Overturning Question 1

[MechEng27]

I have a lighting pole 30 ft high (w/ a cantilever arm at top) that has a total bending moment of 20,000 ft lbs.

This pole is supported by a foundation pad that is 2 ft diameter and 4 ft deep.

If the pad is fully embedded in the soil (will be a clayey Georgia soil with lat. pressure about 120 lbs/ft2/ft I imagine) will the struture overturn? If not, will the structure overturn if it is embedded at just 3 ft (1 ft of pad exposed)?

There is no surface restraint.

Many thanks in advance..

 

[JAE]

That foundation sounds a little light for a 30 ft. pole. The poles I'm used to seeing (midwest US) would be perhaps 24" dia. x 6 ft to 12 ft. and this isn't just for freeze protection.

Gaylord and Gaylord's Structural Handbook has a nifty nomograph that designs pole foundations. For your case, the numbers (20,000 ft-lbs from a 30' pole (666 lbs at 30'), 24" x 4 ft takes me off the chart assuming average soil (allow. average soil stress, psf = 2000).

There is also a program out there called LPILE that analyzes conditions like yours.

 

[Focht3]

To me, the issue is less about the capacity of the circular footing (cuz it ain't a pier with a L/d of only 2...) than about whether it is reasonable to use lateral soil resistance within the "normal trenching depth" of 6 feet or so. Will the pole be well outside any public ROW and at least 10 feet from any buried utilities? Can you assure yourself that no one can excavate within 5 or 6 feet of the edge of the footing for the next 20, 30 or 50 years? In most cases, the clear answer is 'No' to one or both of those questions, and the footing would be considered as inadequate regardless of whether the calculations say the pole will stand up.

You have two other things to consider: the depth of seasonal change in soil moisture, and allowable tilting of the light pole. Since you're in Georgia, I suspect that the depth of seasonal moisture is greater than 4 feet. (I don't have any first-hand knowledge of how deep it is, but your area is subject to drought conditions so it's greater than 2 or 3 feet...) Significant moisture changes will affect the soil around the pole. Too wet, and the soil softens - the pole leans. Too dry, and the soil shrinks away from the footing - the pole leans. (You can't win for losing.)

And such a small footing will experience a lot of rotation before it develops enough soil resistance to resist those 20 kip-feet of moment. An inch of groundline deflection will result in almost 2 degrees of rotation. That's very noticeable. And 2 degrees of rotation may cause your moment to increase significantly...

(That light pole foundation is a bigger deal than you probably thought -)

Finally, I did not check your calculations to see if the footing has enough capacity. I suggest that you completely re-think your approach to the problem. Hire a geotechnical engineer to design the foundations for you. I doubt s/he designs lighting systems, HVAC systems, etc. Know your own limits - and hire an expert. It's likely that if you are working on a commercial site, and the owner has already hired a geotechnical engineering firm. Talk to them - I doubt they bite!






Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Qshake]

Good points FOCHT3.

In addition to the Structural Engineers Handbook, you can very effectively and cheaply gain a sanity check with your State DOT. Go online and check out the standards - which light-poles like this will fall under and see what is the typical foundation. I doubt it is what you've described.

 

[MechEng27]

I had a feeling the 2 ft diameter by 4 ft deep foundation would not cut it.

Also, the issue of excavations within 5 or 6 feet of the edge of the footing is something to consider.

Thanks for the pointers..

 

[boo1]

http://www.dot.state.ga.us/DOT/construction/contractsadm/

Section 680—Highway Lighting
Section 647—Traffic Signal Installation

 

[Ron]

MechEng27...don't know what part of Georgia you're in, but wind load could be reasonably high. That interjects the stability requirement of building code for your area.

Also, the passive pressure resistance of those soils will vary greatly with moisture content as Focht3 alluded.

Qshake's advice to check GDOT is on target. If they don't have anything substantial, check neighboring Florida...the FDOT has quite a bit of info on such.

Your foundation looks way too small. By my rough estimate, you should be looking at about a 36-inch dia. x 12' shaft.