Bracing against LTB
Bracing against LTB
(OP)
Hello, all:
I had a question about restraining flexural members against lateral-torsional buckling. I suspect that there's some misunderstanding on my end about beam theory that I'm not properly thinking through. When beams in frames are braced laterally to beams in an adjacent parallel frame, how does that result in a brace point for that beam? If one beam in a set of frames is loaded to the buckling point all on its own, I can understand how an adjacent unbuckled beam would provide lateral restraint. But if a floor or roof is loaded uniformly, wouldn't all the beams tend to buckle simultaneously? In that case, how is restraint against buckling applied? This is especially bothersome to me with stair stringers (where it's common to treat the unbraced length as the distance between treads), since the treads aren't even connected to the compression flange of the stringers. Can anyone help shed some light on what I'm not understanding about situations like this?
I had a question about restraining flexural members against lateral-torsional buckling. I suspect that there's some misunderstanding on my end about beam theory that I'm not properly thinking through. When beams in frames are braced laterally to beams in an adjacent parallel frame, how does that result in a brace point for that beam? If one beam in a set of frames is loaded to the buckling point all on its own, I can understand how an adjacent unbuckled beam would provide lateral restraint. But if a floor or roof is loaded uniformly, wouldn't all the beams tend to buckle simultaneously? In that case, how is restraint against buckling applied? This is especially bothersome to me with stair stringers (where it's common to treat the unbraced length as the distance between treads), since the treads aren't even connected to the compression flange of the stringers. Can anyone help shed some light on what I'm not understanding about situations like this?






RE: Bracing against LTB
Joe Yura at UT Austin (Texas) has done a lot of research into this and much of his work is included in the latest AISC Specification (Appendix 6).
He has indicated that you can brace a beam against LTB either by resisting twist or by resisting lateral translation of the compression flange.
There are approaches for "relative" and "nodal" bracing in Appendix 6 where nodal braces provide rigid, non-moving restraints against LTB and the relative brace is more like what you describe in your parallel frame description.
For stair stringers, the treads are actually at an angle to the "vertical" webs such that rotation of the member is prohibited, or highly resisted. Therefore they act as a brace - almost like solid bridging between the two stringers.
RE: Bracing against LTB
RE: Bracing against LTB
RE: Bracing against LTB
In stair sections that I have designed, I used cross bracing of the bottom flange to prevent torsion.
RE: Bracing against LTB
RE: Bracing against LTB
RE: Bracing against LTB
RE: Bracing against LTB
The treads (and risers if present) do form a diaphragm of sorts. It is a Vierendeel truss. As long as the treads are adequately connected to the stringers, I don't think you need to worry about buckling or sway.
RE: Bracing against LTB