Desinging Weld for Torsional Loading 2

[Stillerz]

I am trying to design welds for torsional loadings (twisting) on HSS.
I am using Blodgett's procedure for welds treated as lines.
Table 4 on page 7.4-6 of Blodgett has the equation for the force on the weld for "twisting" as:

f=T*C/Jw

Jw can readily be determined from table 5 on pg. 7.4-7 for HSS.

My question is, how does one determine "C" for a rectangular HSS ?

 

[cntw1953]

Without getting into details, the alternative is much superior/desirable than the original, which tends to have excessive stress concentration at the tips. You might be able to get ride of the short weld cloest to the hole, but still comes out with thinner weld size with same capacity, if not greater.

 

[desertfox]

Hi racookpe1978

This file works out the equivalent fillet weld for the "U" shape to give the same weld area as the existing "L" shape,
the new fillet weld 0.227" however I suggest you round it up to 0.25".
We need to analyse the loads next, but I am unsure from your drawings the orientation of the load and also how it can be typically in 9 positions round the hole?
I will assume a single unity load acting at the hole running parallel to the 2" thick strip at the 5.5" distance as given on the original sketch.

desertfox

 

[dik]

Thanks Desert Fox... I've converted the info as attached. The first column is based on original sizes, the second and subsequent are sorted to use with VLOOKUP in excel. Re-sort on the NDX value to restore.

 

[dik]

I'll try this again...

 

[desertfox]

Hi dik

Nice job, if you look round the site you will find tables for "I" beams, channels,angles etc.
There is a lot of other good engineering info there to.

regards

desertfox

 

[desertfox]

Hi racookpe1978

Here is a calculation for the old and new weld stresses.
I used unity load because you didn't specify one, but you can see your new arrangement uses half the weld size but is under less stress than the old one assuming of course the load case I chose is correct.
Let me know what you think.

desertfox

 

[racookpe1978]

This is a pdf sketch of the Pipe Support. It's arrayed around the center of the casing, each pipe support weldment is clamped to the circular pipe inside the array.

Design assumption, since all 9 pipe support plates are welded up identically, must be that the load is shared equally at all 9 bolted points, or that it's easier/better to make one common part than it is to individually calculate each plate and each weld.

 

[desertfox]

Hi racookpe1978

What is the orientation of the load as we look at your plan view? I assume the load is going into the paper but whats its magnitude?
I have calculated that your new arrangement only needs to have a quarter fillet weld as opposed to the half inch previously.
Are the calcs of any use to you.

desertfox

 

[desertfox]

hi racookpe1978

Well if I am right about the direction of the load then the calc's I posted need the load turning through 90 degrees and the welds need to be stressed for bending not torsion, would you agree?

desertfox

 

[racookpe1978]

True: But again, every weld is sized and spec'ed the same, so I'm not really sure what his method really was. Or how detailed his original analysis was.

You are looking horizontally into one end of the machine casing. The weight (load) on these nine pipe supports is a circular 6" diameter pipe ring with a large number of equally spaced nozzles (small flex hose connections) spaced all the way around. So dynamic loads are equal everywhere around the circle, but were probably considered very low compared to the weight of the pipe assembly itself. (Reasonable, a small diameter flex hose will not transmit much force to its nozzle.)

Dead weight loads are "down" - towards the bottom of the image.

The dead weight was probably were considered equally divided among the nine bolted pipe supports, but since each acts vertically on each bolt at each pipe support, each weld is stressed at a different angle at each of the nine. Since weld and each pipe support plate is detailed identically, somebody apparently just figured stresses out once.

 

[cntw1953]

If the main loading is the pipe dead weight, it is not unreasonable to think all 9 bolts share same amount of weight. You can make a spreadsheet to calculate force components along the axes of the support arm at each position. However, the design will be governed by the most critical case, and the thickness be kept the same for all positions to simplify the construction (welding) process.

 

[cntw1953]

By the way, looks like desertfox has done most of the work, a star for him. Cheers.

 

[desertfox]

Hi racookpe1978/cntw1953

Thanks for the stars guys.

Well I think your worst case situation for the welds is the one I have detailed, where the component weight acts at 5.5" from the bolt hole vertically down creating torsion and direct shear but I could be wrong.
Not sure I agree that the weight of the pipe is equally shared amongst nine supports as they will always be some slight variation in the manufacture which destroys the symmetry. Indeed just looking at the sketch of the pipe supports they are not spaced symmetrical, one half of the pipe as four supports the other five and only one support is on the horizontal centre line.
I think if it were my problem I would design the support such that if I had all the weight hanging off one or two of those supports it would do the job, then the additional supports would be belt and braces.
I agree racookpe that whatever component weight hangs off the supports each of the welds is loaded differently.

desertfox

 

[cntw1953]

desertFox:

I agree, the bolts would not have identical load if the supports are non-symmetrical. But you are quite conservative on only one or two out of 9 supports work scenrio, a huge safety factor for static pipe support.
I think a SF of 2 should satisfy most environment.

 

[dik]

Desert Fox...
Each time I encounter this type of data, I convert it to the format that was attached. I use the first column as a dropdown list to VLOOKUP in the sorted data. I add the index to convert it back, in the event I have to add or remove something.

Dik

 

[Stillerz]

my post has been hijacked!!!

 

[desertfox]

hi Stillerz

Yes sorry it wasn't done intentional,but racookpe1978 had a situation very close to yours with torsional stress on weld,I suppose he felt it was related which in truth it was.

regards

desertfox

 

[dhengr]

Stillerz:
Did you ever get your welding for torsional loading problem resolved? Or, are you just complaining about your thread being hijacked to be difficult. I hesitate to tell you what Blodgett is talking about, on pg. 7.4-6, because I thought I told you never to bring the subject up again, in public.

Blodgett’s books and papers are sometimes a little bit difficult to follow, he was a bit short on explanation at times, his notation was lacking at times, some publishers might have done a little classier job of proof reading or exact technical review before printing, but the books would not have been as inexpensive either, and they sure contain a wealth of good info. I knew him pretty well and he was an amazingly intuitive guy on welding, weldment design and your kind of problem. For what he had to work with, he certainly boils the problems down to the basics, but could be quite theoretical too, when the problem called for it.