Mechanical engineer: help?
 

I want to raise an antenna 6 to 8 feet. The rated lateral thrust @ 100mph wind is 39 lbs. It's 86" long and weighs 12.6 lbs. Mounting instructions say 1.9" OD mast. It's currently mounted to an 8inch beam as you can see in the pictures. What kind of pipe would I need to raise it 6 to 8 feet using existing mount? 1.9" OD pipe 8 feet long is what I need but how thick? I've done some reading on pipe and such but am not well versed on the subject. I'm not an engineer. Would Schedule 40 steel galvanized be overkill? Aluminium?

http://forums.pelicanparts.com/uploa...1580155122.JPG
http://forums.pelicanparts.com/uploa...1580155122.JPG

I'm not an engineer.
What kind of mast is it on now?
Aluminum?
Will steel interfere with the antenna function?
What is the wall thickness of the current mast pipe it's on?

Looking at your picture, based on my experience building stuff, I'd move your clamps further apart so that they are above and below the flanges of the beam. IMO this would give you a better mount because it would add a degree of twist resistance via the flanges of the clamps to beam flange interface.

thrust is distributed load which is the same as acting on the center more or less. 40lbf @ 4ft is ~160 ftlbs bending load at the base. Not sure if pipes are rated for bending loads, if so you want it to be above 160ftlbs. The mounting method looks pretty like it distributes the moment pretty well, so not sure if you need to do additional calculations there.

If there is no bending moment load for the pipe available, it will be a relatively simple calculation involving the beam bending equation and the moment of inertia of the pipe which will be easy to calculate vie wikipedia level engineering

also if you want it to survive an infrequent 100mph wine, a small factor of safety for the pipe bending load is probably fine say 200ftlbs. If you need it to survive sustained conditions and not fail by fatigue you will need at least a 2x factor of safety, if it needs an essentially infinite fatigue life it is more like 10x factor of safety.

considering my 86" antenna has a lateral thrust of ~40 lbs at 100mph, what would be the bending force at the base if I added another 6 feet?

EDIT: answer my own question: an extra 6 ft would give me 40lbf @ 10ft is 400ftlbs.

A little more “google-foo” tells me an 18 gauge pipe is thinner than a 2” schedule 40 pipe: 0.049” vs 0.154. I would think 2” schedule 40 pipe might be way too much…but would 18 gauge do the trick?

I see what you mean; that would help.

Schedule 40 is probably cheaper and as it is stronger, seems like a solution?

Not an engineer but spent years designing machinery. What is the thickness of the pipe now? It looks like Aluminium on there now, 1 1/2" sch 10 is 1.9OD and .109 wall thickness, sch 40 is .15"wall and sch 80 is .2" wall, I would go same thickness or one thicker, any thinner and you risk the clamp deflecting it.

If you move the beam clamps further apart you will loose clamping force against the I beam, closer is better.

The "pipe" is the antenna (not the short piece) along with the 2 dipoles. I believe it's aluminum but can't say for sure. Probably 1.9OD.
Moving the clamps apart just a wee bit to allow them not ride on the I beam, but rather mate up square against the beam edges.

Do you have a magnet handy? Steel will be obvious.

1 1/2 pipe is 1.9 Dia, if it is tube it is probably 2"Dia. Aluminum usually has what it is printed on the side.

I would bring the threaded rod as close to the beam as it can get, should work either way.

Oh I have a magnet alright but the antenna is in Edmonton and I'm in Vancouver ;)

Oh, look at the bright side you are in Vancouver. Warm in Edmonton today -2c.

You should mention that the antenna mast must survive -50c, not sure how people live there.

This. It sure looks like aluminum, but could also be stainless steel - but doesn't look like black pipe (painted) or galvanized. I assume you want to replace the current mast with the dipoles on it with the 6-8' longer pipe (not adding new pipe to existing). 1.9" OD is 1-1/2" IPS pipe size as stated above. Heavier wall thickness (Sch 10 to Sch 40 to Sch 80) will obviously give you more strength. I did have the bending formulas for pipe in a spreadsheet (replacing a steel roll cage with aluminum or titanium!) but cant find it right now - will keep looking! Galvanized pipe is more easily obtained, but aluminum shouldn't be an issue for what looks like a industrial/commercial installation. Specs might even be visible on the existing piece.

If the pipe really "IS" the antenna, changing the length changes the resonance frequency.

You may want to consult the manufacturer.

new height?
 

Hendog,

Went back to the original post, reread it, and wanted to make sure that I fully understood the problem.

Are you adding 72" to 96" to the existing 86" or are you raising an existing 72" to a new 96"?

If you are trying to accomplish the latter, then I see you only need 24" of additional height.

What is actually at 72"? The top of the mast or the upper dipole? Is the entire mast 86" long or is that the height of the upper dipole?

Can the dipoles be moved closer to the top of the mast? Looks like you could gain about 6" or so if that is possible.

Can the mast be moved higher in the beam mount? Can't see how much of the mast is below the beam clamp in your photo, but it may be possible to obtain some more length that way.

You said it is 86" and weighs 12.6 lb, likely Aluminium 1 1/2 sch 80. 86" 1 1/2 sch 80 @ 1.26lb foot works out to 9.0lbs so if you add the top cap and other antenna's on there prob about 12lbs.

could be 1 1/2 sch 40 alu is about .93lb per foot.

Steel and S/S would be out of the weight spec at 2.0lb foot for 1 1/2 sch 10

Does not look like Titanium

Picture worth a thousand words
 

This picture is what I'm shooting for. I hope that clears up my intent. The additional mast is 8',however 1' is consumed at the top parallel clamp to antenna and another 1' consumed to I beam mount at the bottom.
So the question is: What kind of pipe can I use which will take the load? I need documentation to support the claim.


http://forums.pelicanparts.com/uploa...1580231873.jpg

Hey Henri, You mention you need documentation, pretty sure if you are trying to shed liability you will need an engineer from Alberta to approve the work anyways, can you find any Mec contractors in that area with an engineer on staff?

This was my first thought. Seems unnecessary to try and "re-engineer" a solution, when the folks that have already done the work are likely just a phone-call away.