Thanks for your thoughts. There is some truth in what you say as small deflection theory can not be used.
You may be interested in viewing all of the comments made on this subject including those received after writing to the Institution of Structural Engineers. These are at:-
Further comments would be appreciated.
Martin Double CADOSS
Sorry for my English, but I am not a native speaker
> IIRC what professors taught us: the reason that slenderness must be
> limited is that theory works with idealized materials and properties
> which is not true for the real world materials.
> So this limit represents the point where material behaviour starts
> showing differences (greater than certain limit) from theoretical > behaviour.