unbraced length Lb question

[delagina]

I always thought that Lb for bending of beam is for both lateral and torsional buckling length.

In attached platform pic, Lb is the whole length of the beam?
Because the perpendicular beam will only brace the torsion but will not brace the lateral buckling?

 

[BAretired]

For lateral torsional buckling, the unbraced length is Lb/2 if the cross beam prevents torsional rotation at mid-span. For lateral forces, the span is Lb and would be shared by two beams in bending about their weak axis. This could be reduced to Lb/2 by using a couple of diagonal braces or by relying on the platform deck to act as a diaphragm.

BA

 

[delagina]

in staad, we have UNT and UNB used for bending and Ly and Lz used for compression.

so UNT and UNB will be Lb/2, and Ly will be Lb ?

I've always used Ly = UNT = UNB and nobody has been complaining.

 

[BAretired]

For compression, the unbraced length is Lb because both beams can buckle in the same direction unless you provide a diagonal brace in each bay.

I don't know how staad treats the input, so I would say that:

UNB = Ly = Lb (lateral unbraced length)
and UNT = Lb/2 (torsional unbraced length)

The fact the nobody has been complaining does not mean it is correct. Perhaps your platform deck or grating is acting as a diaphragm in which case you would be okay with your assumption.

BA