Haynewp,
That is a very good question - sorry my response is so long.
You had me scurrying back to the code and to a very good 1975 reference "Source Book for the Australian Steel Structures Code AS 1250" by Dr. M.G. Lay. (Our ASD code has remained essentially unaltered since then). This was not intended as a design guide or commentary, but to provide "background data, explanations and reference sources for those rules...whose origins and reasons for inclusion are not immediately apparent" [If only there was a similar document for every code that has been written since then
![[ponder] [ponder] [ponder]](/data/assets/smilies/ponder.gif)
]
The Code includes :
Rule 6.1.3
Slender-leg struts When the value B/T for any angle exceeds 208/sqr(Fy) [Austim comment - still metric - Fy in MPa], the calculated stress shall not exceed the lesser of :
(a) 0.50 FY, and
(b) the value of Fac calculated from Clause 6.1.1 [which is the standard equation to allow for flexural slendness effects].
Dr. Lay's booklet has the following to say :
"
Rule 6.1.3 Slender-leg Columns
Column buckling is usually associated with flexural Euler-type buckling of the type covered by Rule 6.1.1. However, stocky members may also buckle in a torsional mode by twistng about their longitudinal axis. This mode is actually closely related to the local plate buckling failure mode discussed in Comments 32-35 and 45 [which relate to the effective width formulae I mentioned in my earlier post - Austim]. It is not very common in structural components as the torsional buckling load is much higher than the squash load for practically proportioned columns. In addition, because of its relation to plate buckling it is usually effectively restricted by the Rules of Section 4 [again, the effective plate width rules].
However, one case which can be of practical concern is associated with the use of high strength (Fy>=350 MPa) angles with slender legs. Whereas I and channel sections cannot usually be made with critically high B/T ratios, the simple angle is available with B/T ratios of up to 16. To avoid the possibility of torsional buckling mode (ie local twisting of the angle legs) the Rule limits the maximum permissible stress in angles with B/T over 208/sqr(Fy) to 0.50Fy. It effectively downgrades the yield strength of slender legged angles when used with short slenderness ratios. Many angles are used at high slenderness ratios, and so are unaffected.
[Haynewp - Dr. Lay must have known you would ask about this

] Other cases of outstands on compression members could also be covered by this Rule. However, it would be too severe for a case such the "web" of a T section made by splitting an I beam. In such a case the slender web would be attached to a much stiffer flange. Control of such cases is adequately supplied by Ruyle 4.3 [effective width rules again] which provides an upper "effective' B/T limit of 256/sqr(Fy). The more conservative 208/sqr(Fy) in Rule 6.1.3 is restricted to poorly supported outstands in pure compression.
The theoretical background to this Rule is given in Reference 84 - (Lay, M.G. Width-thickness Limits for steel angles in compression, BHP Tech Bull, 14(2), Aug 70, 24-28)"
Getting hold of a copy of that reference now would be nigh impossible, but Max Lay's commentary would generally be taken as "Gospel" in these parts without worrying too much about going any further back.
If you have read all this - well done. Time for a coffee.