I have a 35 foot aluminum light pole that has 4 bolts holes in the base of the pole. I need to know how deep and wide should I pour the concrete into the ground. I'm not going to use the light pole for a light, I'm recycling it to make a flag pole. The base bolts holes are 8 inches apart in a square. The base bolts measure 11.5 inches diagonally. I want to be able to fly two flags from the pole and I want to make sure that I have enough concrete in the ground to support the pole, two flags, high winds, ect. I live in western Kentucky.

  • 27 eighty pound bags of Quikcrete should do the trick as long as you have a deep enough hole to accommodate all the mixed concrete. Before pouring the concrete a whole series of rebar pounded into the dirt sides of the hole will secure the concrete block even more. – Michael Karas Jul 11 '19 at 4:27
  • That's 2,160 lbs of concrete are you serious? I was thinking it would take about 1,000 lbs but I was just guessing, I didn't know for sure. – Tony Jul 12 '19 at 3:01
  • Tony - I was kind of thinking that for something like this more might be better. The "Flag Pole Calculator" linked in @jwh20 's answer indicates that you would need a concrete footing 2 feet in diameter sunk almost 12.5 feet into the ground to support a 35 foot flagpole with a wind load pull at the top maxing out at 10kg. (That is with the top of the footing constrained at ground level by a slab, wall or other means. A unrestrained footing would be almost 17 feet deep requirement. – Michael Karas Jul 12 '19 at 4:10
  • Michael thanks for helping me. I have some questions, I want to make sure that I am understanding this right. First question. My light pole has 4 holes in the base plate. There would be concrete deep in the ground and at ground level the concrete would have 4 bolts rising up from the ground level concrete. The base of my light pole would sit upon the 4 bolts. Then using 4 nuts screwed down onto the base of the light pole. Is this called "constrained at ground level"? – Tony Jul 13 '19 at 12:40
  • From what I can tell with regard to the "Flag Pole Calculator" the mounting of your pole to the concrete has nothing with the "constrained" parameter. Clearly the bolts embedded into the concrete to secure the pole have to be long and secured somehow into the concrete and with the rebar in the concrete. Four short bolts just embedded into the concrete just a couple of inches would not suffice. As far as the "constrained" parameter do note that the unconstrained footing computes to a deeper footing from the calculator. I take this to mean that a constrained at ground level footing (continued) – Michael Karas Jul 13 '19 at 17:17

There are lots of variables that really need to be evaluated by a Registered Engineer but you can use this site for a quick evaluation:

Flag Pole Calculator

Of course you'll need some details to figure the lateral force at the top of the pole, this comes from the size of the flag and the expected windspeed it will encounter. You'll also need a figure for the soil characteristics.

Again I encourage you to consult an engineer since the last thing you want is for this thing to come crashing down and injure or kill someone.

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