Regions that experience seismic activity are subject to conditions and loads that can exceed the yield strength of the base metal causing plastic deformation to occur. Some of the design details currently used are designed to force yielding to occur in the member, not the connection. Older designs didn't have that feature; as a result the connections are subject to overload, deformation, and possibly brittle fracture. Moment connections, when designed properly, using the proper materials form a plastic hinge, thereby accommodating the deformation to a limited extent without failing in a brittle manner. Poor design details, the use of welding techniques and filler metal that exhibit poor notch toughness, backing bars and/or weld tabs left in place (which act as stress risers), etc. can reduce the connection’s ability to form a plastic hinge and can experience brittle failure when subjected to overloads caused by the movement of the structure during an earthquake.
The probability of brittle failure is promoted where there are discontinuities that can intensify stress. Sharp notches, changes in geometry, incomplete fusion, backing bars, weld tabs, etc. are typical discontinuities that intensify stress and can amplify the potential of brittle fracture. The structural welding code, i.e., D1.1, is supplemented by D1.8 when the construction is in seismically active areas. D1.8 has additional restrictions that are imposed on the construction to improve the probability the structure will withstand the forces induced by an earthquake. The design, materials of construction, additional restrictions on welding materials and techniques, tighter acceptance criteria, etc. are all intended to improve the probability of survival of the structure and those that occupy the structure in the event of an earthquake. That isn't to say the structure will not sustain damage and it isn't to say the building will not be condemned after the event, but the probability of a catastrophic collapse are substantially reduced. Once the beam has yielded beyond a certain amount, once the beam has deflected beyond a certain limit, once the connection has formed a plastic hinge and experiences yielding beyond a certain limit it is considered to have failed from an engineering stand point.
The samples with arc strikes that both you and John tested most likely exceeded the yield point of the base metal (were the samples permanently deformed when the gross cracking was first noticed?) and would have exceeded the limits of what would be considered a structural failure. The tests clearly demonstrate the arc strikes have a detrimental effect on the structural integrity of the structure in the event of an overload. The presence of an arc strike could be the trigger of a structural collapse rather than a structural failure that leaves the structure damaged, but intact in the event of a major event such as an earthquake.
Any discontinuity is going to adversely affect the performance of a connection subject to overloads, cyclic stresses, or any condition that results in gross yielding of the weld or HAZ. Those discontinuities that are notch like tend to be viewed more critical that those that are rounded. An arc strike that has a hard, brittle HAZ and small cracks is going to be more detrimental than spherical porosity. Discontinuities that can initiate cracks or cracks that can propagate have stringent acceptance criteria invoked by the applicable code.
It is the published criteria that we as inspectors must enforce. Granted, I do look very closely at notches, tack welds outside the weld, and arc strikes very closely to ensure there are no visible cracks. The arc strike blemish must be removed and reinspected. However, there is nothing in D1.1 that says anything more than visual examination of the area must be performed.
What does D1.8 say about the removal of an arc strike and the method of reinspection?
Best regards - Al
So, a couple of points as addressed by you,
Two of my samples fractured completely without any 'bending' of the coupon. The minute stress was applied they just broke right along the arc strike. Now, both were more of a multiple arc strike as when a welder may 'stick' the electrode and when pulling it loose gets a 'drag' series of arc strikes or during striking of the arc miss the weld area and get multiple arc strikes in one area. But, there was no bending to speak of before failure occurred. Overall, I see your point and it is well taken as to the difference between showing ultimate material condition and application in practical use failures.
D1.8 does not deal with arc strikes. Neither, according to my search function, does AISC 341-2010 Seismic Design Manual. Only D1.1 as far as codes go so D1.1 is the governing document as to how they are handled.
And both of us have stated the same thing on multiple occasions, that being only what is stated in the applicable code and/or Contract Documents is to be enforced by us. And it appears your view of D1.1's statement of grinding and reinspecting for soundness is the same as mine in that that statement alone does not mandate MT examination of the area. Only a visual inspection unless the engineer has stated otherwise in the original Contract Documents.
One more note of interest, it is becoming more and more common to see backing bars left in place. It is even dealt with within the codes referenced here. Some have a 5/16" fillet weld to the main member as well as the CJP weld they were there to back up from the other side. Code claims that is all that is needed to dissipate the stress risers sufficiently for seismic activity. This is not for all locations. But many that used to require removal can now remain.
He Is In Control, Have a Great Day, Brent
Interesting. I have not worked on any projects requiring D1.8, so I make no claim to be conversant with those requirements. I'm actually surprised to hear that D1.1 does not include more stringent criteria for arc strikes.
Engineers learn a little more with each major earthquake. As is the case with much of life, there is a cost associated with the benefit of new code provisions (building, fabrication, or welding codes). There has to be a major benefit to offset the cost of any new requirement. If one cannot justify the cost of the new requirement, it will not be included in the new revision of a code. Regardless of the talking heads, cost is a major issue that is considered when considering more stringent code reguirements.
What is the benefit versus the cost associated with removing all undercut, all backing bars, etc.? In the case of aircraft the cost of additional inspection and more stringent acceptance criteria is offset by weight savings and increased safety. Such expenditures are not justified in the construction of a steel framed building. It is more cost effective to increase the size of the structural member when constructing a steel frames building. The safety factor use for aircraft design is 10% (so I've read) compared to the safety factor used in steel construction that can be 150% or more. Steel cables for elevators enjoy a 10 to 1 safety from what I've been told.
Intersting that a couple of your samples fractured without any plastic deformation. Was it a plain carbon steel or a high strength low alloy steel used for your experiment? I would suspect the steel had a relatively high carbon equivalent for that to be the case. I've bent welded samples with undercut (within acceptable limits) without failure. I bent the ASTM A36 test samples in anticipation they would fracture. Surprise, no fracture and no cracks. I'm going to have to find some time and make a few samples using different base metals to see how they respond. To hear they fractured without some plastic deformation is surprising to me. However, even with a slight load applied, the yield point is reached rather quickly. Should there be a small crack, fracture mechanics tells us the stress concentration is a multiple of the unit stress of the cross section. The closer the ratio of YS/TS is to unity, the less ductility the base metal has. The lower the ductility, the more likely the sample will fracture during the bend test. That is why most welding standards permit the use of a larger bend radius for higher strength base metals.
Maybe John can join the discussion and share more details of his experiments with us.
So, again, what was the base metal specification and the minimum specified YS and TS used for the experiments? Inquiring minds want to know.
If we can't discuss these matters with each other, how are we to learn? What better place to discuss these matter than here in the Forum? We haven't had a good discussion for a good while.
Best regards - Al
The material was A36 Flatbar. Not cuts in this case but still made so that the direction of roll would have been correct if used for a weld coupon.
I would not say there was NO plastic deformation prior to the fracture, but, it was very minimal. I have been looking for my pictures. I have changed computers since the time that was done and my article was written but they should have been on my 'passport' (external harddrive/storage device or whatever term correctly describes this thing).
I kept the experiment very simple. I used FCAW and SMAW to create the arc strikes. I did not get 'consistent' ones in order to do a real standard comparison analysis. For my purpose to show how it takes a very small discontinuity to cause failure it was sufficient. It was not my intent to do research that would be code altering. Just to show welders and inspectors how inaccurate it is to say 'It's only an arc strike! What's the big deal?'
And in the article, in other comments, and in this thread, I have always said we can't make anyone go beyond the applicable code.
What is interesting to me, is that discontinuities are not more stringently mentioned in D1.8. Most all items revert to D1.1.
Some comparisons that are covered: intermixing of filler metals (when 1 is FCAW-S), inter-pass preheat (max 550°F), K-area weld free zone, Demand Critical welds, Protected Zone, Supplemental Welder Qualification (for bottom flange at beam flange to column flange moment connections welded through a weld access hole), FCAW Electrode storage and exposure, to name a few.
Most engineers will require the removal of run off tabs but not very often do they require the removal of backing bars. Most would be difficult to do without doing more damage than good. The use of the additional fillet weld on the main member side of the backing bar for stress transferal has apparently been determined to be the safest, best procedure in most cases. AND, you would be right, weight is not really a consideration here.
He Is In Control, Have a Great Day, Brent
Many years ago my company's gas supplier down is SoCal told me that an employee down there was killed in their fill plant when a cylinder exploded. He said the investigation showed an arc strike on the bottle where it failed. I can not verify this story, but that is what he told me.
Blaster,
Poppycock!!!
A cylinder simply will not EXPLODE because of the presence off an arc strike.
46,
Yeah really.
Look at the charpy curve.
What you have is a material that had extremely low ductility even at 125deg F.
125deg!!!!!
At 125F this material was moving well into the lower shelf of energy.
In other words, the material was schyt.
I've tested Grade 91 better than that. DBTT on Grade 91 is somewhere around 80deg or so. Maybe a little lower or a little higher. And its entirely martensite. Or is supposed to be.
The vessel was also almost 1 1/2" thick which should then have been heat treated, and it wasn't.
The Code makes very clear that the possibility of brittle fracture of the material is to be considered during hydrotesting. This clearly was not done.
So again, engineering.
I'm not arguing that there aren't times when concern for arc strikes is warranted. I am arguing that such extreme examples do not argue for a blanket requirement.
And far from disputing my point your article actually confirms it since there were other VERY important circumstances that were as contributory as the arc strike itself without which the failure would not have happened.
JS I understand what you are saying. Nevertheless,the fact remains that there are thousands of vessels such as this around the world. Most, hopefully build to some standard such as ASME or BS but many built before our understanding of the nature of brittle cracking, arc strike etc. was at our current level.
I understand JS's cautions against 'Blanket' specifications that would do far more testing than may be necessary (read following posts in this thread) as well as seeing some of the other conditions also contributing to the failure here.
BUT, thank you Glyn for sharing this as it does also illustrate the need for care. All discontinuities are unique to the application at hand. Good example of several things that got away from them on this one.
Brent
Brent,
We can say lots of thing that happen in labs. But the simple fact is, the laxity of the codes pertaining to arc strikes is based upon a lack of evidence that there is a problem in the real world with certain forgiving materials such as carbon steels.
JS,
I know your education and experiences far surpass those of myself. There are many here of whom I stand in great awe (very seriously).
I also know that, compared to many other materials, the majority of carbon steels in use for structural jobs are indeed very forgiving when it comes to discontinuities of any kind, including arc strikes. Add to that, the over engineering of structures and we have a built in safety factor anyway.
I also am not trying to say that a great multitude of building failures, even in earthquake zones, have taken place and that those which have can be directly attributable to arc strikes. On the contrary, for the amount of disregard that I have seen on the part of welders and inspectors to correct arc strikes even to the requirements of the existing codes, I think it supports your 'forgiving' nature support of carbon steels. Our buildings have done pretty well all things considered.
The point is, arc strikes DO leave more of an effect on carbon steel materials than people are giving them credit for. AND, the AISC and D1.1 codes do deal with them. I personally believe it is incumbent upon both welders and inspectors to at least give them the attention the code calls for and not be so apathetic as I believe many are concerning them. That is the main purpose of my personal testing and article submitted. Draw attention to something that I don't feel should be so disregarded off hand.
I have always been a person who took responsibility to extremes. Ask my wife and children, especially the son who still works with me. But, when it comes to inspections, I try to be very realistic. I don't play god and force my own will and interpretation and desires on others. Let the codes speak for themselves. But at least follow what is there. And, as stated previously in the article as well as here, what I choose to do in my LITTLE fab shop in no way obligates anyone else to do the same. I can do things that would really cost LARGE fabricators. Doing what I consider to be a proper fix to an arc strike does not cost me nearly what it would others. Two things make it worth it TO ME: 1) cosmetic appearance for general contractors who definitely see every part that I put up which effects my local reputation, and 2) a feeling of peace and safety that allows me to sleep soundly at night because I went out of my way to insure the safety of others.
I personally would like to see the codes require MT on arc strikes. Why? Mainly because I think it would cut down on the number of them we see because people would get upset about the cost of 'reinspection' which should be put on the backs of the fabricator/erectors not the owner/client/engineer. They would make their people be more careful. It really isn't that difficult. It is caring and craftsmanship instead of care-less-ness.
Is that going overboard? Maybe so, but where do we draw lines? The way it stands they get totally ignored way too often. The more they get ignored and left the more potential there is for a catastrophic failure at some time. If Canada has deemed it important enough to add requirements should we follow suit? Not just because they did, no. But is there some far better research out there than my little experiment that would lead a person to take a little more care? Maybe. Worth looking into.
I think part of what you and I are looking at as well is how each of us views the usage of the term 'overemphasized'. I think with a good many things many of us inspectors, and yes, I do include myself here, tend to overthink certain items. We all have pet peeves. I try not to make any ONE thing a pet peeve. It is welder and inspector attitudes that I prefer to take exception to. If the code deals with it we should do no less while also not requiring more. Does that make sense? I hope so.
Enough from me.
He Is In Control, Have a Great Day, Brent
Brent,
My main point is that the philosophy behind the 'leniency' of requirements for arc strikes is simply engineering and empiricism.
It is the same reasoning behind the fact that we don't build everything out of super titanium, or 10" thick.
The same reasoning behind 5% or 10% radiography.
The same reasoning behind allowing flaws in welds such as slag, porosity, or insufficient penetration, all of which could be proved to cause a premature failure in a testing regime.
It is the same reasoning behind Section I not requiring radiography at all under 16" pipe in some circumstances.
The same reasoning behind broadening the qualification range of procedure or performance variables.
The same reasoning behind P-No.'s.
The same reasoning behind VT only in D1.1. Except of course for that one paragraph on transverse cyclic stress.
The same reasoning behind Boiler Code not having much in the way of VT.
The same reasoning behind allowing PT or MT when volumetric could be used.
The same reasoning behind B31.1 having no requirements for CVN's AT ALL.
Bottom line the Codes are full of these engineering and empirical 'leniencies'.
Are they always right?
Certainly not.
And it is certainly valid to argue each case individually as you are doing here.
I have no problem with that. And in some cases you might even find myself arguing for greater stringency.
But in this case I think the weight of evidence favors the requirements as they are.
I would like to throw one more research example into this while stating that I think we are really both headed the same direction, I just choose to be a little more conservative in my personal preferences, MAYBE. Hard to tell exactly how you and I would address the same discontinuity on the same member on the same job going to the same location since we have never had to.
Now, the example, as to real life scenarios and research, when I started a job over two years ago that involved Sideplate connections for the moments and CA (San Diego in this case) requirements. The fabricator and Sideplate had gotten together and welded a mock up that then got sent to labs in CA that set it all up and did awesome tests to simulate earthquake action and how the joints, welds, steels would stand up to the stresses actually placed upon them. You should have seen that assembly when it came back. It was very telling on how the ends of welds are critical, as well as certain other discontinuities and weld profiles. Sideplate has included specific criteria for the completion of their jobs in the Job Specifications because of these tests. Anyone who has worked with Sideplate on a job knows what I am referring to. And their requirements are backed up by this testing. Some of the little things that will cause failure may surprise you but overall what you are saying is very true, about the flaws that are acceptable such as porosity, etc.
Structural welds do not have to be perfect. Not on a building and not on a piece of equipment and not even on a nuke job. But, each one does have it's own specific areas of special attention. Undercut that would not effect the nuke may be critical to premature failure on a Log Stacker. Porosity that would result in a leak would not be a problem on a high rise even in LA. Etc.
And as a result, inspectors need to be just as careful as anyone about how they let their own past experiences and knowledge, or lack thereof, affect their view of the code and it's application on the job at hand.
Back to Lawrence's question and my personal opinion from this, I don't think the US requirements, the AWS codes, or our job as inspectors is wanting/lacking because MT is not required in the states on arc strikes after it has been ground per D1.1 requirements. I don't know that it would really be needed for 95% the structural jobs to have MT done on arc strikes as part of the Job Specs. After all, the average building built to D1.1 is less than 10 stories in height and not in an earthquake zone.
I do stress that we at least do our job as detailed by D1.1 and try not to miss arc strikes getting at least ground down and VT done to do due diligence making sure there is no cracking or undercut or other discontinuities requiring attention because of the arc strike.
Always comes back to the engineer being able to decide if a particular job may need more inspections required than the codes dictate. They should know at what point the minimum standards of the applicable code are adequate or not. Goes way beyond my pay grade.
HEY, you have been awfully quiet after starting all of this Lawrence.
He Is In Control, Have a Great Day, Brent
Another aspect of this occurred to me that I feel is worth mentioning. I'm going to preface it this way:
Ever try to tear a phone book in half? With your bare hands? And a pretty good sized one like for Portland, OR? You see this kind of stuff on tv and other media but do you know the dynamics of how it is actually done and that most anyone in here can do it? We used to do this with the Sears catalog at Christmas time, yes, the big one in the 60's and 70's. So, go ahead, grab it and start pulling and see what you can do. Not much. BUT, take it and bend it with as sharp of a crease as you can put from the edge inward. Then, to make it easier, go the opposite direction and do it again. You should be thinking, hey, that's how you tear a sheet or two of paper and get a straight line. Yep, only more pages at a time. And the better the crease, the easier it is to tear. Same with the phone book. It will still take some good work and even working of the pages when you go to tear it but once you get a little tear started it goes easier and easier until you have it totally torn in two.
Now you are saying, 'so, what does this have to do with arc strikes?' Well, while overall I concur with the idea that many of our structural materials are very forgiving and many of the applications are just not 'worth' going overboard or 'overemphasizing' arc strikes and the idea of having to do MT on them in absolutely every situation there is one more thing to consider that is not written into the code.
If I have a beam going into an area with an arc strike on the web it is not nearly as apt to be a problem as a beam going into the same area with an arc strike on the edge of the flange. If there is any kind of stress riser, ie notch, overlap, undercut, arc strike etc right on the edge of a beam it will be multiple fold worse than the same discontinuity on the web of the member.
Now, multiply that with going into the SFRS (Seismic Force Resisting System= the critical areas for resisting and transmitting forces from earthquakes) compared to no earthquake consideration and you start to get situations where arc strikes become definite considerations.
Again, this is not really a consideration for the inspector, he should inspect every part the same per the applicable code and Contract Documents. But, there should still be watchful eyes that understand different areas of concern. That is where the Protected Zone comes into play as well as Demand Critical welds and other factors.
The code and special inspection requirements are up to the engineer. But they have latitude to alter because only they really have a handle on exact conditions applying to the work at hand. Our codes are to minimum standards but they get more rigid as conditions dictate and are then also up to the engineer to alter as they see fit to make the building as sound as necessary.
He Is In Control, Have a Great Day, Brent
Interesting. Don't see a reference to arc strikes but still interesting. Have heard of them going off when pressurized with bad rust out of the cylinders and other items. There was fear of Acet tank walls getting too thin from improper transport and storage by so many for so long that they replaced a good part of the nations stock.
BB
Not the pictures I was looking through my files for but as a matter of fact, you can see that the one with the fracture has very little plastic deformation.
Thanks Glyn. You still home across the pond? Have good holiday?
Brent
Hi Brent, I'm back this side for a few months now but I did have a good vaction over the christmas period thanks!
Blaster,
Yeah, I was a little too, 'enthusiastic' with my critique.
But I think the main thing is to separate the concept of cracks from arc strikes. They are not the same.
The martensitic discontinuity has received a lot of discussion pertaining to arc strike metallurgy. However, martensite is not always a problem just because it is martensite. There are alloys that are intended to be entirely martensite. And many more that are intended to be at least partially so.
Arc Strikes on compressed cylinders is fairly serious, BUT those things are made from CrMo steel, with a good bit of carbon (maybe .3%?). The interested reader can figure hoop stress on one of those things, and add in low cycle fatigue (from filling them up) and it turns into a big deal.
Thing is, it can be a good bit worse than that. You show a 2200 internal pressure, but a standard size 44 cylinder (which is what I call what you cut up, its 44 liters) should be rated to 2400 psi. If I gotta fill that up to 2400, I have to have about 2500 in the bottle before I quit, as they get pretty hot when you fill em, and when they cool down the pressure drops.
If I am using a "+", then I can pump to 2640, and again, I gotta have an extra 100-200 psi in there to account for temperature changes.
If the arc strike, which may or may not have led to hardness and/or cracks, it is going to cause problems for hoop stress, as well as becoming a contributing factor for a longitudinal stress riser. Unless the strike occurs in the "neutral zone" (see fixed end beam diagrams to get an idea of the area with the least stress), this will add to the concern
Now i gotta think about testing the cylinder every 10 years at least, and I test it at 5/3 service pressure. So now I'm at 4400 (hydro) pressure.
Now I look at low cycle stress (fill the cylinder every 2 weeks?)
Last thing is, when one of those lets go, you don't need a postcard from your mom to let you know that sucker just popped. Its a BIG deal
Hello fschweighardt, I might include just a little tidbit to add to this conversation. As Blaster's report was in reference to an Acetylene cylinder I believe there "could" have been another contributing factor. From my understanding of acetylene cylinders it is imperative that they don't have any dents in them, if they do then a void could be present in their internal structure and this could allow for a spot on the inside of the bottle where free-state acetylene gas could have been present. As free-state acetylene under pressure is VERY unstable, even the slightest upset or jarring could have caused an explosion. I am not saying that this is what happened and likely without a very exhaustive forensic investigation it will probably never actually be known. This incident could have also involved a number of smaller issues leading to this large one, kind of like all of the stars and the moons were aligned and Ka-bang.
I also follow your mention of the rated pressures versus actual's, the over-percentage allowances, and even temperature changes. If you have a cylinder that was filled in the dead of winter and ends up being stashed away somewhere in the mix of the bottles and then someone digs it out in the middle of summer. I am going to guess that it's internal pressure is likely a bit on the high side. Now, since the bottles are equipped with over-pressure safeties and there is a safety allowance for it's pressure content there shouldn't be a safety issue. Yet, in the case of acetylene gas bottles that is somewhat of an animal of a different breed since pressure isn't the only consideration. Best regards, Allan
Too follow that thought a bit, the pressures in fuel bottles are considerably lower than those of the high pressure tanks. But, whatever the cause, and whatever the tank, when they let go the resultant explosion is not something I want to be in the vicinity of.
He Is In Control, Have a Great Day, Brent
Have seen that before but very appropriate here. Thanks Lawrence.
BB
I have been personally witness to an Industrial catastrophe back in 1978 that killed 3 and made it to the 3 (back then it was ABC, NBC and CBS) National Network Evening News. After that, I developed a hobby interest as it were of reading up on other disasters from around the world: Seveso, Italy (major dioxin release from a soap plant that could be converted to make Agent Orange), Kyshtym, Russia (quite possibly worse than Chernobyl and led to the downing and capture of Cptn. Francis Gary Powers), the Boston Molasses Flood (in 1919) and dozens of others.
What I found was that the vast majority of these incidents did not happen due to a single failure. But were in fact the result of multiple issues that independently had minimal opportunity to wreak havoc. The same could be said for personal injury accidents in the work place.
Will a single arc strike (arc burn) cause a catastrophic event??? Highly unlikely in my opinion.
Though not per AWS D1.1, API 1104 (20th ed.) A.5.3, Tables A-2 and A-6 do have some rigid definitions of what is acceptable. Due to this, most company specs have simplified it by prohibiting arc burns and require the area cut out. This severe spanking usually results in termination of the welder (2nd time or "3 strikes you're out") who very soon modifies their technique and gets that bad habit corrected.
It is a sore subject for me as I see it as blatant poor craftsmanship, laziness and lack of personal pride in performance (as in take some initiative; grind, weld finish cleaning it up so it doesn't look like you did what you did!).
Left unchecked, this often decays into the area in and around of the weld looking like metal pooping hens have roosted over it.
At this point, now it has become a hazard to the durability of the finished product!
The item in the attached foto was only tacked. The welder was not done arc scratching it! I think I have deleted many pics from various gigs so that the lack of evidence would protect the guilty.
After posting the above, I dug up my 2005 D1.8 Seismic book, thinking "Surely, this will have some arc strike prohibitions written in to it..." Nothing that I could find.
Which makes me wonder... Is API just guilty of perpetuating old school mythology or is AWS guilt of oversight???
The things that keep me up at night tossing in a fevered stupor.
Already stated above sir.
But, I believe they just saw no reason to go beyond D1.1's guidelines unless it was an item of concern that the engineer decided to write into the Contract Documents.
And, AISC is the same.
He Is In Control, Have a Great Day, Brent
Well, I got things well and truly off track, no reason to quit now I suppose.
Acet cyls are concerning for the reasons Allan mentioned, and the usual thing is somebody has secured them with a chain binder and chain, or has used a big ol' bolt on their rack to hold them in place. The way I've seen it, if the cylinder comes back like that, you just bought it.
Topic American Welding Society Services / Technical Standards & Publications / Arc Strike and "Soundness"
Does it have to sound good too?
