Bracing for Torsion

[sticksandtriangles]

I am trying to brace a beam for torsion where I am cutting off a double tee to bear on steel beam. The saddle connection has a good bit of eccentricity so torsion is a big concern.

I was wondering if a detail that I have shown below would be considered a brace for torsion.

Hopefully you can make sense of what is going on.
I would have one angle each side of the connection and some plate that was anchored into the precast double tee for the angle to connect to.
On one hand, I could this transferring shear like a normal shear connection, not helping for torsional bracing at all, but on the other hand I could see this double sided angle providing a brace for torsion if the beam wanted to kick out of plane in torsion.

Any thoughts on the matter would be appreciated.

 

[EngineeringEric]

Are you bracing back to the double tee? or are you braces going up towards the slab on the opposite side?

 

[sticksandtriangles]

Bracing back to the double tee, yes.
Braces are not going back to the slab on other the side. This is a one sided connection of angles only bracing the side with the double tee.

 

[EngineeringEric]

So i think that means you are bracing to itself? If the double is inducing a torsion, i don't think it can also provide the torsion restraint. I may be wrong, but I think it doesn't work, as the beam spins from torsion (bottom web spins away from Tee) the angles will then exert a downward and lateral load pushing the angle over even more.

If the angles went back to the slab, i think you would have a chance to make it work (assuming slab has capacity as well as connection detailing)

 

[sticksandtriangles]

Bracing to itself (the double tee), yes.
The way I picture it, if the beam starts to kick out under torsion, the angles will go into tension and start pulling back onto the double tee which is quite stiff, thus bracing the bottom flange against rotation.
I am not sure either though.

 

[JAE]

I think it can work. You are simply creating a rigidity between the tee and the wide flange such that the wide flange cannot rotate.
If there's no rotation then there is no torsion in the WF.

The downward load from the tee is eccentric to the WF....say a distance e.
That creates a moment (or torsion) at each tee of Pe where P is the DT's reaction and e is the eccentricity of the connection to the WF centerline.

Resisting this torsion is a force couple based on two forces:
1. A force between the new slab (over the WF) and the DT flange
2. A force between your brace angles and the WF bottom flange.

As long as these two forces are developed with proper connections it can work.
The DT flange and the new slab must be in direct contact (under compression)
The sloped angles must connect to the DT and to the WF bottom flange with adequate capacity in tension.

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[BAretired]

I agree with JAE in principle, but in order to remove torsion from the steel beam, the braces will be in tension. The connection to the underside of the DT flange has to be able to resist a tension of P.e/d.cosφ where φ is the slope of the brace. I would prefer to use through bolts, but with the top of DT flush with finished floor, that may not be possible.

BA

 

[KootK]

It's a bit of a double edged sword in my estimation. Converting torsion into flexure is usually the way to go in these scenarios. And the double tees are certainly the stiffest flexural element near by. That said, if you connect both the top and bottom flange of the beam to the double tees, then you force the beam to match the end rotation of the tees which can result in some significant beam torsion near the beam supports in some cases.

I wonder if the detail shown below might have legs. It restrains beam torsion but wouldn't lock the beam into parroting the double tee end rotations.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sticksandtriangles]

I like the idea KootK, but like you show, I would be a bit scared to dig into the web of the double tee with all the reinforcing in there.

That said, if you connect both the top and bottom flange of the beam to the double tees, then you force the beam to match the end rotation of the tees which can result in some significant beam torsion near the beam supports in some cases

I am having a difficult time understanding this statement. I do not believe that the angle connection will be forcing the beam to match the end rotation of the the double tee. I imagine the connection as the double tee being supported off of a pin, with the rotation of the beam not having to be equal to end rotation of the double tee. Maybe you could elaborate.

Thanks

 

[BAretired]

Connecting to the web of the DT seems like a pretty good idea, perhaps with a strap each side and holes drilled through the web horizontally. Could X-ray the web to make sure to miss reinforcement.

If the steel beam is tied to the end of the DT, then the torsional rotation of the beam will be equal to the flexural rotation of the tee.

BA

 

[RPMG]

For steel beams, it's typical to cjp (bevel) weld the flanges for torsion, which is bracing the beam to itself. Any detail will work, as long as the steel bottom and top flanges are connected.

You could just extend the steel beam stiffeners to the concrete web, and anchor it.

 

[KootK]

I like the idea KootK, but like you show, I would be a bit scared to dig into the web of the double tee with all the reinforcing in there.

Like BA said, you could get around this with a permutation of the detail that bolts to the side of the tee stems.

I am having a difficult time understanding this statement. I do not believe that the angle connection will be forcing the beam to match the end rotation of the the double tee.

Consider:

1) when the tee end rotates, the tee flange will push down on your angles.
2) when your angles are loaded in compression, they will kick the beam bottom flange to the right.
3) the top of your beam will stay put as it will be retrained by the floor deck.

All that adds up to your beam rotation matching the tee rotation in my opinion.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.