Tether Pole Footer HELP!!!!!!! 3

[Nhargrove]

First off thanks for any help you can provide me. We are in the process of putting in some tether tracks for 24/7 testing on some of our vehicles. I am having trouble determining the size of footer I need to support the necessary load being put on the pole. I have done some quick calculations and I am getting that I need roughly a 6' diameter footer, 9' deep. The footer will not extend past ground level. The pole is 18" in diameter with a wall thickness of 2" and will, I assume, rest at the bottom of the footer and stand roughly 4' out of the ground. The force that will be pulling on this pole is 3500lbs at the top pivot. My calculation included a safety factor of 6500lbs for a total of 10klbs. Can anyone verify these calculations or help me

 

[JGard1985]

18" Diameter pole, 2" thick? Concrete 6' diameter and 9' deep? Wow!

Not sure about your physical constraints, but I'd take a step back and look at your problem. Can you use gussets and/or bracing to stabilize your pole (reducing section size) and expand your foot print?

How did you size your footer? It's deep...did you consider the restoring force provided by the soil? For a side pull you could go with a shallower wider footing. The wider your footing the greater the restoring moment

Some quick math with 10kips @ 4 ft suggests you will need a section modulus of about 20in^3 (figuring 40ksi yield, and 1.67 SF). For this we are talking about 8" diameter 1/2" wall pipe for elastic analysis.

Can you post your math?

Jeff
Pipe Stress Analysis Engineer

http://www.xceed-eng.com

 

[Nhargrove]

Thanks for the response Jeff,

The pole itself was not factored in to any load calculations. The Slip-Ring that will be bolted to the top of the pipe has roughly the same dimensions and that's where the size came from. So more than likely the pipe is oversized for the application. After looking back at my math, the numbers for the footer are coming out to be a 4' diameter and 8' deep. The calculation is based off an online formula to determine flag pole footer size and that is the closest thing I could find that resembled my application. My background is electrical and manufacturing engineering, so this is def. out of my normal scope of things to do. I am having a hard time convincing my boss to pay for a structural or civil engineer to design this because we already have a tether track in operation and they just want me to mimic it. I question the current setup which is a 12'x12'x12' steel reinforced footer with the same size pole. I hope you can see why I question it. So any help on the design or thoughts would be really helpful.

 

[Ron]

First of all, it is a "footing" not a "footer". Leave that nomenclature to the contractor.

Your load is only 3500 lbs at 4' above the top of the footing? That's only 14,000 ft-lbs or 14 k-ft (pick your poison....ft-lbs or lbs-ft). Not a high load for an 18" diameter, 2" thick pipe! An 8" dia. pipe with a 3/8" wall thickness can easily handle that load.

As for the mass of the footing.....let's assume a square footing with a 14000 ft-lb moment. Start with a 5' square footing and use an overturning factor of safety of 1.5. Let's also assume you have competent bearing soil and the footing will be allowed to rotate about its edge in overturning. The footing thickness would probably need to be about 3 to 3-1/2'.

At first glance, what you have is overdesigned and not necessary....unless there are other factors you have not shared.

 

[Nhargrove]

Here is a sketch of the design/application

 

[Nhargrove]

Hey Ron and JGARD1985,

Did you get a chance to look at the very crude drawing that I have attached? Thanks again for your help

 

[Ron]

Yes. Same response with the addition that you need to check lateral movement from assembly pressure on the soil.

 

[thaidavid40]

Since this is a very dynamic load, I can understand the previous design being so large. This design will likely be controlled by the soil spring constant under the footing. The concrete mass is there to dampen the rocking that will naturally occur. And as for the center post, if there will likely be millions (or hundreds of thousands) of repetitive cycles of load placed on it (each lap around the pole), then the design would also be robust, to keep stresses well below the fatigue limit of the steel. I've done quite a bit of industrial structural design over the years, and the size of the structural elements required has ceased to amaze me long ago. Usually, the cost of larger members is far outweighed by other more practical factors - like longevity.

Thaidavid