I work at a football club and I need to install some long 20 ft metal poles into the ground set in concrete. My question is how deep must the holes be and what diameter hole needs digging. The poles are ex scaffold poles and are 3 inches in diameter. The poles are being installed to hold up netting behind the goals to stop the ball from going into adjoining fields and roads etc. Thanks for any help
Concrete – Installing long 20ft metal poles in concrete around a football pitch
concrete
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You can actually punch surprising large holes in the middle of a beam without diminishing its load-bearing capacity. This is because that capacity is largely a function of only the top and bottom portions of the beam and how far apart they are.
Engineered floor joists are a good example of this:
It would take a structural engineer to tell you exactly how much the load-bearing capacity will be reduced, but I can tell you that this amount will be at least an order of magnitude less than the margin for error in manufacturing the beam.
Don't worry about it. It's what these things are made for.
Set a steel pin (1-2cm diameter) or post-base-bracket in the concrete. Drill a hole in the bottom of the post for the pin, or attach the bracket to the post bottom - no lateral movement, also no accelerated rot. Rot is effectively guaranteed if you set the post into concrete. Brackets designed for the job maintain a small space between the concrete and the post. If you don't use that type of bracket, or you use a pin, set a small square of tarpaper/roofing felt/asphalt shingle on top of the concrete, under the post, to reduce water transport into the post from the concrete. A quick search indicated that post-base-brackets seem to be available in the UK, so that's the best option - tie the lower part of the bracket into your reinforcing mesh before pouring concrete. There are several typical options, this is one type (image from Simpson Strong-tie)
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Best Answer
The primary stress in this case will be wind. Typically you'd embed a pole at a ratio of 1:4. In your case, if the poles are 20' above ground, you'd want to be 4-5 feet down. If your poles are 20' in length (16-17' above ground), 3-4' may be adequate.
I'd probably use form tubes (or cleanly-dug holes) of a 10 or 12" nominal diameter (at least 3 time the pole diameter). You wouldn't want less than 3-4" of concrete outside the pipe to prevent breakout.