Truss Design 1

[cieg22]

Novice looking for help....
How do I evaluate a truss for lateral torsional buckling? I realize that the individual members carry axial loads only, but is there some way to take the unbraced length of the entire truss into account as it acts as a beam?

 

[whymrg]

You need to check top and bottom chords for out of plain between supports (top chord needs to be checked only if you do not have deck). You need to provide bracing (similar to joist bridging) between joints of truss to reduce out of plain length of chords.
Good luck.

 

[Chewey68]

If you are getting trusses from a truss manufacturer, have them provide you with the engineering calcs which should contain axial loads of the truss members.

 

[cieg22]

Thanks, Whymrg & Chewey!

Chewy,
We are actually extending an existing truss, so unfortunately, I don't think the manual will work for this case--although I will keep that in mind for the future designs.

Whymrg,
This is a design for a canopy, and I'm worried about negative bending due to wind. I'm not certain that Lb is less than Lp (if the bending formulas still apply).

I've also been thinking about this a bit more, since the top and bottom chords are not continuously attached (and they do not produce a moment of inertia where d = depth of truss), would I need to consider Lb anyway?

Thank you again for your help! I would greatly appreciate any additional comments you might have.

 

[whymrg]

As long as you have joints braced out of plain, you need only consider bending between joints. So Lb is distance between joints.

And approximate moment of inertia equal to area of chords times square of distance between neutral axes (between chords) and chord center of gravity (like beam without web).

Good luck.