Beam Lateral Bracing (at simply supported ends)

[pioneer09]

What is the lateral brace force at the end of a beam that is simply supported. Take for example cutting in a new opening in a wall that only has dead weight (CMU or Precast). A new steel beam is placed over the top of columns on each end for a steel frame in the opening. What is the required lateral force (out of plane resistance) at the beam end. I would say that beam is unbraced laterally since the CMU wall in this is example is only providing gravity loading. With that being said, using Appendix 6 of AISC 13th, the lateral bracing force for nodal bracing does not seem like the right approach to take. Any input would be appreciated.

 

[1503-44]

That column looks like it provides no lateral restraint for the beam above. You essentially have a cantilever beam that is only being prevented from vertical deflection. The column is a fragile flagpole, not designed to provide any lateral restraint for anything. Not even itself. The beam is the only thing holding that column upright. If the end of the beam moves left, the column moves left. As far as lateral buckling of the beam is concerned, it is a free end cantilever.

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[KootK]

Wouldn't a brace at the top flange of the beam where the cross beam ties into stop the twisting from occurring?

Every little bit helps but be careful that the thing that you're bracing to is itself restrained against lateral motion.

It's a sunny Saturday afternoon and I don't feel much like wandering down "proof alley" but I am 99% certain of these things:

1) Conventional LTB design of a simply supported beam absolutely requires rotational restraint of the beam ends which, in North American parlance, means:

a) A lateral brace at both the top and bottom flange to provide torsional restraint.

b) A roll beam to provide torsional restraint. In this case, it is common, although less convincing, to use a "roll column" instead. Using a roll column here, I would absolutely want some kind of lateral restraint in combination with the twist restraint. I'd be a little more liberal with a true roll beam properly connected.

2) Someplace, I'm pretty sure that #1 is formally expressed as an AISC requirement. If I recall, this was shown to me during a lengthy LTB discussion with WARose.

So I feel strongly that your first order of business is to get that beam torsionally braced at it's supports. Given the nature of your situation, I'd also like to see the bracing tied off to something more substantial than a roof joist. I've shown some options below.

Lastly, another consideration here might be web side sway if you don't have bearing stiffeners. Here's an excellent discussion on that: Link. It looks to me like you do have stiffeners but the shadows in the photo make it difficult to tell for sure.

 

[Settingsun]

What braces the column heads perpendicular to the beam? Bolting the beam down to the braced column head with beam web stiffeners is adequate to restrain the beam end for LTB. If the columns aren't braced, they probably should be.

Is the torsion due to wind on the concrete panels resisted by bracing at 8' centres as mentioned earlier?

You've also got top flange loading. Does the US code penalise LTB for that? I had a feeling it doesn't.