Torsion Compatability

[GalileoG]

Please refer to the sketch attached to this post of two connections as an example.

Say I have two connections. One beam to beam and the other beam to column connection. The beam in both connections experiences torsion, which produces a force-couple on the top and bottom flange (which are connected using "cover plates")

There is an eccentricity between the 3 sided weld group and the single fillet weld on the girder beam/column.

My question would be, how is the torsion produced by the eccentricity of the force (M/d) shared between both weld groups in both connections? How do you determine that?

I would assume for the beam to column connection, I will have a moment on the column to cover plate joint equal to M/d * e (it takes all of it since the column is alot more stiff than the beam) whereas the beam to beam connection, the moment produced by the eccentricity is distributed between both weld groups, but to what extent, I don't know.

Can someone please elaborate? How can I quantify/calculate all of this? Is my basic understanding correct?

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[DaveAtkins]

The weld line at the supporting beam or column will only see shear (in and out of the paper).

The three sided weld group at the supported beam will see shear and moment.

DaveAtkins

 

[GalileoG]

But is that always the case though? Wouldn't some of that moment be shared by the 3 sided weld group on the beam to beam connection because of the low torsional stiffness of the supporting beam?

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[rb1957]

in connection 1, how much moment can the web react ? not much. it'll work very much like a pinned end, and so rreact only (ok, mostly) shear.

conection 2 is better at reacting moment, if the flanges aren't too wide. this will be more like a cantilevered end.

but nothing is fully fixed, and few things are truely pinned.

 

[hokie66]

I don't see any torsion on the welds. The supporting beam would be twisted, but the supporting column would be bent. The supported beam in both cases is in bending and shear, not torsion.

 

[GalileoG]

Hokie, I think you may have misunderstood. My example is if the supported beam has applied external torsion on it and not bending moment.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[hokie66]

Yes, I did misunderstand.

I think for weld purposes both connections are the same. The supporting beam will resist the torsion by strong axis bending, and the supporting column will resist the torsion by weak axis bending. I can't see why you would say the column is a lot stiffer than the beam, when it looks to me like the reverse would be the case.

 

[StructTaco]

Hokie is correct, the beam is in strong axis bending due to the applied torsion. The column is in weak axis bending due to the torsion.

As for the welds, twisting the supported beam will cause shear in the flange welds, that reverses in sign from one side to the other. The web weld will not be very torsionally stiff in relation to the flange welds.

All that being said, and it's time to stop the calcs today:

So for a quick back of the envelope check I would assume a flange force, or shear in the weld = Torsion / Depth of supported beam, then compare that number to about half of the weld length available in the length of two beam flanges. If the allowable shear is greater than the flange force....

Call it good??

Or model it in an FEA program for solid modeling...or look into a steel design text addressing instantaneous centers of rotation methods for eccentrically loaded weld groups...

 

[DaveAtkins]

You've got me confused, Clansman.

The torsion in the supported beam is resolved into two equal and opposite flange forces, in and out of the paper. As I said earlier, each flange force enters the supporting beam through shear along the weld line. The three sided weld group at each flange resists this flange force through shear and moment about the weld group.

DaveAtkins