Mill Certificate vs. Design Yield Strength 2

[ekelley]

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!

 

[Lion06]

I've been in the position of having to use tested coupons of in-place steel to check a substandard design. We used the lowest coupon value for the design checks.

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.

 

[IFRs]

Playing devil's advocate, what's wrong with using the mill reports and the higher yield? Isn't that the whole idea of mill reports? Often steel is dual-certified for multiple grades, what's wrong with using the higher values? Safety is not compromised, etc.

 

[JAE]

Here is a snippet from AISC Specification, Appendix 5. It really deals with the evaluation of existing structures but sheds a little light on the mill cert test results.:

[blue]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.
[/blue]

 

[Ron]

A mill certificate is tied to a specific "heat" or run of steel production. You must be able to tie the material used to the actual run or use the lower minimum value. A steel section that is not traceable to a particular mill certificate should not be designed using a general mill certificate value.

 

[DRC1]

I do not know how the coupons work for square tube, but mill tests are not representative of the strength of the steel for wide flange beams. For beams, the coupons are taken from the center of the web which cools the slowest and thus gains the most strength. It is also generally the the least structurally critical part of the beam. The edges of the flanges cool much faster and are many ksi below the web. Steel design values incorporate this discrepancy into allowable values.
Without knowing exactly where the coupons are taken vs. the stress distribution, I would require design based on the codes.

 

[connectegr]

Does the fabricator have sufficient control to guarantee which pieces/shapes are used in reference to specific calculations and mill certifications? Not in my experience, pieces are marked after fabrication, not before. Are you going to review every mill delivery and certification, to verify that the appropriate mill certifications are used. Generally material is ordered by ASTM designation, the fabricator does not pick specific pieces according to mill certificates. AISC Specification for Structural Steel list the ASTM materials which apply and does not allow for the use of material strengths in excess of the ASTM properties. The ASTM properties provide an intentional safety factor.

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

 

[TXStructural]

There is nothing magic about the ASTM yield strength, and it is not tied to anything except the minimum values the mills involved in writing the spec can consistently achieve. It should be acceptable to have and use a yield strength which exceeds the ASTM nominal strength. When used in seismic-resisting elements, using a nominal strength rather than actual yield can result in elements which are too strong to function as designed - this is the reason certain specs have upper bounds on yield.

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.)

 

[JStephen]

It's not entirely clear what is going on.

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.

 

[mfrad]

HSS sections are cold bent which increases tube yield strength at the four radius corner and then seam welded. Does the mill certificate indicate yield strength or tensile strength of flat plate?

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.

 

[paddingtongreen]

Bear in mind that the fabricator may buy only the material certified to 55K. Then he doesn't have to be traceable to the heat.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BigH]

The way I see it - for what it is worth. ASTM gives the minimum yield and tensile strengths (or the acceptable range depending on the grade). If you "order" ASTM A500 Grade B, the material passes the specification if the strength values (and others requirements such as chemical composition) are met. End of it. However, the designer has upped the ante and specified that the minimum strength must be a certain value - in this case only those A500 Gr B heats that achieve the designed values would be permitted for use in the fabrication. Other heats meeting A500 GrB that do not meet the design value would be excluded from use in the works.

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 . . .

 

[ekelley]

Thanks for all the great responses. I see the merit in both sides of the argument, but I'm in the design per the ASTM minimum values camp. One, because I looked back at AASHTO and they tell you to use the minimum values for design. Granted ASTM A500 Gr. B is not in the AASHTO table, but I interpret it to apply to the ASTM min values too. Two, because it seems like a very difficult thing for a resident engineer or field engineer to verify. There is a submittal process, but not always. And three, because I don't want to prolong the back and forth with the fabricator by asking where the coupon was taken for testing, etc. Thanks again.

 

[weab]

This is a very interesting string. In the past during fast track designs, I would find that structures are overstressed while being fabricated. I was instructed by my superiors to check the actual mill certs and design to those. At the time, it seemed perfectly reasonable. Now several issues are making me think.

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.

 

[paddingtongreen]

@ekelley, you said mill certified in the OP, the mills test the material before forming it into shapes,

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dhengr]

If Fy = 64ksi and Fu = 65ksi, you’ve got something more than just meeting the ASTM specs. going on for most structural (construction) steels. You should also be interested in elongation, maybe charpy tests, chemistry and alloying, heat treatment, etc. Then you also have significant residual stress issues with some rolled and continuously cast sections due to cooling rate, or from cold working in rolling or in forming. Generally, for a given heat of material intended to meet a specific ASTM spec. slight variations in chemistry and alloying or heat treating might cause Fy & Fu to move up or down the scale, within spec., but the spread btwn. them should not change drastically (come so close together) as weab suggests, for normal construction steels. The two tend to move together unless you are doing something exotic with chemistry, alloying, heat treating or cold working.

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.

 

[weab]

dhengr,

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.

 

[BigH]

I disagree that mill certs should be acceptable for design. Might this not cause designs to be adjusted in the field to suit the mill certs of the materials being delivered? Take, for instance, rebar; we have some heats coming in just above the A615 values - then a few days later, we have heats (from a different source) coming in well above. What? Am I suppose to try to remember when placing 100 tons of steel for a concrete placement which heat I am using and adjusting spacing? this would be a royal pain. Statistically, it might be necessary to have 3 or more full tests on a particular heat in order to determine the appropriate characteristic design values.
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.

 

[desertfox]

From my experience if you design using values off the mill cert, usually you know what the values are going to be [because you agree with the supplier before purchase] or at least how much higher than the minimum yield value there going to be, long before you receive the certs and those values are restricted to that batch or batch's of steel in which case that should eliminate BigH's concern of mixing steel of lower strengths.
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

 

[JAE]

This is a bit geeky-technical-literal, but keep in mind that ALL of the code/specifications for Fy in AISC refer to Fy as the [red]SPECIFIED[/red] yield, not the actual yield.

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).