shear center or centroid of weld group? 2

[Lion06]

Why, when analyzing a weld group do you take the torsional moment as the load times the distance from the load to the centroid of the weld group instead of to the shear center of the weld group?

I'm thinking specifically of a C-shaped weld group (as shown in example 5.18.1 of S&J 5th edition, but also shown in many other examples). My understanding is that for zero torsion to exist that the load must be through the shear center. If it occurs through the centroid, and the centroid doesn't align with the shear center, then torsion is introduced. Why is this not accounted for in elastic weld analysis?

Let's say you have a channel cantilevering from a column that is fillet welded on three sides, creating a C shaped weld. Let's, for argument's sake, say that the channel is loaded in a plane through the centroid of that weld group. You would clearly have torsion in the channel, because it isn't loaded through the shear center. Why is it that the torsional moment in that channel doesn't have to be taken out with the connection (the C shaped weld group)? Per typical elastic weld analysis, the weld group is loaded through its centroid, and is therefore not subject to any torsional stresses.

I don't agree with that. Anyone have an opinion on this?

 

[chichuck]

miecz,

I read your printout, and I note that you've assumed Case 7 from AISC Design Guid 9. That case is a torsionially fixed ends. I refer to Appendix C in that design guide:

"...If however, the beam is an isolated span, Ojalvo (1975) demonstrated that a closed box made up of several plates or a channel, as illustrated in Figur C.2, would approximate a torsionally fixed end. Simply welding an end plate or column flange may not provide sufficient restraint."

I can't include Figure C.2, but it is a wide flange with two plates welded top flange tip to bott flange tip, on each side of the W-shape, making an effective tube for a short length at the beam end.

The cantilever channel in the original post does not have this type of box included, so I think that the channel welded to a column flange is not a torsionally fixed end, rather it is a torsionally pinned end. I think the difference between fixed and pinned is the warping, just an end plate doesn't restrain the warping in the section, while th box end does.

chichuck

 

[miecz]

chichuck,

You're right, the connection "may not provide sufficient restraint" to be considered fixed. However, the connection will provide some unknown degree of torsional fixety. The design guide is focusing on the design of the beam, not the weld. For the design of the beam, I believe it's conservative to assume the torsional restraint is pinned. For the design of the weld, it would be conservative to assume the torsional restraint is fixed.

 

[Lion06]

nice find miecz.