# Thread: Hopefully simple math question on square aluminum tubing.

1. ## Hopefully simple math question on square aluminum tubing.

After a lot of reading, I still have a question on square aluminum tubing. I need to build new safety poles. My current ones are overbuilt of steel I beams, heavy, and ugly.

The current working design is square aluminum tubing, 6-7' high depending on the person carrying it, and square plates welded flat on the ends. These will stand on one of the square plates, with the load pushing lengthwise on the other end. Basically these would be like giant jack stands.

I was thinking 4" square by 1/8" thick tubing, with an 1/8" thick by 8" plate welded on the end. The goals with this are to take a minimum of 2000 lbs (that includes a 2x safety factor) in the lightest and easiest to carry package possible. I am leaning toward the aluminum because it will require less maintenance to keep looking nice (this will be used in people's homes sometimes.)

I would happily take suggestions to reduce the weight if I'm still way overkill, but a little overkill doesn't hurt.

I have found equations to calculate the load if it was used as a crossbeam, but I don't seem to find anything on using the square tubing as a column (at least that I've understood that way.)

If anyone here can tell me if that size tubing will work, or if I need bigger or can use smaller, it would really be appreciated! Equations so I can learn a little for next time would be even better.

Hopefully this comes out like I wanted it to as an example:

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Thank you!!!

see:

http://www.engineersedge.com/column_...lumn_ideal.htm

3. Things you need to know:-

Effective Length - this actual column length (L) multiplied by a factor that depends on columns end fixings (both ends fully restrained = L*0.7=Le, both ends free = L*2=Le and every condition in between. Check your local codes for these factors).

Area - This is cross sectional area of your column member.

Second moment of area (I)

Radius of Gyration (r) - SQRT(I/A)

Slenderness ratio - Le/r

Elastic modulus - E

Yield stress - fy

You may also need to add in an imperfection factor, again, you'll need to check your local codes; these will help you in your design of a compression member (<key phrase). You'll also want to look at "compression members subject to bending" if these columns are external.

Base and head plates are a different kettle of fish altogether (depending on the preferred method of calculation in your state).

Sometimes it's just easier (and safer) to employ a local engineer.

4. Originally Posted by Kelly Bramble

see:

http://www.engineersedge.com/column_...lumn_ideal.htm
OK, using that calculation, with a moment of inertia here: http://www.engineersedge.com/calcula...are_case_4.htm

I went with a lowball of 2 on moment of inertia and a 9,000,000 PSI elasticity at 84", I can drop that down to 0.1" thick walls and still hold 25,000 lbs before buckling. I like that design for my purposes.

Please let me know if you see some major failure in my math...

5. I haven't been thru the numbers (and don't really intend to) but my gut tells me I would not load 2,000 lbs on anything with walls less than 1/8" thick. Be aware those formulas are based on laboratory conditions. They cannot account for various conditions encountered in the real world, like sudden shocks, imperfect fabrication techniques, offset loading conditions, etc. If there is ANY potential for human injury here under ANY circumstances I would recommend you obtain the services of a trained professional engineer for this kind of load.

6. Originally Posted by jboggs
I haven't been thru the numbers (and don't really intend to) but my gut tells me I would not load 2,000 lbs on anything with walls less than 1/8" thick. Be aware those formulas are based on laboratory conditions. They cannot account for various conditions encountered in the real world, like sudden shocks, imperfect fabrication techniques, offset loading conditions, etc. If there is ANY potential for human injury here under ANY circumstances I would recommend you obtain the services of a trained professional engineer for this kind of load.
I appreciate the reply. I do not have the resources for a professional engineer (or I would have gone straight to them, rather than a forum), but this will NEVER see 2000 lbs. The worst it will probably see is 800, a critical drop would be 40ft/min, and I plan on going 1/8" thick anyway (you can't safely weld less.) The only reduction I might have made is to drop the square size, but I like the stability of 4" for welding. That math was for absolute buckle at 20,00lbs (not repeated or moving load), and I understand that. Since ALL my values are run under, it tells me it should be safe for my use. It will also see a overkill test on the design using the forklift before I build a final version. I just wanted something for numbers to tell me if I was even in the ballpark before I wasted any money on materials.

If I build one that will take a sudden drop, it will be too heavy to reasonably carry (already have one that's 40 lbs that couldn't quite take the sudden drop that sits doing nothing) and it won't get used. In this case, a lightweight ounce of prevention is worth the 50+ pounds of cure. This is for a protection against accidental run and reduction of potential energy on a hydraulic system. There are brakes (along with blowout protections), but I need something that will make them set BEFORE someone gets hurt. I've never had an accident, but I don't want to take a chance because the safety was just too heavy for one person to move.