Maximizing Strength: Choosing the Right Aluminum Tube Diameter for Marquee Poles

[Aurelius]

TL;DR

How tube strength changes with diameter

I'm planning on making marquee poles out of aluminium tube. I did so last year and they worked well, but they flexed slightly under tension.
To correct this flex, I want to use slightly different size of aluminum tube.

I used 4" tube, with a ¼" wall, at a length of 30'.
Will the tube be stronger with a larger diameter, or do I have to go for a larger wall thickness?

Basically, is there a way of working out
strength /diameter ratio?

Or even more basically, will it be weaker or stronger with a larger diameter tube, keeping the wall thickness the same?

 

[berkeman]

Welcome to the PF.

Summary: How tube strength changes with diameter

I'm planning on making marquee poles out of aluminium tube. I did so last year and they worked well, but they flexed slightly under tension.

What's a Marquee Pole? And why would it flex under tension? Don't you mean some sort of bending stress?

Can you post pictures of your previous poles? Use the Attach files link below the Edit window to upload PDF or JPEG files. Thanks.

 

[Aurelius]

Marquee poles, or big top poles. Tent canopy pole.
I've attached an image of the poles as they're set up, ready for the tarp. They're anchored with guy lines to the top of the pole, and we use a winch to hoist the tarps up the poles.
Hope this is better information.

 

[berkeman]

Ah, thanks. That helps.

But what did you mean about flexing under tension? Do you mean they flex if the rope tensions are uneven?

Who insures the installation of these tents? What wind loading do you need to design for? Where are these tents used?

Are there other commercially-available tents like these? And what pole construction to they use?

 

[Aurelius]

by flex I mean a slight bow in the full length of the pole if the tarps are over tightened, or hoisted too high. The wind loading is 65mph. The pole construction varies from company to company, steel is one option, wood another, aluminium also. We have used aluminum because of the weight reduction.

 

[berkeman]

Who insures these tents? Who assumes the liability if there is a failure (and especially if people get hurt)?

 

[Aurelius]

They're insured by the company I'm constructing for. Also covered by my public liability insurance.

Going a bit off topic though,

Any advice on tube diameters etc?

 

[Chestermiller]

I think you mean that the poles flex under compressive load, rather than tensile load, right. Also, I think you are referring to the stiffness of the poles, not their strength. Strength usually refers to the conditions under which the material breaks.

 

[Aurelius]

Yes I agree, my terminology is not as good as it could be. I am more of a hands on person and this is all a bit technical for me, but I'm learning!
Yes, it's the flex when the poles are put under tension. And the stiffness, I guess that is what I need to increase to reduce this flex...?

 

[Chestermiller]

Yes I agree, my terminology is not as good as it could be. I am more of a hands on person and this is all a bit technical for me, but I'm learning!
Yes, it's the flex when the poles are put under tension. And the stiffness, I guess that is what I need to increase to reduce this flex...?

For a given material, the thing that determines the flexural rigidity of one of your tubes is its moment of inertia about a centerline. For a given economics, the moment of inertia should be as high as possible. The moment of inertia is a function of the tube diameter and the wall thickness. Google formulas for moment of inertia of various beam cross sections.

https://skyciv.com/tutorials/area-moment-of-inertia-equations/

 

[anorlunda]

Be careful, you may not have as much freedom of choice as you think. In the USA, if people get hurt and if the NPFA codes are not followed, it could result in criminal charges. I'm not familiar with the corresponding code in the UK or EU.

NFPA 102

Standard for Grandstands, Folding and Telescopic Seating, Tents, and Membrane Structures
NFPA 102 provides requirements extracted from NFPA 5000 and NFPA 101 for life safety in relation to fire, storm, collapse, and crowd behavior in tents, membrane structures, and assembly seating.

I would first determine the required pole properties from the safety codes, then contact aluminum tube manufacturers and ask which products meet those requirements. Tubes not only come in different sizes and thicknesses, but also different alloys and shapes. Indeed, the manufacturer may already be familiar with tent applications like yours and have a standard recommendation. The manufacturer may also be able to advise on restrictions on drilling holes in the tube to mount stuff and advise on suitable butt plates.

A possible vulnerability of aluminum is that the soft metal could be easily dented during transport or erection. How big a dent is allowed while still meeting the codes? If you had to discard a dented pole, that might wipe out your savings.

I think yours is definitely a case where licensed professional engineers might save you money and liability.

 

[berkeman]

They're insured by the company I'm constructing for. Also covered by my public liability insurance.

Going a bit off topic though,

Any advice on tube diameters etc?

Not really off-topic. Safety is extremely important in projects like this that involve life and potential injuries.

Do your insurance companies know that you are asking for technical engineering construction advice in anonymous Internet discussion forums?

 

[AZFIREBALL]

Also, varying the moment of inertia from top to bottom can save on the weight of the pole.