Resolving Torsion due to Simply Supported Beam with Overhang

[polskadan]

Hi all! Hope everyone had a great Christmas and is ready to get back to the grind!

I have a situation in which I would like to get some input from you forumers. I have an HSS beam that must tie into a girder and then follow with an overhang. Due to height restrictions I cannot afford to stack the beam over top of the girder. Currently I have an HSS beam that frames into an HSS girder for ease of connection (welded moment connections). I would fundamentally like for this system to behave similar to a beam running over top of a girder which relies on the girder only to pick up vertical load and zero torsion. I want the end reaction of the HSS girder to the column to only be a vertical load. With my beam framing into the girder & forming a moment connection for continuity, the beam will induce rotation in the girder that it attaches to. This rotation induces torsion and leads into the issue that I am currently grappling with:

Now that I have induced torsion in my girder, unless the beam would warp to infinity, the ends of my beams will have torsional loads. With my choosing an HSS girder, warping is restrained and thus I have a large torsional force at my connection point of my HSS girder back to the web of my existing column. I ultimately do not want to dump this torsion into the column which would be a moment about the strong axis of the existing of the column, rather I would like the connection to be torsionally released. Realistically however, in order for this to occur, if I create a cap plate with shear tab connection for the HSS girder to column connection, the shear tab would have to rotate/warp in order to not transfer this torsion load into the existing columns. This fundamentally does not seem like something I would like to do. Is this typically a scenario that many people wave their hands over & ignore, simply put, if you did not design your connection to transfer torsion then their will be no transfer of torsion? In my situation it would seem that I cannot ignore the torsion that would be transferred from my HSS girder to shear tab connection.

Ultimately I may have use an Wide Flange Shape for the girder as to promote warping and reduce the torsional load at the connection to be minimal & much less than with an HSS Girder.

Am I viewing the load path and subsequent design assumptions here similarly to how any of your forumers would? Your input is greatly appreciated. Also, I have attached a rudimentary sketch for clarification.

Thank you!

 

[TLHS]

Before you spend too much time thinking about this, consider how much your shear tab would need to deflect to release any torsion at that point, which would establish the load path you're looking for. Some quick math is telling me that it's a really small number. It really depends on your geometry, though.

Model it like it's pinned there, check the rotation at that point and then translate that into a deflection on your connection. If it's negligible, I'd be comfortable designing the connection such that it doesn't carry torsion (but such that it would fail in a ductile fashion against torsional loads) and then distribute it all to the traditional load path through your framing beam.

 

[Ron]

When you induce torsion in the HSS, you have to react the torsion. In this case, you are reacting it through a rigid connection at the column. If you use a floating connection with shear tabs, you will still induce a moment in the column (in the form of a force couple) when the rotation in the HSS hits the shear tabs. Either way you shake it, you'll have to react the torsion through the column or you'll have to prop the cantilever with an angle strut.

 

[KootK]

1) I don't disagree with any of your logic.

2) I do believe that most engineers would let this ride. I would.

3) You could take detailing measures to reduce the torsional stiffness at the end connections. Maybe create a bearing seat at the column and connect the girder to it with a thin bearing plate and a couple of bolts? Throw a flexible neoprene bearing pad in there too if it suits you.

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.

 

[hokie66]

I would consider this to be the same condition as if the cantilever member passed over the girder. The compatibility torsion can be ignored.

 

[Ron]

From a liability perspective, you should consider the torsion, even if you ultimately ignore the magnitude of its influence. If you consider it and ignore, that is your professional opinion and exercise of engineering judgment. If you don't consider it, that becomes possible negligence. It's easy to consider and discount....not so easy to defend a negligence claim. This is particularly true if you are in a high wind area.

Looks like you've certainly considered it...now stick a note in your file that you did so.

 

[DETstru]

Imagine if you didn't have the stub cantilever on the left, you would have a beam framing into the side of your HSS girder at an eccentricity of b/2. This would put a slight amount of torsion into your girder.

The stub is actually taking torsion out of the girder. If you had an equivalent torque from the left to combat the eccentric reaction from the right, torsion would be zero. Now you can't assume that so you're likely designing for unbalanced loads. So yes, there still is torsion there.

I think you need to find the worst unbalanced torsion and resolve it into the columns you're connecting to. Unless your loads are massive, I don't imagine it'll impact your connection whatsoever.

 

[hokie66]

This is the old issue, discussed on the site many times, of compatibility torsion vs stability torsion. In this case, it is simply compatibility torsion, where the carrying beam does not need to resist torsion, but rather the moment is taken in the carried beam. If you must write something to file, it starts "By inspection..."

 

[Ron]

hokie66....agree! That's all it takes to prevent a lot of heartburn down the road!