Capacity of a round weld 2

[anchorengineer]

Hello All,
I'm looking for a formula to calculate the capacity of a fillet weld for a round post with a horizontal load at the top inducing a moment.
Thanks!

 

[paddingtongreen]

KootK, I was about to agree with you when I saw your note on AISC. I got there by considering an equal and opposite pipe on the other side of the plate, and wondering if the shear pattern should really be changed by the addition or removal the plate.



Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[CELinOttawa]

Isn't AISC the code which does not require a combined actions check?

I think we may be talking about an incompatibility of methods...

 

[CELinOttawa]

Okay, so I had a dig through, but don't use the US AISC code very often. I'm happy to be corrected, BUT...

It appears that section G "shear" requires the use of the reduced Ag/2 (just as Kootk found), and F "Flexure" requires a reduced capacity for local buckling, effectively resulting in an approximate S/2 value...

BUT this code does not require a combined actions check. Meaning that it reduces the flexural and shear values, but then does not require combined actions. It is acheiving the same result in a different way.

If I have the energy (and can think clearly enough) over Christmas, I'm going to go through the solutions for CISC, NZS 3404, AISC, and Blodgett all with the same input. Let's see if these disperate codes give similar results... I think we've been mixing methods at cross purposes...

I do really, really like Kootk's derivation of the shear case for the long (what I called "global" case), but I don't buy it as then needing to be subject to combined actions. If you've already isolated the loads to the shear "webs" on the sides, the flexure is in turn isolated in the top and bottom "flanges". Doing both is excessive in this case... UNLIKE in the case of a fully plastic member where you've activated the full cross section, and then AISC still doesn't require combined action. *confused*

 

[KootK]

@Paddington: interesting mental experiment. I agree with the conclusion. The shear forces in the pipe at the support are the shear forces in the weld.

@CEL: the AISC method is just straight up VQ/It mechanics of materials. There should be no compatibility issues.

@ Everybody:

I did a bit of numerical fiddling using the Blodgett example and assuming my theory of shear distribution to be correct. See the graphs below which describe the variation of force in the top left quadrant of the weld starting from horizontal (0 degress) and ending at the zenith (90 degrees).

I plotted four relationships:

1) Weld shear force on its own calculated via VQ/It.
2) Weld tension (bending) force on its own.
3) Combined, vector sum weld shear calculated with my theory.
4) Combined, vector sum weld shear calculated with Blodgett's simplification.

The results are as follows:

1) At the original 108" length, shear stresses barely register on the graph.

2) At 24" length, the shear stresses register on the graph but do not affect the outcome.

5) At 5" length, the shear stresses would finally affect the outcome. Of course, at such a low span to depth ratio, flexural theory probably doesn't even apply.

My conclusion:

An improvement upon Blodgett's simplification would be to simply not include shear in the calculation at all. There doesn't ever seem to be a practical scenario where it would affect the weld size.


5" Length

24" Length

108" Length

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.

 

[KootK]

Thanks for your latest response CEL. I posted mine before I saw yours unfortunately. I'll give the combined actions business some background eggnog processing over the break.

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.

 

[CELinOttawa]

Just saw your's Kootk... Likewise I will give it thought, but I don't like the application of Tau = VQ/It shear flow for this problem. That's a longitudinal shear flow to assure composite action; I do recognise that it is similar to apply this to a weld on a plate, but it just doesn't feel *right* to me.

Let's regroup to discuss after much Turkey, Ham, and excessive beer. I am allowed beers again tomorrow so long as I wake up feeling "normal" again. Yay!

 

[KootK]

I don't like the application of Tau = VQ/It shear flow for this problem. That's a longitudinal shear flow to assure composite action; I do recognise that it is similar to apply this to a weld on a plate, but it just doesn't feel *right* to me.

I sense that a lot of folks have trouble with this. Because Tau_xy = Tau_yx, the longitudinal shear is the transverse shear. And because the transverse shear delivered by the differential element closest to the weld is the weld shear, VQ/It shear = weld shear.

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.

 

[desertfox]

hi Gents

Well I did some digging too and found you only need to combined stresses when they occur at the same point.
In the case of the circular beam or rectangular beam the maximum shear stress occurs at the beam neutral axis whereas the stress due to bending occurs at the beam section extremities.

see this link

http://www.steel-insdag.org/teachingmaterial/chapter31.pdf

see pages 31-5 to 31-11

I also came to the conclusion that the direct shear stress is very small compared to that in the region due to bending but it looks like you've all got there before me.

regards

desertfox

 

[KootK]

@desertfox: these great references that you keep posting seem to be chapters of an online book. Any chance you could provide links to the complete books?

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.

 

[desertfox]

Hi KootK

No sadly I never seem to get any of the full books but if I do I will post it.

This latest link I attached below on pages 3 to 6 is quite clear about how conservative the treatment of welds as lines actually is.

https://eis.hu.edu.jo/ACUploads/10526/CH 9.pdf