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Structural Analysis

Structural Analysis

Structural Analysis

(OP)
My company is looking at an existing steel structure to determine if it is capable of handling increased loading due to the heavier equipment being installed on the operating deck.

The steel structure was originally designed and constructed in the 70s using A36 steel, ASD design to the 7th edition steel manual.  We have CMTRs for all of the steel used in the building.  

We have run into a situation where we need to either reinforce certain beams under the new equipment per the analysis using fy=36ksi or sharpen the pencil in the analysis.  When reviewing the CMTRs for the steel, the steel averaged > fy=42ksi.  When using the CMTR reported yield strength of the steel we limited the number of modifications to the existing steel.  

Any guidance on whether this is a code accepted practice?  Do we need to do some form of destructive testing to verify the yield strength of the steel?  

Thanks.   

RE: Structural Analysis

I'm Not familiar with the term CMTR but I'm assuming it means steel mill certificates? If that is the case I'd say I don't believe you would need to do destructive testing but it obviously couldn't hurt if there is budget fr it. I wouldn't use the average for your steel strength. I'd think you would at least want to use the average minus a standard deviation, but I'm sure there is a procedure out there to calibrate the Fy you have from your as-built conditons to the percentile that AISC uses (for instance I believe ACI assumes the f'c that is used in their equations correspond to the 10th percentile of tested cylinders)

RE: Structural Analysis

(OP)
CMTR = certified mill test report

We want to use the reported minimum values less a small percentage for uncertainty.  

Fortunately, budget is not a problem for this project... we could always take samples from the steel to prove the steel yields at a higher value than 36 ksi.

RE: Structural Analysis

Other than a deviation from the standard of care, there is nothing that will prevent you from doing as you noted.  I would take samples of the steel rather than rely on the mill certificates.

RE: Structural Analysis

Also...since you seem to have a generous budget, why not just reinforce the members and be done with it.

RE: Structural Analysis

I have always been afraid of this....I have heard that even testing coupons can be difficult since the steel can test differently in different locations.
For a nuke plant, I say reinforce.

RE: Structural Analysis

The Mill Cert tests are run pretty fast and tend to be a little higher then you get when the test is run slower.

Part of the safety factor is that the steel will be stronger than Fy.  For temporary structures I have used the tested Fy, but not for permanent.

 

RE: Structural Analysis

I would limit Fy to 36 ksi in the analysis regardless of what the tests or mill certs say.  A36 steel SHOULD always have Fy>36 ksi; using 36 ksi is part of the safety factor as dcarr mentioned.  My recommendation is to reinforce the beams and be done with it.

RE: Structural Analysis

Reinforce it.
 

RE: Structural Analysis

I don't think there is anything wrong with this approach.  We've known for years that most A36 material has a much higher yield strength.  

Running some new material tests so that you can take advantage of that extra strength seems like the most cost effective way to proceed.  

You obviously don't want to cut any corners for nuclear work.  But, this shouldn't be viewed as cutting corners. More like reducing the uncertainty from the original design....  

 

RE: Structural Analysis

Be carefull.  I see you handle says you are Nuclear.  And I know that in the US, the NRC does not allow the use of CMTR's for justificaion of increased loading on a steel structure.  

They do however allow the use of concrete cylinder test results.  But you have to be very particual with this and gather all of the cylinder results for the area in question, then use the minimum. Atleast that is how I have seen it done.

RE: Structural Analysis

One thing to note here is that you just don't want to take your mill tests, or your coupon tests, and use the minimum value.

It may be that there is actually a statistical way to determine an applicable Fy which would be sensitive to the variability of the data.

I don't have a clue where you'd go to look for that statistical treatment of the scatter (i.e. to get a 90% confidence value based on the variability) - so to me - using 36 ksi would be my first choice anyway - or if I absolutely had to - use some value below 36 and the lowest test/coupon value.  So if your lowest test/coupon was, say, 41.5 ksi, I'd perhaps use something like 38 or 39 ksi for Fy.

RE: Structural Analysis

Appendix 5 of the current AISC steel manual, page 16.1-417, section 2 has some recommendations for yield strength for insitu material. Fys=R(Fy-4), where Fys is the static yield stress, R is a reduction factor either 1.0 or 0.95 depending on where it was taken, flange or web, and Fy is the reported yield stress. I would take the lowest coupon test and apply it to this equation to determine you design stress.

RE: Structural Analysis

azcats, wow....two years ago and I'd forgotten that I'd posted that.  I thought there was something in the back of my mind about this topic.  Thanks for digging that up.

 

RE: Structural Analysis

azcats,

Sorry for stepping on your post, I didnt read it before I posted.

RE: Structural Analysis

I guess the other obvious thing to keep in mind is stresses aren't the only part of design.

I'd also tend to think that if you were designing to today's AISC code vs 1970's, there will be some inherent reserve capacity.  

RE: Structural Analysis

(OP)
DWHA, you say that the NRC does not allow the use of the CMTRs for the increased yield strength.  Where does this come from?

This is actually the issue I'm running into because the NRC tells us that it is not in compliance with the AISC code if we use the CMTR for yield strength.  I disagree with their approach, but then again this is a nuclear world where things are very different.  Thank goodness this is just our turbine building that does not serve any safety functions.

We are trying to limit the number of steel modifications because of time.  Even though we have a healthy budget, we simply don't have to the time to be doing the reinforcement on the building.  Obviously we will if we have to, but that causes serious construction delays.   

RE: Structural Analysis

(OP)
And since the 7th edition was the code of record, we are reanalyzing everything using the 7th edition instead of the current year code.

Thanks to everyone for your responses on this post! I appreciate all of your expertise.

RE: Structural Analysis

"And since the 7th edition was the code of record, we are reanalyzing everything using the 7th edition instead of the current year code."

Is this the proper thing to do?  

RE: Structural Analysis

I can't speak for the Nuclear world, but whenever I touch/modify a building component U use the latest Code (or at least the one adopted by the local Building Dept).  I don't think it is proper to use the 7th Edition if you are actually changing the conditions in the Building.
 

RE: Structural Analysis

(OP)
That's where nuclear is tricky, we have to go with the licensed code of record, which per our license is the 7th edition.  Typically, for new design we always use the current code, but for the existing stuff we don't go back and reconcile the design to meet today's code requirements.   

RE: Structural Analysis

'planes are the same ... when you modify a plane you (generally) use the requirements at the time of certification.  the logic to having a "basis of certification" is so the manufacturer is trying to hit a stationary target on a project that lasts several years; you couldn't accomodate a rule change late in the design process.  then for modifications the idea is "if the rest of the plane is designed for X, where's the increase in safety if this (small) change is designed to Y".

for this reason (like it or not) most of the 737s flying today are certified to 1960s rules (with some updates, like fuel sys, cabin sys, ...).  Boeing might have updated the basis of certification for their latest model, but i'd be abit surprised if they did; not that they're doing anything Wrong.

RE: Structural Analysis

thanks, I already hated flying.  

RE: Structural Analysis

@JAE  Thank you for originally posting it.  I've used that reference often in the last couple years.

And no worries AUCE98.

RE: Structural Analysis

EricaB,
I do not know of the top of my head where it is stated.   I just know I had an action that was due today (2/17) to review all pending modifications for the use of CMTRs and create additional actions as req'd.

Have you tried an OPEX search for CMTRs?  If I get some time, I will do some digging and see what I can find. Good Luck!!!

RE: Structural Analysis

The nuclear business is strange. Much has been done, as EricaB says, to the original code, but when we were re-qualifying piping and cable tray supports years later, that work was done to the latest codes. Of course, the loads were different because of better software and updated earthquakes.

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

RE: Structural Analysis

(OP)
We always have the option to reconcile the differences between the codes.. on a lot of stuff, like cranes we have, but on things like piping and steel, we do not... however, we do use the latest codes for new design on some things...

At least I don't glow in the dark.

RE: Structural Analysis

"At least I don't glow in the dark."

...Yet.

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