Lateral support for a beam

[SKJ25POL]

I told my boss that a W beam (if under loading and bending about weak axis)needs to be separately lateraly braced and being braced for bending about strong axis does not assure the bracing for bending about weak axis. May I ask if you think I am right.

We are talking about two parallel beams in air . No floor on them. Please see attachment.

Thank you

 

[structSU10]

I don't think bracing is helpful for weak axis bending. The only thing that can buckle is a slender flange in compression, which cannot really be braced against. You may want the 'bracing' to be there to distribute the lateral load, but I would think an x brace, or chevron type arrangement is more typical. along with a horizontal truss maybe.

You will be worried about lateral torsional buckling for the strong axis, and if this is just a two beam system, you may have to consider global LTB for the two beam system, along with individual LTB. See attached paper for some discussion.

 

[JAE]

Yes - weak axis bending doesn't need bracing against Lateral Torsional Buckling.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[KootK]

@StructSU10: Great paper. Thanks for that. This actually came up in a recent thread:Link.

@OP: It’s true, beams don’t laterally torsionally buckle when bent about their weak axes. Someone should explain why that is, of course, so that you can debate it with your supervisor. Everything in nature tends towards a state that minimizes potential energy. Unchecked, nature would very much like to roll our I-beams over weak axis so that deflection would increase, the loads would move closer to the earth, and potential energy would be minimized. Lateral torsional buckling of beams loaded strong axes reflects precisely this tendency. By the same token, it would make no sense for a beam loaded weak axis to roll over to the strong axis position as that would increase potential energy. Thus, no weak axis LTB. It would be much like hopping on a skateboard and having it coast uphill.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[SKJ25POL]

So genlemen,
You are saying there is no need for bracing for bending about weak axis for lateral torsional buckling?
Did I understand it right?
I appreciate it simple language

Sincerely,

 

[jayrod12]

Yes, no need. Your section would yield before it were to buckle.

 

[SKJ25POL]

Thank you everyone.
I appreciate for your valuable comments.

Have a good weekend

 

[TehMightyEngineer]

A good analogy I've always used is if you take an envelope or similar sized folded piece of paper (extremely slender beam) and try to bend it strong axis it almost immediately buckles. This is lateral-torsional buckling. But, if you take the same paper and bend it weak axis then it will simply bend without ever buckling.

It's worth noting that weak axis bracing IS sometimes required for axially loaded beams and columns if they need it. But this is to get the KL/r_y value up, not for lateral-torsional buckling.

Maine EIT, Civil/Structural.

 

[BAretired]

Simple language, right. The question asked in the sketch is "Does this bracing work for weak axis bending?"

The answer is no, not for lateral torsional buckling. But if a force is applied at the arrow marked '2', the left beam will resist all of the force and the right beam will resist none of it. If the force is applied eccentrically, the left hand beam will rotate.

So the conclusion is that the bracing ensures that the two beams deflect equally at the brace point and prevents rotation of either beam, a simple form of load sharing.


BA