Beam stability against LTB
Beam stability against LTB
(OP)
In AISC, the equations used for determining beam strength based on LTB are developed from the assumption that the beam is restrained adequately against torsional rotation at the supports (ends for a simply supported case). Suppose a beam is laid across a single span and supported by the ground on either end, but it is in no way mechanically fastened (bolted/welded) to the ground. Would you consider that to be restrained? How would you modify the allowable moment (or Cb factor) to accommodate such a case?






RE: Beam stability against LTB
http://www.eng-tips.com/viewthread.cfm?qid=410584
RE: Beam stability against LTB
RE: Beam stability against LTB
RE: Beam stability against LTB
Does this depend on how the load is applied? If the load is applied below the rotational axis (e.g. neutral axis for an I-beam) is this still not stable for vertical loading?
RE: Beam stability against LTB
It is worse than a suspended lift beam, as lift beams are supported at their ends from the top. They can swing, stably.
RE: Beam stability against LTB
RE: Beam stability against LTB
Dik
RE: Beam stability against LTB
Can't find my reference for it, but the internet says BS5950 4.3.2 Table 13.
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The name is a long story -- just call me Lo.
RE: Beam stability against LTB
Would this approach also work for checking a lifting beam for LTB?
RE: Beam stability against LTB
For LTB of lifting beams here are two reference papers:
2] "Stability of I-beams Under Self Weight Lifting" by Dux and Kitipornchai, Australian Steel Construction Journal, Volume 23, No. 2, 1989.
RE: Beam stability against LTB
A beam resting on supports (but not connected) is rotationally restrained by the reaction extending across the beam width. This is then discounted as I mentioned above for web flexibility (versus a traditional bearing stiffener) between the bottom and top (compression) flange.
A lifting beam is rotationally restrained only by its own self weight relative to the point of rotation imposed by the rigging.
Ingenuity's two references are some of the best I've found to address the subject, although they're a little too academically focused to apply to some situations.
For other references, check out WARose's post above. I'm not sure I'm entirely sold on Helwig's method, but it's very practical to apply.
Regardless -- this is a problem without a rigorously defined solution (as far as I'm aware). And I've been looking for years.
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The name is a long story -- just call me Lo.