Ask an engineering question, get an engineering answer: That Depends.
Just how are you going to be loading the angle? How well are you
supporting it from twisting? If you want it to resist a bending moment
(which I assume is what you want), why aren't you using a 'U' or 'I'
section, or a tube?
I'll have to plead ignorant. The angle was chosen because of
availability and ease of working other materials around it, but this
can be changed. The steel will be bolted to 3/4" plywood with an even
load of 450# along a 6ft length. What would be the most efficient
shape and size to use?
Heh, the gang is making you jump through hoops!
Your original premise that a longer leg down might increase the strength
is a good start but angle iron has some quirks in the strength
department. When used to support shelves, the lower leg tends to buckle
when overloaded. So even though the section modulus might be higher with
a longer leg, the thinner cross section will let it buckle under a
lighter load than a shorter leg version. This also means that nobody
really wants to say it will (or won't) work in your application
The other problem with angle iron is that the upper and lower chords
within the angle are wildly unequal. Plenty of metal in the upper
(horizontal leg), very little in the lower leg (just thin metal)
A rectangular tube shape is usually a better choice. Start with one
about the same weight per foot as the angle.
The OP wrote "with the longer leg PARALLEL to the load", ie. NOT at
right angles to the load nor vertical. Therefore my answer was NO.
I'd use a bent-up channel section of the appropriate section modulus.
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