Compression Flange Bracing

[pbailey]

With the attached detail, would you consider this steel beam's top flange to be braced at the spacing of the joists? There might be a little torsion induced by the offset - is there any standard practice or recommendations to reduce this torsion? (The wall above is an exterior wall and the joists form a second floor mezzanine in a single story building)

 

[DaveAtkins]

That beam is continuously braced.

DaveAtkins

 

[KootK]

1) In my experience most designers will consider the top flange to be braced in this situation.

2) You are correct in that the situation creates torsion.

3) In the past, I've attempted some fancy stuff to try to torsionally brace the beam to the joists. Straps, shear wall hold downs installed sideways, etc. I've relaxed some and abandoned those strategies in most situations.

 

[phamENG]

would you consider this steel beam's top flange to be braced at the spacing of the joists?

Depends on how everything is connected. You should run the numbers to see how it works out. Determine the maximum allowable unbraced length for the beam to work. Then, using AISC 360 Appendix 6, determine the strength requirements for the brace connection. Can your fasteners do that? (Remember, you need to get the load into the diaphragm, so you'll be looking at beam to nailer --> sheathing to nailer connections OR beam to nailer --> nailer to hanger --> hanger to joist --> joist to sheathing load paths.)

Torsion can also be checked easily enough. If you have a copy of Steel Structures: Design and Behavior by Salmon, Johnson, and Malhas there's a good, practical approach in there, often referred to as the simplified bi-moment method. Here's a thread on it: thread507-472982

Is that a bearing wall above? If so, you probably don't have much to worry about. Though if there's a giant door pushing on the bottom flange it might be an issue.

 

[pbailey]

Thanks everyone - I think this really clears thing up. I will go through those suggestions.

 

[XR250]

I have seen these twist in practice if the wall does not have enough load to balance out the joist eccentricity. (residential)

 

[Aesur]

I would say the top flange is braced, bottom and beam is not. I like to refer people to the RISA help file for unbraced lengths which makes this much clearer to understand. Link