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Opinion - LTB with Stiffener Plate

Opinion - LTB with Stiffener Plate

(OP)
I've got a project where a big I-beam is required. I routinely try to frame secondary framing into the web of the primary I-beams to reduce the unbraced length of the beam. But I've got an issue where the depths are not close. A W27x84 and smaller "joists" - W16x26. I don't want to increase the beam size any more than I have to....it is spanning 59 feet and I need to find a way to reduce the unbraced length. I don't think the architect will go for kickers from the bottom flange. I would appreciate thoughts on the idea attached.

RE: Opinion - LTB with Stiffener Plate

I suspect this is easier than you're thinking. I doubt that you need to to anything special here.

1) Old school wisdom was that a connection covering 60% of the girder depth was sufficient for rotational bracing. You're about there. I think that there's a real rule somewhere in AISC now but I can't recall what/where.

2) Do you not have a diaphragm on top of the beams that restrains their movement in the plane of the diaphragm? If you do and compression is limited to the girder top flange, then all you need is nodal bracing of the top flange and infill beam depth is of no consequence.

3) Worse come to worse, I think that your proposal would be quite effective.

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.

RE: Opinion - LTB with Stiffener Plate

Have you considered a full depth (or deeper) shear tab on one side (or both) with std bolt holes.

RE: Opinion - LTB with Stiffener Plate

So you are dealing with a negative moment region of the girder?

The W16's would routinely be considered as laterally bracing the top flange if it was in compression - assuming that there is a diaphragm or other means of securing the W16's against lateral movement as well.

With a negative moment condition, the stiffeners could indeed be considered as bracing the girders bottom compression flange assuming there is a rigidity between the W16's and the stiffeners.
AISC has requirements for the minimum stiffness required - in your case you are using the flexural stiffness of the W16's (and to some extent the plates) to keep the girder braced.

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RE: Opinion - LTB with Stiffener Plate

Actually, if your dealing with new construction, I think that your detail could be improved. Just bolt your webs to the full height -- or near full height -- stiffeners. This would avoid field welding and preserve the rotational ductility of your connection.

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.

RE: Opinion - LTB with Stiffener Plate

(OP)
Well I don't want to add anything if it's not necessary. There is no negative moment here - beam is simply supported. I think I tend to be conservative when it comes to LTB. This is a roof beam and it will have a metal deck on top. I suspect we'll puddle weld. Just wasn't sure if I could count on the deck to brace the compression flange. Needs to resist 2% of the flange force, right? If I need to do anything I like the idea of a full-height shear tab better than what I sketched.

RE: Opinion - LTB with Stiffener Plate

The W16 beams and a metal roof deck will brace the girder I would think. To verify just go through AISC's checks.

If it is a roof beam, do you have any net wind uplift? (D+W) Something to check also to verify that there isn't any negative moment in the girder.

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RE: Opinion - LTB with Stiffener Plate

Agree with KootK. The more common connection detail for this would be a full depth stiffener with the W16 web bolted to the stiffener plate.

RE: Opinion - LTB with Stiffener Plate

(OP)
Thanks for the comments! Appreciate the help.

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