calculating loads

[Andy57]

To all you smart guys,
I haven't visited the site in a while but I need some help. I am trying to determine the load a 3"x3"x1/4" 6061 square aluminum tube will carry over spans from 12" to 120". If anyone can assist it is greatly appreciated.

Andy

 

[Trevor]

Uniformly distributed load or a point load? Are the ends rigid?

 

[Andy57]

The load would be spread out not pin pointed. Each end has a welded plate and that plate is bolted to the structure.

 

[nodaksn3]

Andy- What are you going to do with the info? Are you trying to bridge over something? Or, making ramps? 3" x 3" aluminum won't carry much, especially if you're spanning 10'. I'm a PE, and wouldn't give out any kind of advice like that, especially without knowing all of the details. If you're mixing Land Cruisers and aluminum sections, my advice would be: be very careful. Ned

 

[amaurer]

as a rough estimate, 4x margin to yield is safe-ish (hope these are static loads!), and i'll ignore your welded supports and pretend they're pinned which should be conservative.

if you have a uniformly distributed load, keep it below about ~350lbs per inch on a 12" beam (4200 total), or ~10lbs per inch on a 120"er (1200 total).

if you do ANYTHING that deviates from a perfect-beam-perfect-load situation you need to go lower. For example, if your load is not uniformly distributed (honestly, unless your load is a giant air bag, it WON't be uniform), or if you manage to "pinch" the beam and buckle it. this also says nothing about how well (or not!) your beam is attached to the supports.

if it was me, i'd consider my worst case, that is, all of the load right on a point in the center of the beam, and stay below THAT... in this case, stay under 2200 pounds on a 12" and under 220 pounds on a 120"er.

(all of that has 4x yield margin)

 

[eleblanc]

I get similar results in 2 software. per amaurer comments, perfect load, perfect assembly.

2000# equaly distributed over 120", both end suface pinned.

 

[Fast Eddy]

I get similar results in 2 software. per amaurer comments, perfect load, perfect assembly.

2000# equaly distributed over 120", both end suface pinned.

Cool. Etes-vous une fusée scientifique?

 

[eleblanc]

Cool. Etes-vous une fusée scientifique?

Nah with the right software this is easy to check.

 

[rsbcruiser]

I ain't no fancy PE just an ME so i don't have to deliver it so refined fellers, 1 ton on a 3 by 1/4" butter beam, Nuts, aluminum SUCKS for dynamics dude, plus all this is hog wash cause you is gonna weld the sucker therefore destroying the heat treat. Trust me here, worked for a moron who built aluminum trailers, QUIT after imagining myself in Court trying to expain why my Employer was an Idiot and why I worked for him. I'd go a factor of safety say 10 or better. Understand aluminum fatigues so once you get comfortable and quit watching for cracks THEN it will fail. Bet all those alum. trailers you see on the road are post weld heat treat.

 

[amaurer]

I ain't no fancy PE just an ME so i don't have to deliver it so refined fellers, 1 ton on a 3 by 1/4" butter beam, Nuts, aluminum SUCKS for dynamics dude, plus all this is hog wash cause you is gonna weld the sucker therefore destroying the heat treat. Trust me here, worked for a moron who built aluminum trailers, QUIT after imagining myself in Court trying to expain why my Employer was an Idiot and why I worked for him. I'd go a factor of safety say 10 or better. Understand aluminum fatigues so once you get comfortable and quit watching for cracks THEN it will fail. Bet all those alum. trailers you see on the road are post weld heat treat.

Some good points here. T6 heat treating of 6061 increases yield stress from 8ksi to 40ksi, a factor of 5. So, if you ruin the heat treat on the whole bar of aluminum, you can only apply 1/5 of the above loads while preserving your 4x safety factor.

That said, if he just welds the ends of the bar to a plate its probably not a big deal. The contact area is pretty large, so the stress isn't nearly so concentrated as it is in the cross section due to bending. I fully realize this is hand-waving and your point is valid as for why this wouldn't cut it in a "product", but depending on his application it might still work just fine; for example, if the weakened areas are all loaded in compression...