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unbraced length Lb question

unbraced length Lb question

RE: unbraced length Lb question

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

RE: unbraced length Lb question

(OP)
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.

RE: unbraced length Lb question

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

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