Steel Beam Bracing 3

[BH6]

I am trying to make sure I understand the bracing requirements for a steel beam. I have read many of your posts, however, I still am unsure of the following. I have a steel canopy with the largest span of 46'. I have designed the beam as fully braced (from the steel roof deck). I also checked it with the full uplift (42psf)completely unbraced. However, I am unsure if I still need to provide torsional bracing and if I do how to design for that. It is a monoslope roof with a low slope and the beams are tilted to follow the roof. Thanks for your help.

 

[CivilSigma]

To prevent beam LTB, you need to prevent lateral displacement of the beam under design loads.
I'm not sure if the deck metal cladding is stiff enough to resist that.

If you need a more detailed analysis, you should check that the strength/stiffness of the steel deck is sufficient in resisting the lateral movement of the beam when laterally unsupported.

I think I may have found a comprehensive paper on this problem, I hope it helps.

 

[CivilSigma]

The paper with design examples:

http://www.ecs.umass.edu/cee542/handouts/Fundamentals_of_Beam_Bracing.pdf

 

[BridgeSmith]

I'm not sure if the deck metal cladding is stiff enough to resist that.

The restraint necessary to prevent LTB from starting is fairly small. It's the connection of the decking to the beams that is usually the cause for concern for us (bridge designers).

Assuming beam spans are simple spans, so the compression flange is fully and adequately braced, torsion shouldn't be an issue, unless there are external torsional forces applied.

The ends of the beams, of course, need to be adequately braced to prevent roll-over.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BH6]

Thank you for your help.

I have been looking over the link that you sent. I can check the stiffness of the deck for DL, LL, Snow, etc. but what about when there is uplift (the uplift is greater than the DL).

1. The beam is okay designed as completely unbraced so Lateral is okay.

[Assuming beam spans are simple spans, so the compression flange is fully and adequately braced, torsion shouldn't be an issue, unless there are external torsional forces applied.

The ends of the beams, of course, need to be adequately braced to prevent roll-over.]

2. What about for torsion when there is uplift and now nothing is bracing the beam along the compression side now?
It is a canopy and completely open so when there is uplift does a simple shear plate connection at the end of the beams provide enough restraint (There are no external torsional forces applied--except for the beam being at a slight angle)?
How would you check for that? The deck is now along the tension side of the beam. Do I only check the beam as completely unbraced to take the uplift load and assume the stiffness of the deck (along the tension flange) and shear plate connections at the end of the beams handle the torsion?

3. Does the beam being on a slope of 1/2" per foot need to be checked for any additional rotation because the loads are still just being applied perpendicular to the beam.

 

[BridgeSmith]

What about for torsion when there is uplift and now nothing is bracing the beam along the compression side now?
It is a canopy and completely open so when there is uplift does a simple shear plate connection at the end of the beams provide enough restraint (There are no external torsional forces applied--except for the beam being at a slight angle)?

You said it checks out for the uplift case. I'm not sure what you meant by that, but if that didn't include lateral torsion buckling (LTB), you'd have to check the capacity for the restraint conditions you anticipate and see if it's adequate. The checks would need to performed according to your governing code. I can't help you with that; I only know the AASHTO bridge design code.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[CivilSigma]

I'm not sure what code your using, but both Canadian and American steel design codes provide you a reduced moment capacity based on unsupported beam span length.
If this capacity is smaller than the moments caused by the applied forces for the entire beam length, either in gravity or uplift, then your section is adequately designed.

Also because the beam is sloped, a magnitude of the perpendicular gravity force will apply compression axial forces. Check that the interaction design equation is satisfied for combined bending and compression.

The only remaining design consideration is detailing the beam to column connection. Make sure that it provides lateral torsional restraint at the beam ends.