Beam Lateral Bracing
Beam Lateral Bracing
(OP)
I have continuous beam (negative bending moment) with the top flange (tension) braced by a diaphragm. Can I consider the bottom flange braced for compression if I run stiffeners from the top flange to the bottom flange?






RE: Beam Lateral Bracing
RE: Beam Lateral Bracing
My first instincts were that it's not braced but, a collegue is telling me that if the tension flange can't move and the compression flange is tied to the tension flange by stiffeners, then the compression flange can't move either.
RE: Beam Lateral Bracing
RE: Beam Lateral Bracing
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Beam Lateral Bracing
Here's how I think about it: The top flange is braced against lateral movement. The bottom flange buckling to the side will be rotation about the top flange though. The top flange is probably not braced for that rotation.
RE: Beam Lateral Bracing
AISC Appendix 6 provides strength and stiffness requirements for bracing and these would have to be applied to see if the stiffener-diaphragm mechanism could do the job.
Usually decks are only screwed or spot welded to the beams so an adequate force couple may not be developed enough to do the job.
RE: Beam Lateral Bracing
RE: Beam Lateral Bracing
A sidebar question.....
Assuming you have simply supported girders spanning to columns and roof purlin (say a Channel or WF section) is bearing on the girder (bolted with 4 bolts or welded parallel to flange).
If a stiffener is welded to the girder at the locations of these purlins, can the girder be assumed to be braced at purlin locations for uplift?
It's no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don't know which half - LORD KELVIN
RE: Beam Lateral Bracing
RE: Beam Lateral Bracing
Beam bracing must control twist of the section, but need not prevent lateral displacement.The LTB of the bottom flange may be idealized as due to two vertical equal and opposite forces at the two edges of the bottom flange and not as a horizontal force at the bottom flange since it would cause twist + bending about the minor axis. The full depth stiffeners would carry this push / pull to the top flange where further load path would depend on the adequacy of the fastening of the deck to the flange and flexural capacity of the deck as woodman88 suggested.
For the subject continuous beam, neither the deck at the top by itself can prevent twist of the beam, nor the stiffeners by themselves can prevent twist, though stiffeners would increase the effective radius of gyration about y-axis and hence increase Lr. However, an adequate diaphragm in combination with a couple of stiffeners at the inflection points can control twist of the section and the beam can be considered braced for LTB purposes.
RE: Beam Lateral Bracing
I am happy to have you agree with me, but I don't believe I agree with you. To prevent lateral movement of the bottom flange (caused by a presumptive horizontal force), the stiffener acts as a cantilevered bending member. The moment in the stiffener has to be resisted at the top by a transverse bending member, and that resistance can't be provided by a mere diaphragm.
RE: Beam Lateral Bracing
In my last post I have said adequate diaphragm - by that I meant adequacy of the fastening of the deck to the flange and flexural strength of the diaphragm. It may or may not be enough in every case.
RE: Beam Lateral Bracing
RE: Beam Lateral Bracing
When web stiffeners are used as a brace, they must be developed in bending as mentioned by Hokie. If the web stiffener and the member to which it is connected are strong enough and stiff enough, then it qualifies as a brace. Otherwise it does not. In other words, it is covered by the structural steel code.
BA
RE: Beam Lateral Bracing
Thanks to you both, BAretired and Hokie66.