Andrewstructure
Structural
- Dec 22, 2009
- 43
Continuation of prior discussion located here:
Received from the Steel Solutions Center:
"There are some situations for which Cb can be less than 1.0, but they are rare and specialized. The 2010 Spec. Commentary spells this out a bit better than does the 2005 edition. Near the end of the Commentary to F1, the draft states “Cb may be conservatively taken equal to 1.0, with the exception of some cases involving unbraced cantilevers or members with no bracing within the span and significant loading applied to the top flange.” Dr. Todd Helwig, who is the first author of the underlying paper on the subject (Helwig, Frank, and Yura, (1997) “Lateral-Torsional Buckling of Singly Symmetric I-Beams,” ASCE Journal of Structural Engineering, September) has written to the SSC. He advised that it is realistic for Cb to be less than 1.0 for some singly symmetric members, so one should not take
the value at 1.0 in those cases. He also mentioned that Iyc/Iy has to be pretty small and Cb won’t be very far below 1.0 in these cases. My opinion is that this is a fairly academic question in general because it’s tough to envision a realistic case that Cb would be much below 1.0.
The SSRC Guide to Stability Design Criteria for Metal Structures 6th Ed., Section 5.2.9, next to last paragraph indicates that the typical AISC approach overestimates the strength for certain cantilever conditions and refers the reader to the second quarter 2004 AISC EJ paper by Dowswell entitled “Lateral-Torsional Buckling of Wide Flange Cantilever Beams” and a 1983 book by Trahair. If you’re interested in that kind of situation, I’d recommend referring to the SSRC Guide."
Received from the Steel Solutions Center:
"There are some situations for which Cb can be less than 1.0, but they are rare and specialized. The 2010 Spec. Commentary spells this out a bit better than does the 2005 edition. Near the end of the Commentary to F1, the draft states “Cb may be conservatively taken equal to 1.0, with the exception of some cases involving unbraced cantilevers or members with no bracing within the span and significant loading applied to the top flange.” Dr. Todd Helwig, who is the first author of the underlying paper on the subject (Helwig, Frank, and Yura, (1997) “Lateral-Torsional Buckling of Singly Symmetric I-Beams,” ASCE Journal of Structural Engineering, September) has written to the SSC. He advised that it is realistic for Cb to be less than 1.0 for some singly symmetric members, so one should not take
the value at 1.0 in those cases. He also mentioned that Iyc/Iy has to be pretty small and Cb won’t be very far below 1.0 in these cases. My opinion is that this is a fairly academic question in general because it’s tough to envision a realistic case that Cb would be much below 1.0.
The SSRC Guide to Stability Design Criteria for Metal Structures 6th Ed., Section 5.2.9, next to last paragraph indicates that the typical AISC approach overestimates the strength for certain cantilever conditions and refers the reader to the second quarter 2004 AISC EJ paper by Dowswell entitled “Lateral-Torsional Buckling of Wide Flange Cantilever Beams” and a 1983 book by Trahair. If you’re interested in that kind of situation, I’d recommend referring to the SSRC Guide."
