Moment Connection on Each Side of Supporting Beam 1

[acwooten]

When you are providing a moment connection for a cantilever beam that is supported by a main supporting beam, what is the reason for providing a beam with a moment connection on the other side of the main supporting beam, in the same location as the cantilever, even though the additional moment connection could be a simple shear connection?

Is it simply to provide resisting moment on the opposite side as to bring the resultant moment at that location closer to zero and minimize torque at the main supporting beam?

 

[BAretired]

Yes.

BA

 

[TehMightyEngineer]

It could also be utilized as lateral support for the structure as a whole (a moment frame lateral force resisting system).

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[JAE]

Where is the supporting cantilever moment getting resisted?

Wide flanges have very little torsional stiffness.
Slabs on top of the system could perhaps resist thee cantilever bending moment but how to get the resultant forces up into the slab?


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[acwooten]

JAE,

I don't think I understand your question. Maybe I should clarify mine.

I have a main beam (B1) that is simply supported on each end. A beam (B2) is cantilevered off of one side of the main beam (B1) to pick up a hanging load that couldn't be located directly below the main beam (B1). This cantilevered beam (B2) will just be welded all around to the main beam (B1). I've included another beam (B3) at the same location along the main beam (B1), but on the other side of the main beam (B1). This beam (B3) will also be welded all around to the main beam (B1) and then frame into a secondary main beam (B4) a few feet away with a shear connection to create a bridge walkway, mainly made up of the 2 main beams (B1 & B4). The question was why is it necessary to create a moment connection where the (B3) beam is at the same location as the (B2) beam along the main beam (B1).

 

[WARose]

Maybe I am missing something here.....but are you saying the torsional load on this girder is balanced by a torsional load from another beam framing into the other side of this girder? (All the B1-4s kind of lost me. A pic would help.)

If there is any unbalance (of torque) that results in the ends of the girder seeing any torque.....be advised: simple shear connection won't transfer that load. (I make that point because you say the "main beam" is "simply supported".)

 

[acwooten]

Hopefully this will help.

 

[acwooten]

Whoops, here it is.

 

[BAretired]

I think the attached drawing shows the situation. Unless B1 can resist the torsion without excessive rotation, B2 and B3 must both be moment connected to B1. There will still be some rotation and B4 will lift up by the reaction from B3.



BA

 

[WARose]

Ok, I think I follow you now. So that beam on the other side of the cantilever frames into a girder running parallel to the other girder (with a simple shear connection) right?

If that's the case, yeah, that works. You will get a bit of compatibility torque at the ends.....probably not much though (depending on displacement).

But to answer your question: it has to be (i.e. fixed on both sides) to balance out the torque.

 

[acwooten]

Ok, so my original assumption "Is it simply to provide resisting moment on the opposite side as to bring the resultant moment at that location closer to zero and minimize torque at the main supporting beam?" is basically correct. Thank y'all for responding.

And thanks BAretired and TehMightyEngineer for understanding without the additional explanation!

 

[JAE]

The connection from B3 to B1 must be a moment connection unless B1 can take the torsion (which for wide flanges, they can't.

The connection from B3 to B4 can be a simple shear connection.

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