Long span truss chord bracing

[JMB92]

I am working on designing a long span truss. From past experience I have seen most buildings with long span trusses have the roof beams sitting on top of the top chord and bolted into the flange. If I have a W8 chord and am wanting to use the roof beams to brace the chord against buckling for compression (not just compression flange bracing for bending), is attaching to the top flange sufficient? Or would the roof beams have to frame into the web of the truss in order to brace it for compression buckling?

 

[SteveGregory]

You may want to select a number of top or bottom chord panel points for full truss depth x-braced frames. It may be advantageous to locate these to coincide with the intersection of a compression web member and the bottom chord. These frames would probably be attached to the webs of your top and bottom chords.

 

[JMB92]

I was planning on having X braced frames every several trusses to act as sway frames and then linking them together with struts. This is per the recommendation of Design Guide 7 from AISC. However, the roof beams are much more frequent than the sway frames would be and I would be able to use a more efficient member.

 

[MIStructE_IRE]

Look at it another way which might be more familiar. If this were a tall column subject to axial compression only, would you rely on external sheeting rails alone to resist compression buckling? I’m not sure I would. As described above, i’d pick some discreet points to ensure proper restraint. You could also consider some V restraints which would brace both flanges.

 

[KootK]

I believe that the conventional approach is this:

1) chord is braced against buckling in the plane of the truss at each truss panel point.

2) chord is braced against torsional buckling at each truss panel point.

3) chord is braced against buckling perpendicular to the plane of the truss at each roof beam.

That's a pretty robust setup as your lowest energy buckling mode is probably constrained axis torsional buckling about the top of the top chord.