Lateral Torsional Bracing – Floor Voids

[Eng868]

Good day all,

I am presently working on the design of a multi-storey structural steel building.
In structural steel frames, composite floor decks and secondary beams are commonly used as lateral torsional bracing for main beams. However, when there is a void in the floor (e.g. for a staircase), there is no adjacent floor system or secondary beam available to provide bracing (see image below).

How do you brace the beam in this situation?

 

[human909]

You don't. As you have said there is a void. So lateral bracing is not possible. (Well unless you can wrangle something from the facade on the opposite side of the beam to the void) But given it is not loaded in bending then is the lack of lateral bracing a problem?

Also the following question is why do you have a W16x57 there? I'd expect a beam that is less deep if it doesn't have vertical loading. And if it has significant axial loading then choose a stocky section.

 

[KootK]

How do you brace the beam in this situation?

You get creative if you really do feel that the beam needs bracing:

1) Run posts moment connected from this beam to the beam a floor above or below. This is akin to the "roll beam" bracing concept just done as a "roll post".

2) "Brace" the beam by increasing it's torsional stiffness. Weld a channel or two to the beam web. Or just switch the beam to a stocky HSS.

 

[Eng868]

Thanks for the responses.

Some other info:

• The beam is located on the perimeter of the building. Cladding or a masonry wall will go there, thus induce vertical loads.
• I grouped the beams in the analysis to reduce variability in section sizes. Hence, W16x57.
• I considered using cladding as a brace, similar to the suggestion by KootK. However, I was not sure if this was a conventional solution.
• Once I am satisfied with the ltb requirements and a solution, I will advise the architect on the use of cladding, masonry or otherwise.

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Any other suggestions are welcomed.