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HSS Weld Resistance in Bending

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Zupdog

Structural
Jul 3, 2023
1
Hello,

Fairly simple question I hope,

Say you have a cantilever HSS, Fillet weld all around.

When calculating the resistance of the weld for bending moments would you only consider the resistance of the "flange" sections. See attached weld line diagram.

Considering only single axis bending.

Thanks
 
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I generally consider the entire perimeter, and do not correct for the direction of the weld stresses.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
To expedite calcs, I would only consider the upper and lower "flange" sections, while neglecting the directional strength increase. If that results in either 1) a weld larger than 5/16" (single pass fillet weld) or 2) the base metal of the HSS can't withstand the required weld (e.g. D_req for flange force > D_max_basemetal), I would consider the weld group as a whole (4 workable flats only). If that doesn't work, spec a thicker HSS that can handle the fillet welds you need.
 
It depends on what it’s welded to. If it’s welded to a rigid or otherwise stiff enough object I’ll use the whole perimeter.

If however it’s an HSS welded to a similar HSS I check the web alone. If that doesn’t work I’ll refine the assessment.
 
Tomfh said:
If however it’s an HSS welded to a similar HSS I check the flanges alone. If that doesn’t work I’ll refine the assessment
Seems like bending of the wall of the carrying HSS would be the limiting state here. In which case, the vertical "web" might be taking all the demand.
 
Sorry I mean I check the webs alone, for the reason you mention…..
 
My SMath program uses the perimeter, and includes 3 points for the corners... and is expedited. Just a matter of plugging in Fx, Fy and Fz and Mx, My, and Mz (torsion) and the size and type of weld. This is for an end plate type of connection. If you are welding it to another HSS, that's a different problem.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
Like Tomfh, I feel that this decision cannot be made without consideration of the stiffness characteristics of the support.

a) Super rigid everywhere? Do as you like.

b) Stiff only behind the flanges? Get it done there.

c) Stiff only behind the webs? Get it done there. This is what I do at dog legged stair stringers.
 
I'll typically do the following. (Horizontal member with vertical load and moment about a horizontal axis.)

Attribute the shear to the welds along the vertical walls. The top and bottom walls won't have much shear to transmit through the welds.

Resist the moment by a couple between the welds at the top and bottom walls. The vertical walls would have a linear stress distribution -- conservatively neglect that.

As an aside, in the new AISC Specification, the directional increase isn't used at the ends of HSS in tension. See the new section J2.4 just below Eq. J2-5. Also, the Commentary to J2.4, starting with Page 489, last paragraph.
 
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