Compression Flange Bracing

[Vrpps EIT]

Hi All,

There is this situation of Joists running parallel to steel beam at roof level, but there is a difference in height between the End steel beam and the roof level. So the top flange of steel beam is not being braced or connected by the roof diaphragm (sheathing)directly and the LTB of steel beam controls the design. So I have attached a view of it with wood stud infills (like knee walls)

on top of the steel beam to match up to the roof level. In this case, if a bracing is to be considered for steel beam how to go with it?

My options are:
1). Could I lay the 2x6 LVL studs at every 12"O.C on flat face (horizontally) on top of the wood plate of steel beam and bolt it to the top flange of the beam on one end and nail it to the LVL beam on the other end, so that a horizontal blocking sort of arrangement is achieved which provides a restraint to the buckling of steel beam.

2). or Do I need to have L-Angle connection? between the steel beam and LVL beam?

Appreciate your time for your views, open to any suggestions

Thanks,
Vra

 

[phamENG]

Can you up size the beam to eliminate the LTB concerns? If there is a load path for wind uplift from the roof to the steel beam you may want to do this anyway as bracing the bottom flange of the beam in that case will be even harder.

If you're bent on trying to brace it, check AISC 360 Appendix 6 to determine necessary strength and stiffness of your brace. Then, use NDS to size the brace member and connections. Not sure about where you are, but in my neck of the woods "2x6" LVLs are not a thing. Best the carpenters around here would be able to do is buy a 1-3/4"x11-7/8" LVL and rip it. You may be better off with a 6x6 if you need the added strength.

 

[Celt83]

You may be better off using an HSS rectangular section in lieu of the Wide Flange in this situation.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Vrpps EIT]

Celt83,

Actually a 14x4(.3125"thk) HSS worked even when it was completely left unbraced, is it because the HSS geometry are stronger in lateral buckling that the controlling equation is only yielding and not LTB? Thanks

 

[Vrpps EIT]

Hi phamENG,

The 2x12 LVL connection that you mentioned is like this right?

 

[phamENG]

Never said anything about a "2x12" LVL, just mentioning potential limitations on material availability. I'm pretty sure what you're showing wouldn't work, though. Toe nail connections would not be sufficient. You may be able to make something like that work with a different connection.

Celt83 is probably right, though - an HSS member is typically used in this situation. Remember, you don't just have vertical load in that beam. The jamb studs for those windows (or the mullions, depending on the size) will be delivering horizontal wind loads and seismic loads to the beam that it has to resist in the weak axis. The brace you're showing will prevent lateral movement of the top flange, but won't prevent twist and the resulting torsional stresses. HSS is a bit simpler from an analysis perspective in this situation. The end connections can be interesting in wood, but they'd probably be easier to figure out than all the other mess you have to do for a wide flange in this case.

 

[KootK]

1) I like adjusting the beam size to ameliorate LTB whether it's a larger WF or a switch to HSS.

2) I don't love your latest detail. I'd not want to be relying on a toe-nailed connection in tension to brace a steel beam.

is it because the HSS geometry are stronger in lateral buckling that the controlling equation is only yielding and not LTB?

Yes, the HSS will have a much higher St.Venant torsional stiffness than most wide flange beams which will increase LTB capacity significantly.