Aluminium Beam Design 5

[BenAustralia]

I've read through our Aluminium Code here in Australia (AS1664.1), and it isn't clear what-so-ever. Not much time has gone into writing it.

Anyway, I've never specified/designed Aluminium before.

I have a small roof to do. Rafters are Aluminium RHS, with Al. RHS struts inbetween like blocking. Basically it won't buckle.

Can I design it as per normal Mild Steel? I.e. Just calculate the stresses in the extreme fibres and ensure it doesn't yeild?

Or am I missing something important?

Obviously E and yeild stress is different, but Ixx etc shouldn't be any different.

Ben

 

[Jerehmy]

do you have mathcad. I can give you a spreadsheet using ADM 2010 for comparison

 

[Jerehmy]

but to answer your question, yeah. if the laterally unbraced length is small enough, either extreme fiber stress or deflection will govern design.

 

[EngineeringEric]

From working the US Alumn steel manual, AL design is differnt than Mild Steel. For a beam you look at each section of the beam separate. The T-Flange, the C-flange, and the web. Then you check them each for their stresses with constraints. It has been a while since i did AL so things might have changed but it was 'simple' calculations. Find equation in a table for each portion of beam and then plug and chug.

Again this is a different code, and possibly outdated... I would be leery of treating AL as mild steel, if it was that simple then they wouldn't need beam equations in the AL design manual, right?

 

[Jerehmy]

https://www.dropbox.com/s/0uhi0h31kpqcypm/c-BEAM.pdf

example calculations for a channel.

 

[EngineeringEric]

Nice document J, and as i remembered a little more complicated than AISC Beam Design. I do like the transparency of checking each element individually and then finding the governing cases, similar to steel but just different enough to be complicated first time around.

Do you make all of those MatchCADS? or is there some share site like steeltools for excel programs?

 

[Jerehmy]

Thanks, it took a little bit to make and work all the errors out. I screwed up the effective member widths for a while, was using full widths for the buckling criteria. I prefer the calcs to a table because it helps me understand the beams behavior.

It's using Mathcad 15 but I printed it as a .pdf so Ben can view it in case he doesn't have Mathcad.

 

[a2mfk]

General comment is to be careful in dabbling in aluminum design, it is a bit different than steel. ESPECIALLY when welding is involved, and fasteners in general.

 

[BenAustralia]

Thanks for the info!

In my case I basically have a small roof that is pretty braced out, so a simple flange yield check and acceptable deflections and I'm good to go. Forces are very low.

One thing they don't explain at all as far as I can tell, is what is meant by "Intercept", "Slope" and "Intersection" values. Stupid question I should know or something not explained too well?

 

[Ron]

Not sure about the Australian Aluminum Code, but remember that if clips have been welded for any of the connections, the allowables stresses at or near the weld get reduced significantly in the American code.

 

[Jerehmy]

Yeah welding reduces strength at the weld location by roughly one third in a lot of cases.

The constants are for different strength curves for each temper of aluminum.

An aluminum construction manual from 1967 is available online here

https://law.resource.org/pub/us/cfr/ibr/001/aa.construct.manual.1967.pdf

it hasn't changed much at all (I've only gone through it briefly, but I didn't see anything.)