Freestanding Portal Frame - Member Effective Lengths

[tally25]

Hi guys,

I'm currently undertaking design checking of some existing free-standing single-span steel portal frame structures (i.e. no intersecting members such as purlins, girts, or other members that provide restraint) and it has left me in two schools of thought as to how one calculates the effective lengths of its members. Note that the column base connection is fixed (via a moment base plate attached to moment resisting bored piers), and likewise the column to beam connection is fixed (via moment end plate with stiffeners).

The two ways I can consider this are as follows:

[ol 1]
[li]Consider the beam and columns as a single cranked beam - the system is considered as a single beam with fixed ends at the base plate locations, with full restraint provided at these points. No intermediate restraints along the length of the frame. i.e. L = 2 x column length + beam length[/li]
[li]Treat the columns and beam as separate design members - this is what I would expect the traditional approach to be, however I am unsure of what flange restraints to provide at the beam/column interface? I assume you would have to justify the columns providing lateral restraint to the beams via cantilever action and while satisfying clause 5.4.3.1 of AS4100 at the very least, but this method seems a bit more of a stretch than method 1[/li]
[/ol]

Any thoughts on how to approach this? Any literature anyone can point me to?

Thanks all.

 

[KootK]

#2 would be my approach although I don't know of any specific literature. #1 seems to punitive. You might google around the world of pipe rack support frames as those are similar. Although there is often bracing out of plane.

I'd imagine that K=2.0 on your out of plane column buckling, in combination with whatever out of plane loads need to be dealt with, would address the lion's share of this.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.