guidance please, angle iron radio tower

[brad_bailey]

Hello, folks! I'm a budget minded person that understands that there is no reason why some things cost as much as they do aside from the convenience of pre-made.


Such is the case of antenna masts. When I look at angle iron, quick mental math looks like I can spend time building one for whatever height that is less than half the cost of building new... And I'd still have to pay shipping for pre-made. I'm very crafty, just not a structural engineer.


I need help with selecting the appropriate size material. Please forgive laymen's terms and incorrect verbiage while I try to explain what I'm seeing in my head. The picture I'm trying to attach is a visual idea of what I'm trying to recreate on a smaller scale. (Not the equipment, but the tower itself.


Design goals:

1. I'm wanting the tower to be square in shape, at 16". This seems like it will be perfect for using the side of existing buildings to help anchor/guy the tower.
2. From studying old cell towers, cranes, and warehouses it looks like the cross bracing I want to use is X bracing out of strap/flat bar iron where the diagonal pieces ( /'s and \'s ) fasten to the vertical angle at 45°.
3. The total height will be 70' above ground, but I'd like to be able to extend in the future with more sections to get 90'.
4. I would like the full 90' length to be able to withstand winds of 110mph. Anything higher than that and an antenna won't be on the list of things to worry about. The equipment at the top is small and typically round, making drag factors mostly the material used to construct the mast.
5. Each 10' section will be bolted to the next at multiple points. I'm not the most articulate, so I'll try to describe what I see in my head. Each bottom piece of angle will have two ears on it, for a total of 16 small pieces of strap used to make what's effectively a female socket for the next section to slide into. The X bracing would then span past these connection points making it so the middle of the X is in line with the ends of the angle. The idea trying to be that neither the cross bracing nor the connection ears are solely responsible for the connection.
6. The base where I attach it to the ground will have a hinge on it, likely a rod and pipe type setup on one side of the square and the other side just pins. It will be guyed at intervals along the vertical at 50% height, 75%, and top.
7. Using a winch on one side to raise the tower from horizontal rest to vertical standing. The other will have a 2nd cable of some sort to control the speed once the tipping point is passed so it doesn't slam into standing position.

Expected answer, after everyone has asked additional questions, would be something like:



For mild carbon, use 1.25 x 1.25 x .125 angle and 1 x .125 strap.

[Kelly_Bramble]

70' tall huh?

I really don't think any structural engineers are going to stop what the're doing and bang out the calculations for you.

You're asking bit much, don't you think?

[brad_bailey]

I really don't think any structural engineers are going to stop what the're doing and bang out the calculations for you.

I don't mind doing the math myself, I'm totally ok with doing that. I don't know what I'm looking for though.

More to the question, I honestly wouldn't have thought it would have been a huge issue because angle is only readily available in a handful of sizes and thickness. I'll admit to a large portion of ignorance, but not stupidity. .5 X .5 X .125 would seem outrageously too light, whereas 4 x 4 x .5 seems way over-kill.

1.5 X 1.5 X .25 seems like it would be closer to the Goldilocks area of being plenty enough without being excessive. How would a person know for sure though without asking?

More over, I'm not asking for exact calculation with a +/- 15% error margin.

[Cragyon]

I don't mind doing the math myself, I'm totally ok with doing that. I don't know what I'm looking for though.

It is a statically indeterminate truss structure, I'm guessing the math is way out of your knowledge basket at this time.

More over, I'm not asking for exact calculation with a +/- 15% error margin.

I estimate 40 hours + work.


Buy an existing tower or hire an engineer.