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Yield Stress and Steel Design 2

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SmithJ

Structural
Apr 11, 2003
72
Hello All,

I am having an issue at work that I need some input in. The manufacturing company I work for gets it's steel from a number of different sources. Apparently, our 36ksi steel comes in with mill certs from 45 thru 49 ksi. The practice here is to design at the mill cert yield stress, but I remember reading that the design yield stress for steel is the STATED design yield strentgh, not the mill cert test results.

Is this accurate? Am I unduly penalizing the design if I insist on using 36ksi design when the mill cert comes in at 45ksi? Also, if I design using 50ksi steel, but we find a 36ksi beam that comes in at 48ksi, is that an okay substitute? (strength wise it would be okay).

Thanks in advance for your toughts.
 
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Smithj,

It depends on what type of steel members you are getting. For example most of the wide flange beams come in ASTM 992, grade 50. If you request A36, you will get grade 50 with mill cert that is dual certification.

You can design to yield of ultimate stress as long as you use safety factor that is acceptable. I am not sure what steel members you that you are designing and what code they are subject to (AISC, AISI or is it cold formed?).

These days, I find it hard to believe that mills roll A36 steel wide flange beams. May be aggles and channels. Alos the minimum grade is 50 KSI.

I suggest you visit the AISC web site because they have tremendous wealth of material that can be of assistance to you in this area. Their web site is:


Regards,



That is my quick one minute resposne.



Lutfi
 
Per AISC, the value Fy to be used in design is the specified yield strength - not the actual.
 
AISC defines Fy in the Nomenclature Table. As JAE says, it's the specified yield strength.
 
Indirectly in ASD, and directly in LRFD, usable design strengths are determined statistically. If you pick off the top 50% of the sample, then you have drastically lowered the mean and the 95% confidence level. As a consequence, the formuli and design factors that have been developed to use these design strengths are no longer valid.

For a comparison, consider wood design. Wood is a hugely variable natural product, and even the older standard test samples (1"x1"x12"of clear wood) can have failure stresses at the 5% confidence levelbe several times the value at the 95% confidence level. Sawn wood is divided into grades such as Select Structural, #1, #2 and so on, based on a certain level of defects, and given its own set of design stresses.

There is also a grade of "#1 and Better", a random mix of Select and #1 materials, with its own set of statistics, and with design stresses between those of #1 and Select - even though some of the pieces in that group only qualify as #1. (Note that this is not the same as sorting through the pile at the lumber yard to pull out the sticks marked "Select".) Hand-picking the Select members out of the load would change the statistical basis for the design strength of that group, reducing it to the level of #1. Mills that are producing "#1 and better" are not permitted by the grading agences to possess the grades stamps for either "#1" or for "Select," so as to prevent messing about with those statistics.

All of which is to say, if it is spec'd and certified as A36, no matter what the coupon yield strength, then use Fy=36 ksi.
 
haynewp - it is simply the definition used in the AISC specification - if you look under chapter E for instance, I believe under the formulae for columns there is something like: Fy - Specified steel yield strength.

I know I've read something about this before - relating to specified yield vs. actual - but I don't recall where - perhaps its somewhere on the AISC website.
 
So from what I am reading, It seems that we can only use the specified minimum yield stress as defined by code. If I design using 50 ksi flat bar, then plate that has coupon strength of 48.5 ksi is not acceptable because it is still A36 steel.

Now my issue is convincing the company I just joined to change it's standard practice. This is one of the times I wish I was still in the consulting world. Is there anyone else out there in manufacturing that has a similar difficulty?

Thanks,
JS
 
So would it apply to this situation as well?

I am reviewing drawings during the time when A992 was being phased in, and the drawings specified WF beams to be 36 ksi and the dual certified A992 was actually supplied. I can't check the beams using 50 ksi?




 
haynewp

I would think that if you originally specified the steel to be A36, but as the project progressed, the contractor supplied a dual certification steel, then it would be up to you as EOR to accept this alternative product as a substitution to the A36.

Now once that happens (the EOR saying OK to dual) then the new specified steel yield could be considered as 50 ksi. But the original poster was implying (I think) that they were looking at the mill certs and simply using an actual yield...like 48.3 or something.

Steel must meet the full requirements of its associated ASTM criteria (which is more than just yield). And in doing so - is classified as falling under a particular type of material that is either acceptable to the EOR or not. And each ASTM has its related and specified yield value that must be used in design.
 
Thanks,

So it should be the same if I were checking an existing building where I wasn't the EOR. If the structural drawings have 36ksi steel specified, but the shop dwgs show A992 was actually suppplied, then I should be able to check the beams using the 50ksi for the new loads.



 
SmithJ,

Backing up here a little bit to your last post, you ask about standard prctices..

Is there anyone else out there in manufacturing that has a similar difficulty?

Unfortunately I have to report that generally the answer to that question is yes, at least in my experience. Some companies, in particular the smaller less sophisticated ones, feel that their experience places them above engineering specs and building codes.

I don't want to change the topic in this thread, but your question struck a note with me.

Regards,
-Mike
 
How's this:

If you are trying to predict the failure load of the structure, use the actual coupon yield. If you are trying to demonstrate compliance with code, use the specified strength of the material actually supplied if mill certs are available. If mill certs are not available, use original specifications.
 
mrMikee -
I think you have a correct vein of thought - industrial settings which are creating products for use do not necessarily fall under a "building code" or also a particular material specification unless there is some other outside regulatory entity that may require it of your product.

Since I'm in the building business I naturally think about codes and material specifications - that's because in my field I am obligated to meet those codes and those codes require me to meet AISC.

I am under a standard of care type of responsibility while industrial settings may be more under a warranty/negligence situation where product liability issues are relevent.

In the industrial setting, then, a designer may be totally correct in using the actual Fy as long as they feel they have a handle on what risk/liability they have regarding probability of failure.
 
JAE,

Many companies manufacture gadgets or some sort of thing for which there is no real design specification, and it is up to these companies to figure out what to do and how to do it. Their decisions I think are many times are based on warranty costs and liability issues, as you said. Maybe there are guidelines published by nationally recognized authorities that could be useful, but probably not.

I once worked for a company that manufactured both truck mixers (cement trucks to most people) and concrete plants which consisted of bins and support structures. There is no national specification for designing truck mixers and clearly the AISC spec for buildings is inappropriate. The designers of the mixers did the best they could based on analysis and physical testing. On the other hand concrete plants are a structural product made from structural shapes, bolted to foundations, and subject to the wind blowing and the ground shaking. It is my opinion that they certainly are covered by building codes and design specifications. The question in this thread concerning the yield of steel would be handled differently for each of these two businesses.

It has been my experience that it is hard to work for some of these companies because of the opposing philosophies. You can't design machines with building codes and you can't design structural products by the seat of the pants.

Regards,
-Mike
 
I'm in the UK so the actual steel references don't mean much to me but I think I understand the problem.

If a mill certificate indicates a steel grade higher than the minumum specified, it means that the SAMPLE tested had a higher yield strength. It does not mean that the entire steel batch had exactly that yield strength. The specified material designation is based on a low acceptible failure percentage which means that the steel mills will be deliberately producing steel with a higher strength to ensure that the worst steel still meets the requirement.

If you want to use the mill certificate yield strength then somehow you also need to introduce a factor which allows for the (possible) variations within the batch. Now the mill certificate is derived from testing to ensure compliance with a steel specification. It seems likely that if you are to use the mill certificate then you will need many more test samples. This may or may not be acceptible but still introduces an extra cost.

Now genuine dual or multi certified steel can be used for any of the grades for which it is certified, without additional testing. This is only to your advantage if you have specified a lower strength and then find you need more.

Let us assume that through whatever method is deemed appropriate you determine to use steel at the yield strength given on the mill certificate. This implies that you are skimming off the 'best' stuff. The remainder of the steel now has an artificialy distorted yield strength distribution and you could argue that it no longer meets the specification even though it is the same steel!

This is very similar to concrete testing but with concrete testing you don't usually get to choose after you have the test results...
 
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