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Capacity of a round weld
2

Capacity of a round weld

Capacity of a round weld

(OP)
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!

RE: Capacity of a round weld

This, my friend, is the thread for you: Link. It's pretty fresh too.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

See the attached. Simply calculate the section modulus of the weld, treating it as a line of unit width (line properties) with a diameter equal to the outside diameter of the round post. An M/S calculation will give you a lbs./per inch bending stress (S is in units of inches squared). Then take the horizontal load and divide it by the length of the weld (pi*dia.). The resultant load on the weld is the square root of the sum of the squares (bending stress is perpendicular to the weld; horizontal stress is parallel to the weld). Compare the resultant maximum stress to the allowable lbs. per inch of your weld.

RE: Capacity of a round weld

Quote (spats)

Then take the horizontal load and divide it by the length of the weld (pi*dia.)

I question this spats. The shear stress in the member will be concentrated in the portions of the tube wall most parallel with the applied load. As a result, I believe that the same will be true of the welds. Of course, the shear load impact on the welds is likely to be insignificant unless the pipe is very short.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

If you want to question it, I suggest you contact Lincoln Electric. However, I doubt that Omar Blodgett is still around. My dad used to work with him, and my dad is 91 years old.

RE: Capacity of a round weld

Quote (spats)

If you want to question it, I suggest you contact Lincoln Electric. However, I doubt that Omar Blodgett is still around. My dad used to work with him, and my dad is 91 years old.

I hope that Omar is still around. I thought that he was still contributing to www.weldingdesign.com. Either way, I see no need to bother him with this.

While I can't claim a family connection to Blodgett, I'm enough of a fan that I own hard copies of absolutely everything that the man ever saw fit to put to print. If you can point to an example in any of Blodgett's works that supports your method, post a reference to it. I'll run down stairs lickity split, scan it, and post it here for discussion.

Alternately, if you're up for it, we could just debate the issue using our collective understanding of structural behavior.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

Omer. Not Omar... The old southern US name, not a Muslim name...

Not that it matters in particular, but if you're trying to find one of his (excellent!) books, you might want to use the correct name...

RE: Capacity of a round weld

Oh, and re this:

Quote:

Alternately, if you're up for it, we could just debate the issue using our collective understanding of structural behavior.

Let's all try to play nice...

RE: Capacity of a round weld

Pretty picky! Blame my IPad voice recognition. By the way, I attended "Omer's" weld design seminar in Cleveland in 1972. I traveled half way across the country specifically to see him. As I said, my dad worked with him... I want to say it was the late 40's or early 50's. At that time, they were both involved in the seminars. Of course, Omer was the design guru. My dad is a mechanical engineer, and was more of a welding procedures and equipment guy.

RE: Capacity of a round weld

@CEL: Challenging someone to a technical debate on a technical forum is not unkind is it? I'm sure spats' ego can handle it. I know mine can if it turns out that I have to eat crow on this one. It wouldn't be the first time. In fact, it wouldn't even be the first time this week.

Picking on spelling mistakes on the other hand... that's just pretentious and annoying. I type on my phone damn it; stop picking on me.

Instead of playing nanny / referee, why not step up to the plate and share your thoughts on the issue du jour?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

I apologize if I pushed your button KootK. I didn't know if you were looking for a fight, or trying to stimulate discussion. I guess I have a little arrogance in me, like most structural engineers I know.

RE: Capacity of a round weld

Haha... I have no thoughts, having thoroughly wacked my head on some ice while leaving the office.

It is a fine ER visit for me to end my "wonderful" week of tummy bug. *sigh*

And I did not mean to be pretentious or difficult, just pointing out the correct name for one of our living legends. Respect, that is all.

RE: Capacity of a round weld

I'm neither offended nor seeking a mean spirited brawl spats -- no apology required. I've been learning from you for years now and I know that a) your opinions are not to be taken lightly and b) you can hold your own in a debate. I'm confident that you're wrong here. I simply want you to agree with me or straighten me out such that I agree with you. It's part of my education as an engineer.

I wish that there were a warm & fuzzy emoticon for "I love you dude... but I think that you're frickin' out to lunch on this one".

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

@CEL:

First rule of eng-tips club... we talk about eng-tips club.

Second rule of eng-tips club... we TALK about eng-tips club.

Third rule of eng-tips club... if it's your first comment on a thread, you have to weigh in!

Get healthy, come back Monday, and answer the question damn it!

For anyone not getting the pop culture reference, these are not actually the rules of eng-tips club.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

Here is a calculator that can basically do that and a little more:

NPW Weldcheck

A confused student is a good student.

RE: Capacity of a round weld

and yes this utilizes the methods of Blodgett.

A confused student is a good student.

RE: Capacity of a round weld

Oh me, oh my... I fear that I shall have no digi-buddies left by the end of the night.

@Medeek: I love you dude... but I think that your software is out to lunch on this one. I think that it suffers from the same error that spat's method suffers from IMHO.

Your fvy and fvz values are constant at all locations. That would be appropriate for a solid shaft where the weld group could be expected to move as a singular, rigid body. However, for this problem where we are dealing with a hollow shaft, I would expect the fvy and fvz to vary about the weld group.

It's quite similar to the situation with a wide flange section. If we applied a strong axis shear to that section, we would not expect the fyz values in the flanges to match the fyz values in the web. Rather, we'd expect the bulk of the shear force to be resisted by the web and relatively little to be resisted by the flange shear stresses. It's the same with a rectangular HSS and similar with a round HSS.

@Spats/CEL/Medeek: please note the change to my signature.



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

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 that I want to either change it or adopt it.

RE: Capacity of a round weld

No, No, No!!

Half of the post is in tension and half is in compression....it is a couple from the moment.

For the half in tension, the weld is in shear....typical fillet in shear....1/2 the circumference taking the tension of the couple (the compressive portion of the couple can be neglected).

Now you have to consider that the horizontal load must be resisted in shear. The whole circumference resists this. You now have a combined stress in two directions. You can consider them independently, but you have to check the resultant of the vertical and horizontal vectors across the throat of the weld as well.

RE: Capacity of a round weld

You might be onto something with what you are saying. The problem is how to quantify this. I'll have to crack open my ME book to think about this some more. This would be a good candidate for FEA.

However, if you apply a simple thought experiment perhaps it might become more clear. Assume we have a hollow pipe welded around its perimeter, at the other end of the pipe we have a load evenly distributed around the pipe perimeter that is perpendicular to the pipe's long axis. Now imagine that we slowly decrease the length of the pipe until it is almost zero length, everything else remains the same. You would find that the shear load around the perimeter of the pipe is now evenly distributed along the weld. There may be a flaw in this thought experiment but for now it makes sense to me.

A confused student is a good student.

RE: Capacity of a round weld

I'm wondering if the original poster ever thought about what he was asking ...
"I have a pipe. How thick does it has to be? How long does it have to be?"

Do they actually graduate engineers that are that ... "basic" today?

RE: Capacity of a round weld

You will note that my weld calculator has shear, moments and torsion. For the situation described by the OP, you would need to enter in an appropriate moment and a shear load.

A confused student is a good student.

RE: Capacity of a round weld

Concur. But does the original poster understand enough about design to use it safely?

RE: Capacity of a round weld

Quote (Ron)

Half of the post is in tension and half is in compression....it is a couple from the moment.

We discussed this option pretty thoroughly during round one Ron: Link. Perhaps you've reviewed that and are sticking to your guns anyhow but, just in case, I thought that it would be prudent to bring it to your attention.

Quote (Medeek)

However, if you apply a simple thought experiment perhaps it might become more clear. Assume we have a hollow pipe welded around its perimeter, at the other end of the pipe we have a load evenly distributed around the pipe perimeter that is perpendicular to the pipe's long axis. Now imagine that we slowly decrease the length of the pipe until it is almost zero length, everything else remains the same. You would find that the shear load around the perimeter of the pipe is now evenly distributed along the weld. There may be a flaw in this thought experiment but for now it makes sense to me.

The flaw is that, for loads applied further from the support, the stresses that comprise the resistance to those loads have time to reorganize themselves to reflect the inherent stiffnesses and flexibility of the cross section. Your mental experiment could also be applied to a wide flange section. In that case, the result would be the same. However, I think that we can all agree that the welds along the web should be designed to constitute the bulk of the shear resistance.

Quote (Ron)

Now you have to consider that the horizontal load must be resisted in shear. The whole circumference resists this.

I've quoted Ron but this seems to be the sentiment of pretty much everyone other than myself. Try this "proof" to the contrary on for size:

1) The forces in the welds at the support are simply the stresses in the circular hollow section at the support multiplied by the wall thickness.

2) The shear stresses in the circular hollow section at the support -- or anywhere else -- are not resisted by all segments of the cross section equally. This can be verified through VQ/It analysis and by one our own threads on the topic Link.

3) IF (ARGUMENT 1 = TRUE) AND (ARGUMENT 2 = TRUE) THEN (Welds more parallel to the applied load will be disproportionately loaded in 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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

Interesting discussion and background info. I tried a FEA to take a get an idea. The model consists of a 6x6 piece of solid steel (base) and a 36" tall CHS 4"x1/4" pipe gaped 1/16" from the steel base to avoid full bond between the the CHS/base. There is a 2,250lb load applied to the top of the CHS. It is giving me a headache right now looking at all the various stress reports, so I have not decided which side I am on. I thought I would post it anyway.

RE: Capacity of a round weld

Now we're having fun. Could you rerun the second plot to show shear in the direction of the load Brad? Better yet, can you generate the same stress component on a tube cross section located about 2D from the support?
 

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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

"Could you rerun the second plot to show shear in the direction of the load...."

yes, I'd like to see that too.

regards,

Dan T

RE: Capacity of a round weld

Are you considering the different strengths of a fillet weld loaded axially vs. transverse to its axis?

RE: Capacity of a round weld

Brad,
While you are at it, can you also run a square and rectangular tube?

RE: Capacity of a round weld

The FEA model clearly shows the couple.

KootK...I agree that the weld will not take shear uniformly. The weld more parallel to the line of shear force will have lower shear stress because it likely has a larger cross section than the weld perpendicular to the line of force. It should be noted that the only truly parallel force is the tangent to the circle on either side of the ring.

If the half-circle were divided into 3 sections, the middle third would be the perpendicular force resistor and the two outer thirds would be the "parallel" force resistor.

In any case, the weld will be under both tension and shear on the side where the force is applied, so a unity check should be done.

RE: Capacity of a round weld

I have attached a few more stress plots. You cannot isolate the effects of different forces in this software, so I think I will shorten the column and increase the shear forces to show the effect of the shear better since we all agree on the effect of the moment. I did that last night, but I need to add a stiffening element around the top of the pipe since it started to deform excessively. For some reason solidworks uses the Y direction as the vertical. I don't know why exactly, but it is annoying. I will make a few other changes as well. I can see the 1/16" gap is influencing the stress distribution, and the fine point of the weld is indicating higher stresses than it should.

Engineers, egad. I only started the solid simulation a short time ago, so this is a good case study. The problem with solid modeling is the sheer amount of data.

RE: Capacity of a round weld

Quote (Ron)

The FEA model clearly shows the couple

To me, the second plot shows an elastic, M/Sx style stress distribution with the peak stresses at the extreme fibres as one would expect. Those stresses could not be represented, in statically equivalent fashion, by tension and compression forces located at the centroids of two half circle weld groups. I suspect that there is some "noise" in the plot as we appear to be seeing Von Mises stresses rather than purely axial stresses. Also, for the sake of this argument, it might be better if the peak stresses were not so close to Fy. Yielding will muddle the issues here.

Quote (Ron)

If the half-circle were divided into 3 sections, the middle third would be the perpendicular force resistor and the two outer thirds would be the "parallel" force resistor.

We are in similar ballparks here. I would divide the entire ring into quarters and say that the two side quarters take most of the vertical shear and that the top and bottom quarters take almost none. That mechanical thread that I linked above (Link) yielded two interesting conclusions:

1) The max vertical shear stress in the tube will be located at the neutral axis (big surprise) and will take on a value if 2P/A.

2) If one plotted vertical shear force resistance over the height of the tube, the graph would be a straight line at a uniform value. Since there's more section available per vertical unit height at the top, I interpret that to mean lower stresses at the top and bottom of the section and higher stresses at the sides. And that's consistent with the 2P/A estimate above.

As for a practical weld sizing strategy, I'd probably just design the weld for the vector sum of 2xPxt/A and MxCxt/I. And not bother with the fact that the maximum stresses do not occur at different locations unless I find my weld size very objectionable.

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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

There is another point that was made in that mechanical thread that I feel is significant. To paraphrase:

For weld design, the end game should be only two force components in each unit segment of weld:

1) A force parallel to the longitudinal axis of the tube, reflecting bending stress.
2) A force tangential to the tube representing VQ/It stress.


That makes sense to me. There should be no component of weld force in the radial direction as radial flexibility in the tube walls will tend to relieve that force component. This would lead one to assume that the welds at the very top and bottom of the section resist none of the applied shear. And that would match the expected VQ/It result.

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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

Attached are the results of a much shorter column. Small weld edge radii were added to reduce the appearance of stress concentrations and the weld mesh was reduced to .1". The load in the Z direction was increased from 10kN to 40kN (9kip). The Txz plot does show higher weld stresses where the weld is more parallel to the load. I attached the stress direction diagram from the software in case anyone is interested. I have things to do now.

RE: Capacity of a round weld

The procedure outlined by spats is the method I employ in my practice and I believe it is supported by various structural steel textbooks, including publications by Omer Blodgett/Lincoln Electric. For example, this method is used on page 12 (Example 12D) of "Solutions to Design of Weldments", a publication of the Lincoln Electric Company. In Example 12D, a 9'-0 long, 12" diameter cantilevered pipe is subject to a 10 kip concentric, concentrated load at the free end. The solution uses the entire perimeter of the circular weld when determining the shear stress due to direct shear.

RE: Capacity of a round weld

Hokie93....I would also use the entire circumference to compute the shear stress....my example above was in response to Kootk's comments, which would perhaps yield a more precise stress distribution but probably not necessary in the whole scheme of things.

Blodgett's examples are all done with "slide rule accuracy", as his work preceded commonly available computers by many years, thus the tendency to group things together and estimate the distribution of stresses to be more universal.

All the arguments aside, Blodgett was a welding design guru and his principles are valid and useful.

RE: Capacity of a round weld

I think the line weld method is correct. If the fillet were a straight line there would be no question. In this case, the weld throat is longer than the face against the pipe.

Considering the moment and assuming that it is the major load, different story if the shear is the major load.
The problem that I see is in the materials, we check the shear on the weld/pipe interface area against the strength of the pipe material, we check the throat against the weld material.

Considering the shear force, I think the it is transmitted through the sides of the pipe, these are stiffer than the front and back faces.

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

RE: Capacity of a round weld

I find myself in my least favorite of positions: I no longer believe that I am right but, at the same time, I also do not understand why I am wrong. Now I'm just... confused.

The Blodgett example that Hokie dug up is shown below (thanks Hokie). It does indeed corroborate Blodgett's intention of uniformly distributed shear for this situation. Ditto for the website that Desertfox linked us to above.

Equally damning, in my opinion, is page G-9 of this document: Link. There, the shear capacity of a round HSS is calculated using the the entire cross sectional area (0.6*Fy*Ag). Just. Like. The Blodgett example. This, even though the previous example for a square HSS utilizes only the webs in shear. Maybe the intent is to examine some heavily plastified ultimate strength condition, I'm not sure.

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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

I have a concussion... So if you need to, please take this with two grains of salt and correct me in the morning.

One of my favourite references for teaching students connection design is the (very) old Ketchum engineering manual. The other is an all-inclusive timber-steel-concrete analysis and design text from the late 1940s. I can't remember the second text's name at the moment, but it has one of the most Wooten like statements when you start the welding design chapter.

I paraphrase: Like all states of stress in steel, the true state of stress in a weldment is an incredibly difficult analysis, leaning towards an impossibility. As such in practice we apply conservative values to simplified analytical methods and this has proven to be acceptable in practice.

My point being: You're absolutely right, Kootk. There is a great deal of simplification in connection design. I simplify a lot of my connections a great deal, often producing something a great deal stronger than needed. It doesn't matter. Trying to apply a more refined approach to a sufficiently solved problem is like trying to make concrete more grey. I need a reson to care...

I am VERY interested in the answer. Don't get me wrong; I am keen to have the *right* answer. I just don't want any EITs, or anyone else for that matter, thinking this actually matters.

RE: Capacity of a round weld

I really don't like the calculation of the weld area for the circular pipe... 2PI()r is the circumference alright, but the 6/8" assumption had to be known before the calculations began.

I prefer to use the more conservative assumption of d/2 as the r in the circumference calculation, and then assume a weld size to turn the linear value into an area.

Thoughts? Anyone disagree with this head case?

RE: Capacity of a round weld

CEL...they are the same. (Pi)x 2r is the same as (Pi) x [2 (d/2)].

I think what you meant was to use only the leading 1/2 of the circumference in your resistance calcs. Then it becomes....

(d/2) x (Pi) x 0.71 x (assumed weld leg size) for the weld stress.

RE: Capacity of a round weld

I'm in agreement about stresses in welds being highly complicated and that calculations on these welds using assumptions and simplifying theories is done everyday but these assumptions have been verified over many years by research and experiment.
Here is a link to a welding paper that indicates why assuming the shear stress is distributed evenly in and around a weld.

http://www.ignou.ac.in/upload/Unit-4-60.pdf

Further links to weld design here

http://www.mitcalc.com/doc/welding/help/en/welding...

http://www.mitcalc.com/doc/help/en/C_safety.htm

The latter link talks about the factors of safety based on knowledge of loading materials etc

RE: Capacity of a round weld

Quote (Desertfox)

Here is a link to a welding paper that indicates why assuming the shear stress is distributed evenly in and around a weld.

Can you guide me to a particular page or paragraph Desertfox? There's a fair bit of information there and I couldn't spot the bit that you've referenced.

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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Capacity of a round weld

hi KootK

Start here

4.3 TYPES OF WELDED JOINTS, STRENGTH

RE: Capacity of a round weld

No, I meant I like using 6" for the r, not 6&3/8. I know there is a minimum weld size assumption, but I like to exclude this...

RE: Capacity of a round weld

Also: Am I the only one who thinks it is silly to use the additional 3/8" in concert with the crazy gross simplifying assumption of πr^3 for (π/4)(D^4 - d^4)?

RE: Capacity of a round weld

@CELinOttawa, the 6 3/8" is the OD of the 12" pipe.

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

RE: Capacity of a round weld

Thanks Paddington; This is hilarious - I had no idea that US standard 12" pipe was actually 12 3/4 inches... Funny what a big difference in reading a problem such information can make. I thought that the check was including a minimum weldment size to use a larger area and calculate within the centre of the weld; Obviously not!

Funny, I thought out HSS were weird sizes (273mm, 324mm, 35Xmm - Can't remember the 35something at the moment) because they were US pipe sizes as well... I've never seen anything around 319!

RE: Capacity of a round weld

There's probably upwards of 100 collective years of structural engineering experience being brought to bear on this thread. At the risk of being offensive, I'm struck by the fact that not a one of us seems able to reconcile the form of Blodgett's design equation with our fundamental understanding of mechanics of materials. And this is an equation that we all use and espouse to our colleagues.

I don't feel that it's prudent to dismiss this discrepancy simply because of the limited computing power available to Blodgett or the fact that "knowing" weldment stresses accurately is hard. My intuition is that Blodgett's design equation does make theoretical sense, just like all of his other recommendations. Rather, I think that we are missing something here regarding the theoretical background to Blodgett's equation. This modern design document corroborates Blodgett's assumption of uniform shear stress (Link). For me, that is sufficient circumstantial evidence to conclude that we are somehow lacking in understanding.

Quote (CEL)

I just don't want any EITs, or anyone else for that matter, thinking this actually matters.

This statement could not be more wrong if you'd written it with a gag-ball in your mouth. We answered OP's question sufficiently within minutes of his asking: do like the Romans do and defer to Blodgett. The rest of the discussion has been about trying to understand the basis a design equation that we all seem to be using blindly. I think that's important to us all and, especially, to junior engineers who need to think critically and to see their mentors for the fallible creatures that they are. As great an engineer as Blodgett is, even his work is not above scrutiny and validation. And, unless I miss my mark egregiously, Blodgett wouldn't have it any other way.

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.

RE: Capacity of a round weld

Quote (Desertfox)

Here is a link to a welding paper that indicates why assuming the shear stress is distributed evenly in and around a weld.

Interesting links DF. The first paper provids justification for using average shear stress along the length of a longitudinally loaded weld despite localized effects due to starting and stopping and elasticity theory. In my opinion, it does not address the issue that I've been harping on; namely, why weld stresses can be assumed to be inconsistent with VQ/It shear stresses in the supported member.

The first website that you linked to did provide a nifty kernel though. This sketch is consistent with my original thoughts on the matter:



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.

RE: Capacity of a round weld

We aren't being inconsistent, and the increased accuracy DOES NOT MATTER. I don't need a ball gag to know that if the profession has been doing something for a hundred years without failure and with close agreement from experimentation, refining it isn't necessary.

And this:

Quote:

do like the Romans do and defer to Blodgett
is *NOT* at all what I have advocated. I didn't even use Blodgett in my defense of the standard approach, but rather one of his predecessors (while also methioning a preceeding great who's sum total of Weld comment is "Welding of steel shall not be permitted"). I have advocated that the standard formulae of f=P/A, f=Mc/I, and S=Tr/J.

So, for the record, I'll get all sensitive and point out that you are falsely misconstruing the argument as an argument from authority. This coulnd't be more false if you'd borrowed the ball gag to make it...

Finding a more accurante answer is a whole lot of potential fun. It is something we can keep pounding away at. It is something I'll be here in the thread dedicated to trying to help us all understand better. It is also, NOT NEEDED.

Now if you'll all excuse me, I have a meeting with a shower... And I'm really a little creeped out about Kootk's prescient comment about this family's local application of Wooten's first rule...

Also, in English, does falsely misconstruing constitute a double negative in English? Je ne comprends pas...

RE: Capacity of a round weld

Some thoughts on the discussion...

As I see it, the refining of this issue is interesting, but will not change the answer. This is because we already use a combined strength equation on the components of bending and shear, which naturally pro-rates the strength of the weld into an amount to handle each type of load. Thus even if the weldment is behaving in a manner we do not explicitly address, our solution takes care of the behaviour in a manner accurate enough to permit safe practice.

Am I making sense here?

RE: Capacity of a round weld

Hi CEL

There is nothing in the attachtment

RE: Capacity of a round weld

And it would help if I squared the terms under the square root... *sigh*

RE: Capacity of a round weld

Thanks for the sketch and the ideas CEL. I'll consider that my Xmas present.

I'm afraid that it is you who has misconstrued my purpose. I'm not trying to improve accuracy, shave down weld sizes, or advocate an alternative design method. Not at all. What I am attempting to do is to understand the theoretical basis for Blodgett's method and any simplifications that he may have taken. This is now the sixth time in this thread that I've described my goal as developing understanding. Not refinement. Not replacement. Understanding.

Based on this thread, it has become clear that none of us understands the theoretical basis of Blodgett`s shear stress simplification. So how is it that we`re fit to use it? Or to adapt it to other circumstances? And how do we know that it's conservative? I'm not looking to replace or refine Blodgett's method. I'm seeking to understand Blodgett's method. Surely that is a worthy goal and something that 'matters' to any engineer who cares to actually know their craft.

The M/Sx form of Blodgett's equation suggests that we're sticking to the elastic domain. Given that, and ignoring all olf the messy stuff like residual weld stresses, restraint etc, it seems to me that determining the analytically "correct" stress distribution is relatively straight forward. It's just VQ/It. See the sketch below which is also attached as a PDF file.

It is instructive to note that, for short tubes where bending stresses do not dominate, the predicted maximum shear stress of P/A would be out by a factor of about two.

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.

RE: Capacity of a round weld

(OP)
I love asking tough questions but never in my wildest dreams did I think it would lead to this kind of discussion... Wow!

RE: Capacity of a round weld

Kootk: I get that you're looking for a better understanding. I simply think that the understanding we have is sufficient, and I am quite happy with the level of understanding as sketched out in my attachment.

The level of understanding you seek is interesting, but not necessary.

There, I've said it enough times, so let's just get busy trying to find the answer you want...

More PDFs if I have time tomorrow.

RE: Capacity of a round weld

@anchorengineer: yeah, you never know what will capture the imagination. My first comment linked to a thread where I thought that this was already settled.

@everybody: some late breaking news. I misread the AISC provisions for shear strength of round HSS. AISC section G6 and commentary use Ag/2 which is exactly consistent with the derivation that I just posted. So My status is no longer "confused".

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.

RE: Capacity of a round weld

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

RE: Capacity of a round weld

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

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

RE: Capacity of a round weld

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*

RE: Capacity of a round weld

@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.

RE: Capacity of a round weld

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.

RE: Capacity of a round weld

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!

RE: Capacity of a round weld

Quote (CEL\)

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.

RE: Capacity of a round weld

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/chapt...

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

RE: Capacity of a round weld

@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.

RE: Capacity of a round weld

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%209.pdf



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