ASD: 1.03 Allowable Overstress
ASD: 1.03 Allowable Overstress
(OP)
I have been used to allowing a 3 percent overstress when working with the AISC code. However, I recently realized that I can not adequately identify the basis for this overstress allowance. Does anyone know the origin of this 1.03 or where I can find information that would explain it.
Thanks.
Thanks.





RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
What I was taught in school was not to sweat a couple of percentage points because none of the calculations are that accurate to begin with. However, one should be prepared, in the case that anything goes wrong, to explain in a court of law why one technically did in fact violate the code.
The significant digits argument is much more compelling.
Hg
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
We always reject the calculations for the reason that if, God forbid, something were to ever happen, we would have a hard time defending the overstress in court.
It always turns into a nasty dogfight with the metal building guys.
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
What kind of software do you write?
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
OK I can see the "draw the line" dilemma. Conversely, it seems very unlikely to be concerned about overstressing a steel beam in flexure, say 5%, without first exceeding a deflection allowable.
RE: ASD: 1.03 Allowable Overstress
I don't know about all you guy's but I don't like my calculations to show any "OVERSTRESS" because any laywer worth his salt can hang you.
If it makes a difference, get more precise on the way you specify your loads. Unless you always design to the gnat's eylash, there is some room to make you results come out under the "ALLOWABLE" load. If you are designing to the gnat's eylash, then you should be particular about "EXCEEDING" the code.
Regards,
JPJ
RE: ASD: 1.03 Allowable Overstress
I think it's important to understand that there is no actual "overstressing" of the material. It's a question of mathematics. As much as we might admire the accuracy of our calculations, they are not accurate to two decimal figures. Consequently 1.03 rounds down to 1.0. So does 1.049 for that matter!
dbuzz
RE: ASD: 1.03 Allowable Overstress
dbuzz,
From a purely technical point of view I agree with you, however, from a legal point of view, you leave yourself open to a line of questioning in which you will have to admit that according to your calculations, you have exceeded the allowable stress.
That’s what the jury will hear and you will have lost a great deal of credibility with the jury.
I know that when I am designing some structure or another, I typically use conservative but easily applied loads to reduce computation time.
When my results show members exceeding the allowable stress, if I don't want to increase the member size, then I go back in and rework the loads into a more precise configuration.
I am not suggesting you have to increase the member size, I am just saying you should run another iteration with your loads more precisely located so the results do not show any overstressed members.
Of course I don't design to the gnat's eyelash, so I usually have room to get the member stress below the allowable limit.
I just like to keep my calculations clean. During an AISC seminar I atteded, I heard that recommendation given, and so I am just passing it along to all who read this post.
Regards,
JPJ
RE: ASD: 1.03 Allowable Overstress
1) The metal building business relies on competitive design - the amount of steel in our preliminary designs wins or loses jobs. "Bump it up to the next size" costs us profit margin and/or contracts, and improves my chances of looking for a new job. This is not the case for the consulting engineer who bids for a contract based on engineering time and does not shoulder any financial impacts based on the steel used in the design. So the metal building industry goes to great lengths to find a competitive advantage. Voila - our death grip on using the 1.03 ratio.
2) A metal building engineer will not bear any financial burden if a lawsuit occurs - the company will bear it.
How many of you have been involved in the oft-invoked "Lawsuit" that seems to govern design decisions? Would you consider a PE negligent that allows a 1.03 stress ratio on a member where all loads and conditions have been considered properly? Seems that jury behavior is a bigger factor in design than the structural behavior nowadays.
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
I faintly remember a presentation where the origin of the magic 1.03 came up, but for the life of me don't recall the discussion. But it seems to be a line that somebody has drawn in permanent marker because it is a pretty widely used/known value, even outside the metal building industry as evidenced by this thread.
I understand your firm's hard-line approach to the 1.0 ratio, especially in today's litigious atmosphere. My engineering judgment - and surely all of those here would agree - tells me that a member calc'd to a 1.03 stress ratio won't fail if loaded similar to what is designed for. It's a shame though, "Lawsuit" has made engineering judgment a thing of the past it seems.
RE: ASD: 1.03 Allowable Overstress
"The reader is cautioned that independent professional judgment must be exercised when data or recommendations set forth in this Specification are applied. The publication of the material contained herein is not intended as a representation or warranty on the part of the American Institue of Steel Construction, Inc. - or any other person named herein - that this information is suitable for general or particular use, ..."
So the ASD spec tells us specifically that we MUST (not Should or May) use independent professional judgment. Seems an adequate defense for "Lawsuit". They also disavow any statement of the suitability of the material in the spec. So if Engineer designs to a stress ratio of 0.80 and the member fails, Engineer has to defend whether the spec is even correct and why we chose to use it?
RE: ASD: 1.03 Allowable Overstress
When I first got out of school, I occasionally followed the old convention "less than 5% overstressed O.K.", mainly because the other "old school" engineers I worked with followed this convention.
However, when I went to an AISC seminar, one of the speakers stated that this convention is not considered good engineering practice.
One consideration worth considering is someone in the fabrication or construction of a member you've designed, may not do their job correctly. If the member fails, you will have a tough time avoiding blame.
If you're calculations show you're overstressed, especially if there is no way to relieve that overstress by another iteration, I don't see how you can avoid getting splattered by the blame of someone else's mistake.
Remember that the "5% over say O.K." rule came about in a different era when running another iteration was a time consuming process. Also design methods were deliberately conservative.
In these days, it is easy to design members very precisely and run another iteration if needed. Our design methods yield results that accurately model actual conditions. Design codes have shaved most of the fat out of the design process.
I certainly don't think anyone should start out by setting the stress limit in their computer analysis to 103%.
Perhaps the metal building industry needs to reign back their practices, and keep stress below the allowable limit.
Unfortunately, one firm cannot do it alone. It may take a serious lawsuit to cause changes. I for one don't want to be on the wrong end of that suit.
I guess I have to disagree with anyone who thinks it is OK to design to a stress limit over and above the allowable.
Regards to All
JPJ
RE: ASD: 1.03 Allowable Overstress
As a structural engineer, to reject their calculations based on 1.03 seems to me, to be very petty and nit-picking. I worry about things more worthy of a 1.03 stress condition in a prefab metal building.
I can recall plenty of instances of inexperienced engineers in other types of construction inadvertently making errors far exceeding 1.03. If we as, an engineering community, stop debating this "1.03/fear of lawyers" issue, and spend more time mentoring new engineers, lawyers would not have so many "victims". It amazes me that there is so little mentoring happening in our industry. If hospitals treated their medical staff likewise, I would be reluctant to enter any hospital.
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
RE: ASD: 1.03 Allowable Overstress
These upgrades almost always require larger and heavier equipment. If we were to start at 1.03 and have to add something larger in the future we would be looking at 1.1+ real quick.
Also, I can't tell you how many times I've been through an older mill and cross bracing has been removed. You can tell it has been and no one knows when/why. Another area we have concerns about is corrosion. Painted, galvanized, and both. After a certain period of time the steel is becoming heavily corroded and the customer wants to put heavier equipment in.
We do not go above 1.0 and sometimes we are even on the other extreme because our "good engineering judgement" says there will be more cost down the road than saving some structural steel up front. This is built into our lump sum estimate.
I realize and appreciate that pre-fab metal building designer/supplier is working in a different environment than I am.
I am definitely not arguing about economical use of materials and in many applications, including many buildings where current and future loads are relatively well defined, I understand why people use 1.03 and this discussion is being made.
It is just different than the situations I normally encounter. There have been more times than not when I have been very thankful to the engineer in the 1960s or 1970s that didn't cut everything very tight.
RE: ASD: 1.03 Allowable Overstress
On the other hand the material cost is the cheapest part of the construction process. I was taught that no one will ever know if you had 10% too much steel, but everyone will know if you had 1% too little steel.
RE: ASD: 1.03 Allowable Overstress
Overstress for rehab work has been written into codes and is therefore defensible in court. Our model code, BOCA Article 16.14.2, reads "The addition to an existing structure shall not increase the forces in any structural element of the existing structure by more than 5%, unless the increased forces on the element are still in compliance with this code for new structures."
Under construction conditions, the area is generally closed to the public, and the contractor, who will bear the cost of any failure, is responsible for the design.
The 3% rule is nothing more than a way to gain a competitive edge over those who don't follow it, namely, the rest of us who don't design prefabricated steel buildings. There is no justification for it, only rationalization.
RE: ASD: 1.03 Allowable Overstress
I totally disagree. If I can't spec a W24 beam because it is too deep to fit, then a W21 with 3% overstress is perfectly fine. Since I know how to do an accurate tabulation of the loads, etc, my beam design is more accurate and proper than the one-significant digit guy who designs girders with 3 point loads as a uniform load. The Factor of Safety includes allowance for engineering error such as that guy incurs.
RE: ASD: 1.03 Allowable Overstress
As you say, the Code safety factors include allowance for inadvertent errors. However, I just don't see increasing allowable stresses because you don't make a particular error. What if you make a different error? By the way, when I calculate the bending moment for a beam with 3 point loads (uniform spacing at quarter points), I get the same moment as for a uniform load. Am I missing something?
RE: ASD: 1.03 Allowable Overstress
Since the cost of steel is greatly increased, and loads in commercial building construction are accurately and easily computed, I allow 3% over as a routine. When I do a custom wood-framed house, the loads are much more difficult to estimate. In that case, I am very conservative with my beam design.
Try this example:
Girder 30' long. 3 point loads, at 5', 15', 25' from one end.
Point loads due to beams with 40' trib, total floor load = 80 psf. Therefore, point loads are 32 kips each. Girder end reactions = 48 kips. Analyzed as 3 point loads, bending moment in girder = 400 ft-kips. Analyzed as uniform load = 80 psf x 40' trib = 3.2 klf, bending moment = 360 ft-k.
RE: ASD: 1.03 Allowable Overstress
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Hopefully these links work... If they dont, visit www.aisc.org, find modern steel construction and look in the back issues for the pdfs.
Cheers
RE: ASD: 1.03 Allowable Overstress
Hg
Eng-Tips guidelines: FAQ731-376
RE: ASD: 1.03 Allowable Overstress
As for the "arbitrary" 3% rule, I would find use whne checking existing element stresses after remodel such as live load increase, new equipment, etc. In fact, there is no document that allows 3% overstress. As far as I know, it is at the discretion of the engineer. I'm used to allowing 5% overstress...
RE: ASD: 1.03 Allowable Overstress
You're right, the uniform load produces a smaller bending moment for this layout. Hard to imagine anyone would appoximate this solution with a uniform load. On second thought, it's not so hard to imagine.
RE: ASD: 1.03 Allowable Overstress
I don't usually use an over-stress factor in most cases, but when I do, it's about 3-5% or less, and that's because I know my structure... where I'm conservative, my load paths, etc. I won't if I can't justify it.
RE: ASD: 1.03 Allowable Overstress
Another case, engineer significantly (50%) underestimated loads on beams.
Another case, engineer divided by 1.33 (this was checking 16" on center rebar spacing) instead of multiplying by 1.33 for required rebar area.
RE: ASD: 1.03 Allowable Overstress
I thought it interesting that a textbook published for the purpose of teaching engineers would use and apparently approve of this practice. Not that I'm strictly against it and in fact have done it on occasion myself. Sometimes it seems reasonable when the option is to go up another size in steel that would add another 10% to the weight and cost. It depends on the situation in my opinion.
Regards,
-Mike
RE: ASD: 1.03 Allowable Overstress
Back in school our prof suggested that we not permit any calculated overstress at the design stage. This would allow a margin for stuff to happen during the design, fab and erect stages. I always follwed his advise and have never come to regret it. He was absolutely right... stuff happens.
I am also reminded that Abraham negotiated well with God about destroying Sodom. Eventhough the place was doomed, he got it down to just ten just men. Well, I've often wondered if the "3% rule" guys would someday get comfortable with 4%? How about 5%? Seemed like a slippery slope to me.
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: ASD: 1.03 Allowable Overstress
being wed to a number with out feeling you can ask why or look at it in the overall scheme of things defeats some of the purpose of engineering. I have seen a lot of money spent to beef up memers that if they failed, would not threaten the stability of the structure, but it was code. I have also seen nonredundent pimary members subject to dynamic loads that are a guess at best designed up to the very limits of the code, sometimes even allowing some overstress. A part of engineering we forget is to stop and look at how our design fits the big picture in quite contemplation.