ASD Limiting Width Thickness Ratios

[jrfroe]

I'm in the process of designing a very unique built-up steel section that will be loaded in combined axial compression and flexure. Imagine a steel plate about 48"x1/2", and the ends of the plate have flanges (18"x1/2") welded on one side of the plate only (so we now have sort of a 48"x18" channel). On the other side of the large plate we'd have two 6"x1/2" plates also welded to the large plate, but set back 12" from the edges (so that they are 24" apart). So the final section looks sort of like a large channel with two extra flanges welded to the back of the web.

So the trouble I'm having is involved with interpreting ASD Table B5.1. I'm thinking for the side plates (flanges?) I could use b/t < 95/sqrt(Fy/kc), and for the 48" plate I could use h/tw < 760/sqrt(Fb) for the non-compact limits. In the section I have, what d and h could I use to determine kc, d/tw, and h/tw. For h, should I use the clear distance between outside flanges (47"), or can I use the clear distance between adjacent flanges (11.25" or 23")? Similarly for d, should I use the full depth of the section (48"), or can I use the distance from the outsides of adjacent flanges (12.25" or 24.5")?


Thanks,
Jason

 

[UcfSE]

I agree with the 95sqrt for the flanges and stiffeners. For the web, I would use the full clear height of the web.

 

[UcfSE]

You should also check the proportions of your channel per Table A-B5.1 on page 5-99, if you would consider this section a channel. I suppose that is where the engineering judgment comes in.

 

[haynewp]

The things I think of when reading your question is to check for local buckling both as a compression member and as a flexural member. And to check how the formulas you are using compare to the newer AISC formulas, since I think you are looking at the old ASD. You may still be able to use the old ASD by your model code, but I would just do a quick check and see how things compare since you are dealing with such a large section. They may be essentially the same but in different form.