Bracing of a torsionally loaded WF beam

[jpw2913]

We have a question going around the office..if a wide flange beam is braced on the compression flange (in this case the top flange) by bar joists, but an upper structure (raised aisle) is also attached to the beam at about midspan that will impart a torsional moment, will a web stiffener below the upper structure where the torsional moment is applied be enough to consider the beam braced against rotation when the upper structure is laterally loaded (wind)? Thanks for any help!

 

[Ron]

Why not just weld a kicker off the joists to the bottom flange of the beam. In my opinion, the web stiffener does not provide adequate torsional bracing...it will just shift the torsional response to another location.

 

[jpw2913]

My initial thought was that since the top flange was braced via the bar joists, beam rotation would not be a concern. The upper structure is welded to the top flange of the beam in question. Will rotation be an issue?

 

[weab]

If I understand correctly, all the stiffener will do is assure that the beam rotates and keeps its I-shape. Your beam must withstand the torsion between torsional supports. Torsion is an issue.

 

[JAE]

From your description I would agree with weab. You might post a sketch so we can understand fully what you've got.

 

[jpw2913]

After reading over "The Fundamentals of Beam Bracing" by Yura, I agree that the web stiffener won't offer any resistance to rotation, it will just keep the beam from distorting. I'll post a sketch in the morning to clarify, but I think I just need to find the torsional moment imparted on the beam then check the the torsional capacity between bracing points (bar joists @ 5' oc)....agreed?

 

[jdgengineer]

Bar joists will not resist the torsion without something bracing the bottom flange as well. Think about it, you have something bracing at the top, and the whole section wants to twist. The bottom flange is completely unrestrained and will want to move.

I agree with Ron, add some kickers to brace the bottom flange. If kickers are not possible, consider a closed shape. If a closed shape is not possible, check torsion on the I-beam per AISC Design Guide #9. The full procedure can be fairly complicated in my opinion.

As a simplification, (depending on your condition, span, end restraints etc) sometimes it is ok resolve the torsion force into a tension-compression couple taken out by the top and bottom flanges. You can then check the flanges alone for the weak axis bending of the flange spanning between points of torsional restraint. This is a simplification and can be easier than calculating the warping torsion equations etc.

 

[JAE]

jdengineer is completely correct on the torsional strength checks - but keep in mind that wide flanges have very little torsional stiffness so your upper structure will move - a lot.

 

[Lion06]

I agree with everyone else. Are we all assuming that the stiffener would happen at the same location as a bar joist? I think that's the assumption, but it's not clear from the description.

If that is true, then the only way a stiffener does anything is if the connection between the bar joist and WF is adequate to transfer the moment (it won't be). This would be similar to trying to brace the bottom flange of a WF with a stiffener and studs into a slab - it works with studs, not just a puddle weld to the deck.

A kicker is the best option here.

 

[ToadJones]

Can you arbitrarily add kickers to the bottom chord of a bar joist?
What checks need to be done?
I have a similar application right now

 

[Lion06]

I would not kick to the bottom chord. I would kick to the top chord and let the diaphragm take out the lateral force.

 

[Ron]

TJ...yes. The small axial load from the kicker would be distributed to two OWSJ's, and for torsion mitigation, one would be in compression and one in tension, with alternating potential.

 

[SAIL3]

Lion...unless the top chord is restrained against rotation the torsional load would still not be addressed completely..the lateral load would eventually go into the diaphragm anyway.
TJ...using the kickers(best option), as suggested, to the bottom fla would change the end conditions of the bar joist from simple to fixed and impart an applied mom from the wind.
The WF would still rotate under wind load but would be self- limiting and only as much as the bar joists deflect...
if there is only one kicker(one side only), the WF would rotate
under dead and live on the bar joists, again, self-limiting..