Torsional Restraint of Singly-Symmetrical Section in Compression

[Serhiy2]

Good day. I'm analyzing the top chord of the open web steel joist in compression. The top chord is cold-formed hat channel and mode of failure in compression is torsional buckling. This top chord is connected to metal roof deck and I'm trying to figure out what can be considered as adequate torsional restraint of this member and what capacity this torsional restraint should provide.

When estimating the torsional restraint moment, would it be reasonable to take it as lateral buckling force in this member (2% of compressive force) acting at the shear center and multiply it by the distance between the shear center and center of gravity of the member?

 

[KootK]

1) Per your other thread, I feel that the chords only have convincing torsional restraint at the nodes where the webs tie in.

2) As far as attempting to use the deck to torsinally brace the top chord, perhaps something like this:

a) Envision the deck as bracing the top chord via "roll beam".

b) Visit the bracing chapter in the AISC steel manual where they provide strength, and more importantly stiffness, requirements for bracing.

c) See if you can get enough out of the deck and the connections to manage #2. Again, though, see my warning in your other thread about the viability of using the fasteners this way. In my mind, you'd need either:

i) Fasteners staggered to produce a couple or;

ii) Reliance on your ability to form a couple between one fastener in tension and the deck pushing against the chord in compression.

3) The 2% business was originally developed for nodal bracing situations. I believe that I've seen some international codes jury rig that for use in torsional bracing situations but, personally, I'd be much more comfortable using the AISC stuff that I mentioned.