Choosing sufficient size and thickness of aluminium angle iron

[stevie888]

For a given weight-per-unit-length, a square box section has the same strength as an L angle in a simple deflection analysis, but has the benefit of not twisting.

Wrong.

 

[alligatorlizard]

Tried a totally different AI - completely different answers again!! OK, lesson learnt there. Also found a bunch of different "deflection calculators" but nothing that seemed to both suit this scenario and that I could figure out all the necessary data for. May just have to spend a bit of time learning the principles.

But back to drawing board in a way - is this idea even doable without immensely heavy beams? Deflection seems to be the main issue, need to keep this to less than 10mm, closer to 5mm probably more ideal to avoid stress on fixings.

Square box section seems best bet, would prefer aluminium (6082 grade) as less weight hanging over my head, but mild steel an option. Channel-shape also might work? If anyone would recommend anything different (either material or shape), please let me know!

 

[Harry Bloomfield]

Square box section seems best bet, would prefer aluminium (6082 grade) as less weight hanging over my head, but mild steel an option. Channel-shape also might work? If anyone would recommend anything different (either material or shape), please let me know!

Unistrut, might be an option, or could you add a wall to wall wire support, above you lightweight structure, then add adjustable suspensions from that, to support your structure?

 

[endecotp]

For a given weight-per-unit-length, a square box section has the same strength as an L angle in a simple deflection analysis, but has the benefit of not twisting.

Wrong.

Correct, I was wrong.

I do not claim to be a structural engineer. As I wrote in my first reply, I looked at this mostly out of curiosity about whether the Artificial Stupidity was being useful or not.

If any actual structural engineers would like to contribute to the thread, that would be great.

 

[endecotp]

The AI was saying around 8mm max

I don’t care how much deflection the useless “AI” thinks it wants - how much deflection could YOU accept?

I suppose the main consideration is that too much deflection would pull on the wall plugs of the mounting brackets.

Don’t fix it like that then. Imagine a shelf on each wall that the ends of the beam sit on.

 

[alligatorlizard]

I don’t care how much deflection the useless “AI” thinks it wants - how much deflection could YOU accept?



Don’t fix it like that then. Imagine a shelf on each wall that the ends of the beam sit on.

Yes - the shelf idea! Was thinking it through last night: if the stress on the fixings due to deflection is the problem, how do I eliminate that? Solution I came up with is mount a 50x50mm batten on either wall, then use L brackets screwed into the top of the batten, perpendicular to wall, to form "channels" to keep the beam from slipping side to side. This way each end basically just rests on the 50mm batten, hence is free to bend all it likes without stressing anything except itself. Also, thanks to the perpendicular L-brackets that will loosely clamp the beams in place, I can then choose a rectangular box section, say 50mmx25mm, and have it with the 50mm sides vertical - as I understand, the horizontal side doesn't help much with deflection, so may as well reduce this to save weight. I think I can get sections this size (or similar) in 3.2mm thickness, if that's enough. Here's where I need a bit of help with the calculations

As for how much deflection I want - honestly, I do not know what is considered acceptable in this situation. Intuitively I would say about 1cm? Though now we've solved the deflection-stressing-fixings issue, I suppose it could deflect more if needed, and if the aluminium can cope with this. Definitely want to keep the weight and size of beams as low as possible, but I suppose it's a trade off between larger/heavier beam vs deflection.

Still hoping someone who's used to working with these materials will come across this thread and may be able to give a ballpark size/thickness for this application.

 

[alligatorlizard]

Here are some basic sketches of the new plans (batten arrangement, side view, and top view) now that the idea is to have the beams resting on wooden battens either side of the room (so as to eliminate the problem of deflection of the beams stressing any fixtures). Just need to figure out what size/thickness of aluminium box section is needed to support 12kg of weight collectively between two beams (or to simplify, 6kg each, as weight will be evenly distributed) - however deflection now no longer as critical.

 

[alligatorlizard]

Have started a new thread, as the plans and main questions have now changed (thanks to advice given in this thread!)

Please feel free to close this thread.

The new one is: https://www.diynot.com/diy/threads/...-of-hollow-box-section-aluminium-beam.645962/