Mill Certificate vs. Design Yield Strength
Mill Certificate vs. Design Yield Strength
(OP)
During a luminaire pole submittal review I noticed a fabricator had specified the material for a square HSS as ASTM A500 Gr. B. Rather than listing the yield strength as 46ksi, they indicated it as 55ksi on the drawing and used 55ksi as the yield strength in the calculations. I commented on the error, noting that the element was not structurally sound when using a yield strength of 46 ksi in the calculations, and returned it.
I received the following in response: "We buy the steel to be certified from the mill to have a minimum 55 ksi for the member. It meets the chemical properties of ASTM A500 Gr. B, but is tested to higher yield."
I'm not very happy with the response. I don't know if the typical yield strength is 55 ksi or not. I know the yield performance of this material is not bracketed like an A992 steel would be, but is the performance affected by the higher yield? The resistance factors (or allowable stress factors) specified in the code are based on the probabilistic properties of the material, right? Is using the precise yield strength indicated on a mill certificate a bad idea for design? It seems they are taking a shortcut rather than just sizing the post appropriately. At a minimum, I would request the mill certificate, but I would rather reject it. How would a typical resident engineer know to check that the mill certificate yield strength exceeded the yield strength spec'd for the material? Seems sneaky to me. Has anyone had any experience with this sort of thing?
Thanks!
I received the following in response: "We buy the steel to be certified from the mill to have a minimum 55 ksi for the member. It meets the chemical properties of ASTM A500 Gr. B, but is tested to higher yield."
I'm not very happy with the response. I don't know if the typical yield strength is 55 ksi or not. I know the yield performance of this material is not bracketed like an A992 steel would be, but is the performance affected by the higher yield? The resistance factors (or allowable stress factors) specified in the code are based on the probabilistic properties of the material, right? Is using the precise yield strength indicated on a mill certificate a bad idea for design? It seems they are taking a shortcut rather than just sizing the post appropriately. At a minimum, I would request the mill certificate, but I would rather reject it. How would a typical resident engineer know to check that the mill certificate yield strength exceeded the yield strength spec'd for the material? Seems sneaky to me. Has anyone had any experience with this sort of thing?
Thanks!






RE: Mill Certificate vs. Design Yield Strength
I think that you're ok if the mill tests come back at 55 ksi or better, but you'll be relying heavily on those mill reports because, as you noted, ASTM A500 Gr. B is only a 46 ksi minimum yield.
Are you the EOR? Is the fabricator doing the design of these luminaires and you're just checking it? If that's the case, I would ask them to size the poles based on 46 ksi. That's not an unreasonable request.
RE: Mill Certificate vs. Design Yield Strength
RE: Mill Certificate vs. Design Yield Strength
2. Tensile Properties
Samples required for tensile tests should be removed from regions of reduced stress, such as at flange tips at beam ends and external plate edges, to minimize the effects of the reduced area. The number of tests required will depend on whether they are conducted to merely confirm the strength of a known material or to establish the strength of some other steel.
It should be recognized that the yield stress determined by standard ASTM methods and reported by mills and testing laboratories is somewhat greater than the static yield stress because of dynamic effects of testing. Also, the test specimen location may have an effect. These effects have already been accounted for in the nominal strength equations in the Specification. However, when strength evaluation is done by load testing, this effect should be accounted for in test planning because yielding will tend to occur earlier than otherwise anticipated. The static
yield stress, Fys , can be estimated from that determined by routine application of ASTM methods, Fy, by the following equation (Galambos, 1978; Galambos, 1998):
Fys = R(Fy − 4) (C-A-5-2-1)
[S.I. : Fys = R(Fy − 27)] (C-A-5-2-1M)
where
Fys = static yield stress, ksi (MPa)
Fy = reported yield stress, ksi (MPa)
R = 0.95 for tests taken from web specimens
= 1.00 for tests taken from flange specimens
The R factor in Equation C-A-5-2-1 accounts for the effect of the coupon location on the reported yield stress. Prior to 1997, certified mill test reports for structural shapes were based on specimens removed from the web, in accordance with ASTM A6/A6M (ASTM, 2003). Subsequently the specified coupon location was changed to the flange. During 1997–1998, there was a transition from web specimens to flange specimens as the new provisions of ASTM A6/A6M (ASTM, 2003) were adopted.
RE: Mill Certificate vs. Design Yield Strength
RE: Mill Certificate vs. Design Yield Strength
Without knowing exactly where the coupons are taken vs. the stress distribution, I would require design based on the codes.
RE: Mill Certificate vs. Design Yield Strength
As IRFs mentioned material can be dual certified. But, the design strengths are still limited to the higher ASTM designation properties, not the certificate properties. The purpose of mill certifications is to verify that the material performs within the RANGE required by ASTM.
In nearly all cases, I would avoid using mill certificates for design properties. As StructuralEIT noted, "in-place" steel would be the rare occasion where test coupons are the only information available.
http://www.FerrellEngineering.com
RE: Mill Certificate vs. Design Yield Strength
You would still use the same load and resistance factors, and you do want to see the mill certs for the steel. They should be able to provide the actual certs for the heats they use, since they do track that info. It a material barely tests to 55 ksi yield, you might hesitate to design for 55, but if it tests to 58 or 60, you might be more willing to accept that the lower bound would exceed 55.
That said, it seems like it might be an unnecessary economization of material, but the designer might have other reasons (weight, fatigue, etc.)
RE: Mill Certificate vs. Design Yield Strength
In some situations, it is possible to buy certain grades of steel with higher-than-standard strengths. If this is what is going on, the company's purchase orders would generally specify accordingly. In principle, there wouldn't be anything wrong with this, assuming the design codes involved didn't prohibit it one way or another. (Doing this might also affect weld qualification tests, etc.)
If you buy a piece of steel, then design for the as-tested yield strength instead of the minimum-specified yield strength, that's a little different; you are simply eroding the factor of safety that the design codes have incorporated, and this is generally not the intent of the codes.
RE: Mill Certificate vs. Design Yield Strength
Questions to ask include: is weld capacity a limitation; width to thickness ratios with higher apparent yield strength also reduce beam and column strength;there are flange web buckling criteria to consider; is fracture toughness for exterior exposure the same as 46 ksi yield.
RE: Mill Certificate vs. Design Yield Strength
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Mill Certificate vs. Design Yield Strength
I have run into this before where a specific specification is given - and when the mill certs came in, they weren't complete for the specified material (missing some chemical tests) - when queried the designer simply stated he was interested in the strengths and didn't "care" about the chemical composition - then why didn't he just say he wanted a carbon steel having a specific strength and leave it at that. The other problem I have continuously is substitutions of, say, JIS for ASTM . . .
RE: Mill Certificate vs. Design Yield Strength
RE: Mill Certificate vs. Design Yield Strength
What is the code intent? Currently A992 calls for yields of 50min and 65max, ultimates of 65 min. Unless I misunderstand, theoretically I could get a piece with 64ksi yield and 65ksi ultimate. This would be brittle but still meet the standard. Would this matter? Isn't the code based on yield (mostly) anyway? Or do the strength reduction factors account for the belief that there is usually some yielding to save the day. Is there an assumed differential between design yield and design ultimate? Would the strength reduction factors be different if steel were more brittle? Everyone knows and perceives steel as being ductile. However in the 64ksi yield 65ksi ultimate situation, my question is this. For code purposes, does it matter if there is little difference between yield and ultimate? What am I missing here?
The strength reduction factors should account for the possibility that your material is not quite as strong as you believe. To relate that to this conversation, I would think that a coupon test of a lot would give you a reliable yield when the strength reduction factor is used. If the mill cert says 55ksi, it may be only 50 or 52ksi which is accounted for in the factors. Correct?
Therefore, I don't see where any safety factors are reduced. I'm sure that I'll be corrected as necessary.
RE: Mill Certificate vs. Design Yield Strength
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Mill Certificate vs. Design Yield Strength
Generally, if we were ordering materials, either shapes or plates for our commercial structural fab. shop, we ordered to ASTM specs. and grades, and I am not sure if they always got mill certs. or if they spent the time and effort so they could track a specific WF to a cert. or a heat. I know they couldn't always track mat'l., but I believe they did for bridge work and power plant work. But, when we were ordering material for my dept., a special structures and large equipment design & build dept., we did always get certs. and spent the time to mark mat'l., at receiving and warehousing, and then kept my mat'l. segregated in the warehouse so that we could track all of our material into the structure. I didn't make a habit of designing to the cert. stresses, but there certainly were some times, in a pinch, during design that I went back to the certs. and took advantage of the fact that Fy & Fu actually came in 8 or 10% higher than the spec. min. And, at that time I had a fair handle on what we could normally expect over and above the ASTM mins. on the mat'l. we were getting. I'm also one of those old guys who would allow a few percent overstress when we were confident in our loads and design details. So, one way or another we got the cat skinned. These were what we used to call engineering judgement and experience calls, a phrase which seems to be all but gone in today's world of engineering.
RE: Mill Certificate vs. Design Yield Strength
I agree with you totally. I was just trying to make a point with Fy=64, Fu=65. I would never expect this. I expect mild steel to be ductile. If not, then I would expect strength reduction factors to be even smaller. My point is that I believe that mill certs should be acceptable for design.
RE: Mill Certificate vs. Design Yield Strength
If one "designed" by heats, in a different vein, would one also design concrete depending on the "last 3" test results? - Oh, I am 20% over the design value, so now I'll cut the cement content to get to "just over" the design value - oops, damn, its changed again. Don't think so.
RE: Mill Certificate vs. Design Yield Strength
The only time I have known people using values after the design was finalised was in the event of a design error thats just come to light prior to manufacture, which in those circumstances might save a lot of additional design work.
So I would say that designing off the mill certs is okay if you set your stall out that way in the first instance,secondly again its okay if you spot an error further down the design process line, provided that those certs are for that particular batch of material your using.
desertfox
RE: Mill Certificate vs. Design Yield Strength
If you try to use certificated yield values and something goes wrong - an expert witness on the stand could easily testify that you, as the engineer, did not follow the standard of care required of engineers (what another reasonable engineer would have done in your place - follow the code).
RE: Mill Certificate vs. Design Yield Strength
RE: Mill Certificate vs. Design Yield Strength
point taken, I don't work with rebar so I won't have that problem
RE: Mill Certificate vs. Design Yield Strength
JAE- I like the "geeky-technical-literal" issue that you bring up - and it's important. I'm at home and don't have a code with me. But is there a definition of "specified"? If not, then the spec yield is the specified yield. If I have an actual yield, I see no reason not to use actual yield because it is now SPECIFIED.
I absolutely don't agree about the standard of care. I'll leave it at that.
Now with that said: I have never heard of using mill certs in original design, only as design and construction progressed, and extra strength is required.
RE: Mill Certificate vs. Design Yield Strength
I've seen it done not in your line of work but for components in mechanical engineering.
desertfox
RE: Mill Certificate vs. Design Yield Strength
RE: Mill Certificate vs. Design Yield Strength
RE: Mill Certificate vs. Design Yield Strength
And, as I explained our commercial structural fab. shop did not do this, nor did our rebar fab. dept. Obviously, BigH's example of rebar which is dumped into a bunker by bar size and grade is a good example of where this wouldn't work very well at all, nuts and bolts from a box or a keg might be another example where individual mill certs. wouldn't be practical. In fact I do recall examples where we used mat'l. other than that shown on the structural drawings and specs. because we had it in stock and it lead to a connection or detail more to our shop's liking or the erector's liking, on a large job with much detail repetition. That was about the only time I really did work with the structural dept. I would get involved doing the joint designs, calcs. and a report which had to be submitted to the EOR to get their approval to change the detail. Furthermore, I knew many of those engineers, otherwise connections came detailed on the plans, or our structural detailing dept. designed their own std. connections.
JAE's admonition to "follow the code" is well taken, particularly when you have no control over the material to be used other than an ASTM Spec. You could probably be sued these days for using the wrong grade pencil lead for your calcs. too; and the codes and analysis methods are getting so complex that I suspect that there isn't a design out there that one couldn't find some fault with if they combed the codes, calcs., plans and actual construction deeply enough. I spend about the same percentage of my time defending engineers as I do arguing against their work, and I don't often damn an engineer for using well reasoned engineering judgement and experience, that is seldom the cause of a failure or problem. However, the reason "SPECIFIED yield" is used in AISC is because the designer only knows the grade of steel he speced. for his job and his design is done, prior to and, irrespective of the exact Fy that the fab'er. might get in that spec. At the same time the laws of Strength of Materials, etc. do not go out the window or become dormant just because you use an actual Fy which is 8% higher than the spec. minFy, assuming a situation arises where you can make that decision. It could be argued that a prudent engineering working with the care normal to and expected of the profession would not increase one beam, out of dozens of the same sized floor beams, to the next size just because it might be a couple percent over stressed in one small, well understood, area and then stand the chance that it be misplaced during erection. Desertfox and I are often in agreement, and we are in the same camp on this one too.
RE: Mill Certificate vs. Design Yield Strength
Using rebar as an example (all steel production works in a similar way), one mill might produce a ASTM A615 Grade 60 bar which yields at 61 ksi, while another may produce a bar meeting the same spec which yields at 77 ksi. They both meet ASTM A615 Grade 60, and the code would allow design of a structure based on the nominal grade (60 ksi), but would not prohibit design using the actual yield (assuming sample size and QA/QC warranted that increase.) The mill may or may not choose to call it ASTM A615 Grade 75, depending on many factors.
If a designer chooses to design for a known yield strength, and the mill reliably supplies this strength while meeting all other requirements, there is nothing wrong with doing so, as long as that grade is permitted for the use (ACI 318 does limit maximum design strength for certain components.)
The reason we design for ASTM specified values is that we know it can be produced and will most likely be available.
I do not believe it violates a standard of care to use a steel strength which is demonstrated by sufficient testing and QA/QC, since this is how compliance with an ASTM is determined. However, you would not design for 55 ksi yield but specify ASTM Axxx Grade 46 (46 ksi yield) hoping for grade 55. You would specify ASTM Axxx grade 55 (55 ksi minimum yield), even if the ASTM only lists Grade 46 (or Grade B, or whatever applies).
RE: Mill Certificate vs. Design Yield Strength
http://st
Have a look at this link its a code for construction in stainless steel, if you go to section 2.2.2 "Design Values Of Propeties" scroll down and you will find it says you can use the prof stress off a mill spec. and further on it says:-
(iii) Design using mill certificate data
Measured values of the 0.2% proof stress are given on the mill (or release)
certificate. The design value of the 0.2% proof stress can be taken as:
py = sm0.2 / 1.2
where sm0.2 is the average value of the 0.2% proof stress as given on the
mill certificate.
It is suggested that the ultimate tensile strength should still be based on the
minimum specified ultimate tensile strength given in BS EN 10088-2.
RE: Mill Certificate vs. Design Yield Strength
- halt of the gauge, meaning there is well-defined yield plateau when a sample is tensioned until yield - this is the traditional method of test, but many steels (including stainless and ductile, high-strength steels) do not exhibit well-defined these yield points.
- EUL, or elongation under load, which has been used for rebar here until recently. A value such as 0.35 EUL means that the the yield value presented is the stress at which samples reach 0.35% elongation. Graphically this is determined as the intersection of the test curve for the steel with a vertical line at 0.35% of length as measured from the origin of the stress-strain diagram.
- Percent offset, which is frequently 0.2%. Given a sample with a proportional, elastic curve (essentially all steels), a line parallel to the straight portion of the elastic curve (representing the modulus of elasticity) is drawn 0.2% to the right on a stress-strain diagram, and the point at which this line intersects the test curve for the steel is the 0.2% offset yield strength. (This is the method used by most of the steel industry, including in the cited stainless spec, as well as ASTM A955 for stainless steel rebar.)
The difference between the EUL and 0.2% offset varies from steel to steel, but will be essentially the same for a steel with a well-defined yield plateau. For steel without a well-defined yield, percent offset best represents the actually yield strength of the steel for most applications.
(The current draft of ACI 318-11 attempts to circumvent the ASTM process by specifying EUL as the test method.)
RE: Mill Certificate vs. Design Yield Strength
I didn't scrutinize all the responses in detail, but please be aware of the formal meaning of a MTC:
It is a document issued as confirmation that the steel supplier has succeeded in producing the grade mentioned.
The test results are "the evidence" that the production facility still produces within the limits (defined for that specific steel grade) controlled by statisical analysis from the beginning on for that specific production process.
The ultimate consequence is that imho the engineering should be done based upon the minimum guaranteed values as defined for the grade and should not be based upon the reported data on an occasional MTC.
RE: Mill Certificate vs. Design Yield Strength
Nonetheless, I have seen MTC values used for existing structural elements only when there is a great deal of confidence in the design loads and there is full material traceability.