Eng. practice of allowing 5% overstress 33

[radair]

It seems to be standard structural engineering practice to allow up to 5 percent overstress in structural design. It's been this way since I graduated college in 1980 and I've seen the practice commonly used in the tower analysis field for the last 15 years.

I've been asked my opinion by a government agency as to why this is a safe and acceptable engineering practice, including citing any relevant structural codes. They are not questioning my work but are asking me for a signed & sealed letter of opinion. It seems to me that this would be a better question for their state engineering board of licensure.

Can any of you help?

 

[swearingen]

JStephen, I do agree with frv that the metal building industry is probably pushing the envelope a little harder than most of us feel comfortable with. I do also agree that shooting for 103% doesn't make sense. You shoot for 100% and if you get something in your model that cracks 103%, squint your eyes, think about what that particular member is doing and how it fits in to the big picture and make a call - yeah, it's OK, or no, I'm gonna have to whittle my loads or move the member or upsize the member.

RWF7437 has a good one there. "Learn the rules so you know how to break them." This statement is perfectly accurate here and particularly applicable to my industries. The codes we work under are primarily geared towards standard commercial occupancy structures. We often have to make judgements about how to apply the codes when dealing with petrochem, papermill and material handling structures. Knowing their basis allows us to make intelligent, safe decisions on how to apply them to our structures, including some overstress tolerance.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[hokie66]

I think this one is about 50-50. That is the way buildings are: some a bit light, some a bit heavy. But you don't really know whether the Unity = 1.05 or the Unity = 0.95 ones are the light ones or the heavy ones unless something goes wrong. Depends on a lot of things, and we should be humble enough to realize that "Structures are smarter than engineers". Sorry, can't remember who said that.

 

[civeng80]

Brick sizes in Australia are 110mm wide x 76mm high x 230mm long. Their is a tolerance +/- I forget what it is. I ordered bricks some years ago to build brick piers and the joints were hopelessly large. I phoned the manufacturer who quoted the brick tolerances to me and stated that they headed for the thinner brick size within their tolerance(obviously increasing their profits in so doing!) Later they sent me bricks to the right size. Obviously these people use tolerances for their own advantage and to increase profits.

May not this also be the case with the 5% overstress? As hokie66 said it is a tolerance but to others its a way of simply saving $s.

Just a thought !

 

[Tomfh]

hokie66,

Wooten perhaps?

 

[youngstructural]

I think was Wooten...

And in my opinion this conversation has completely missed one of the key legal tests: Duty of Care (also often refered to as the engineer's standard of care). The duty requires an engineer to apply reasonable care and skill which can be expected of an ordinary engineer of ordinary competence. This is measured by comparison to other engineers practicing at the same time in similar circumstances, as well as to standards applicable to the works. Since the application of any standard or code is a matter of interpretation, if other engineers of similar training and experience can be shown to make the same assumptions in their work, you have fulfilled your duty of care, or met the standard of care, depending on your prefered terminology. This does not, however, preclude an argument that you were excersing your professional skills outside of the reasonable scope with regard to your level of experience.

Thus, if an engineer applies reasonable skill to works of reasonable scope and complexity given their level of experience, AND further that the interpretations and assumptions applied are used by other engineers practicing in industry, that engineer will likely be found to be in the right of law.

I refer you all to the excellent, must read (in my opinion) text of "Law for Professional Engineers" by D.L. Marston. On p34 Marston quotes a halmark Canadian case regarding the liability of an engineer, which in turn quotes from Halsbury's Laws of England, the text reads:

"Every person who enters into a profession undertakes to bring to the exercise of it a reasonable degree of care and skill, and represents himself as understanding the subject and qualified to act in the business in which he professes to act. The employer buys both skill and judgement, and the architect ought not to undertake work if it cannot succeed, and he should know whether it will or not.

[then further down it continues quoting with]

As to the amount of skill required, the architect or engineer need not necessarily exercise an extra ordinary degree of skill. It is not enough to make him responsible that others of greater experience or ability might have used a greater degree of skill, or even that he might have used some greater degree. The question is whether there has been such a want of competent care and skill, leading to a bad result, as to amount to negligence."

I feel quite strongly that we all must apply our engineering judgement to all situations (nods head to strguy11, civilperson, cap 4000, HgTX & swearingen), and that an the unfortunate event that you ever wind up in court, not matter what you write down a lawyer will try to discredit, embarrase, bully, intimidate and abuse you. At the end of the day it all boils down to ethics...

Can you, honestly, say that you have fulfilled your duty of care to the client and public at large with a 5% "overstress"?

I say Yes.

Except for in Cold Form Steel.

Regards,

YS.

P.S. Not trying to be funny with the CFS comment... That stuff's fun, but totally unforgiving.

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[frv]

Great post as usual, YS.

 

[civeng80]

Yes I totally agree!
Excellent post YS, summarises the situation beautifully.

 

[apsix]

Yes, often it's hard to believe that YS is really a 'young structural'.

 

[hokie66]

Tom & YS,

My memory must be failing. Who is Wooten? I couldn't remember who I quoted, and now don't even recognize his name.

civeng80,

Glad to have a convert to the "Yes" side of this debate. Or are we the "No" side? Now I can't remember that either.

 

[youngstructural]

Hey Hokie;

Wooten was a senior and very respected structural engineer; I can't remember who he worked for (I think it was a steel fabricator in the US, meaning that he would have been heavily involved in steel connection design and trying to refine column sizes, as you would know well being a transplanted American) but his paper "Wooten's Third Law of Steel Design" is an absolute classic...

Very, very eminent, authoritative, and perhaps most importantly for a paper with the preceding adjectives: Practical.

Regards,

YS

P.S. Frv & Apsix: I hope more people agree with my argument and our point of view ; Technically it would increase the probability of our standard practice being accepted as fulfilling our duty of care!
P.P.S. I'm twenty-eight and have just under five years experience... Definately a youngstructural and in need of many of your opinions and guidance. I just hope to think that guys like StructuralEIT & I can contribute from time to time....

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

Wooten, J. (1971) "Wooten's Third Law and Steel Column Design",. Engineering Journal, 2nd Quarter, pp. 1-3. 2006 ASCE/SEI Structures Congress

I strongly recommend that anyone who hasn't read the paper finds and reads it as soon as they can. Some of it is out of date, but the judgement behind it is the key!

I'm pretty sure either someone else or I have posted a link to it previously on an eng-tips thread... But it is quite late here in Christchurch and I'm to bed. Have a search; Should be there.

I look forward to your replies...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[Tomfh]

Here it is:

 

[Dinosaur]

miecz,

When I am designing new, I try and proportion the main members so they are between 0.95-1.00 of the allowable limit. Sorry for the confusion.

 

[hokie66]

Thanks for that, Tomfh and YS. Oh that I could write like that. Should be required reading for anyone who wishes to participate on this site.

Unfortunately, and especially for those who are afraid of lawyers, everyone is not as eloquent as Mr. Wooten.

 

[civeng80]

I must say its the first time Ive ever heard of this bloke called Wooten. I'll read the article posted some time about him and his third law.

I for one am afraid of Lawyers particularly their fees. Would not like to find myself in a Court of Law for Professional negligence or anything for that matter. I guess if an engineer was called in a court of law it would be for something serious, but law now is so broad and Lawyers hungary for money that when they see a crack where money lies it gives them reason to sue and if unsuccessful appeal etc. I agree with Hokie and ys a 5% overstress does not cause a structure to fail and I think many of us would be able to defend ourselves on that one under cross examination. There would be much bigger problems to face! But I guarentee you that the question would be asked of why you let the structure be overstressed by 5%.

I would be interested to hear from anyone in the group who has appeared in Court for any reason not just engineering and give their comments of whats its like dealing with Lawyers.

Cheers!

 

[hokie66]

I have appeared in court a number of times, but always on the plaintiff's side in engineering issues. I found the lawyers on my side to be fine, upstanding citizens, and the ones on the other side conniving mongrels.

 

[civeng80]

Exactly how I felt in my situation hokiee66. I think in my case the opposition were conniving mongrels and lyers but fortunately for me also a bit incompetent. The court I faced was VCAT and that in itself was danger (for my particular case anyway) but my QC and Barrister were there mainly to keep VCAT straight. My Lawyers always told me that we must win with the truth and that was fine by me.

Anyway amazing how popular this topic is and how touchy legal issue are.

Cheers!

 

[frv]

Star for the link, tomfh!

YS had mentioned Wooten's third law a few times in other threads. I could only infer by context the implications of said law, as I was unfamiliar with it myself. This article is exquisitely written. Hell, I think if anyone is ever sued, they should print up several copies, distribute them to all involved, get up and walk out of court.

In any case, this applies to every profession. I have several friends who went through med school. It is not uncommon for first or second year med students to diagnose themselves with an array of exotic diseases, when all they have is the sniffles.

 

[civeng80]

frv
Are you refering to the article posted by Tomfh above?

If its thats good I'll download it !

Cheers!

 

[frv]

Yes I am. And it's only three pages.

It just goes a little more into detail about the implications of Wooten's third law.

I'm extremely curious about the first two, though!