in the lateral bearing values for flagpoles, or signs not adversely affected by a ½” motion at the ground surface due to short-term loads.


I have some questions:


1.                   In your opinions what constitutes a constrained condition? The commentary indicates constraint “such as a rigid floor”. Does this mean that there needs to be a physical connection (dowels) between the “rigid element” at the top of the flagpole footing? Would paving (concrete or asphalt) placed right up against the top of the footing constitute a constrained condition?


2.                   In your opinions is the ½” motion referred to above as the combination of the pole deflection and the movement of the footing against the soil?


3.                   If the footing is constrained, and therefore not actually able to move ½”, can you still take the increase? It seams to me that the intent is that as long as you have a flexible structure that can sustain these deflections the increase is allowable even if those movements are not possible because of the constrained condition at the top of the footing.


Thanks for your comments.


David  James Puskas, P.E.

Senior Engineer - Structural


BSW International, Inc.

BSW International Engineers, P.C.

Thomas F. Keeter, P.E., Inc.

One West Third Street Suite 800

Tulsa, Oklahoma  74103


Work Telephone:  918-295-4213

Cellular Telephone:  918-640-2778


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