Unbraced length of tube 2

[MotorCity]

Does anyone know where to find the Lp and Lr values in the LRFD manual for square or rectangular tubes? I frequently use the tables for the wide flange sections. There are equations to calculate these values but it states that these equations are only valid for wideflanges and channels.

I realize that tube sections are more resistant to torsion and thought that it may not be an issue. Is it required to check for lateral torsional buckling and consider the unbraced length of tubes? If not, why not?

 

[shepherd]

Motor City,

Commentary Section 5.1 of AISC "Load and Resistance Factor Design Specification for Steel Hollow Structural Sections" (Free download I believe)states that lateral torsional buckling is not a significant issues and gives an example showing this assumption. Based on this, I usually don't worry about if for HSS.

Shepherd

 

[MotorCity]

EUREKA!

 

[Taro]

Charlie Carter of AISC sent this response to a similar question on the SEAINT listserve on 5/6/04:

Lateral-torsional buckling (LTB) is a phenomenon of importance for W-shapes and similar members of open cross-section because there is comparatively low torsional resistance. In an HSS, however, there is comparatively high torsional resistance and the warping component is eliminated almost entirely.

In truth, there is a small reduction for LTB in a rectangular HSS bent about the strong axis. However, it is noted in the Commentary to the AISC HSS Specification that the reduction in bending strength is less than 7 percent for a typical span up to 40 ft on an HSS 20x4, which is about as bad a strong-to-weak ratio as you could get for an HSS. In view of this and the likelihood that serviceability limit states will control beam design anyway, it was decided that LTB need not be a design consideration as a matter of practicality.

So yes, I think it is acceptable practice to not reduce for LTB in HSS beams. Local buckling effects may get you below Mp, though.

Charlie