×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

ASD: 1.03 Allowable Overstress
4

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.

RE: ASD: 1.03 Allowable Overstress

Never heard of such a thing.  I've used a 10% overstress as reasonable by judgement when checking already installed work where there was an error in the original design or a changed condition.  But I don't think you'll find that (3% overstress) codified anywhere.

RE: ASD: 1.03 Allowable Overstress

Do we really know ANY loads to within 3% accuracy?  In design, why would you want to walk so closely to the edge of the minimum design loads?  Why not just use the next size up?

RE: ASD: 1.03 Allowable Overstress

I was taught to use the three percent rule for checking. in other words if I had a structure that either looked fine and i needed to check it for current code and the current code loads resulted in three percent overstress, i did not necessarily need to reinforce it although I might anyway depending on redundancy. Also if I wqas adding a load to a member and the load was less than three percent of the stength of the member (ASD) I did not worry about doing a full analysis of all of the loads on the member - again this is limited to situations where i had reasonable certainty that the member I was dealing with was not overstressed to start with. My boss who learned structural design in europe and practised in canada and the USA taught me this.

RE: ASD: 1.03 Allowable Overstress

check in aisc-asd volume ii - connctions. it's somewhere there.

RE: ASD: 1.03 Allowable Overstress

For unity checks it's a case of significant figures.  If the check states "less than or equal to 1", then 1.01 doesn't comply.  However, if the check states "less than or equal to 1.0", then 1.049 rounds down to 1.0 and complies while 1.050 rounds up to 1.1 and does not comply.  Therefore you can "accept" up to 5% "overstress".

RE: ASD: 1.03 Allowable Overstress

Not the answer you want to hear, I'm sure, but--

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

I doubt this is actually in a code.  The code is worded to give minimums and requirements, not suggestions.  I don't think it would also say "but if you want to add 3% go ahead".  I believe the 3% is historical in origin and is based on your own judgment and how comfortable you are signing and sealing something that technically violates code.  I've heard of the rule for checking retrofits and stuff like that and used it as well.  Personally for new construction I say just use the next size up.  We all know that our analysis methods and theory aren't accurate to 3% but that isn't necessarily a legal argument.  

RE: ASD: 1.03 Allowable Overstress

At one time I thought I read something about this 3% allowable overstress being applied to bring ASD results more in line with LRFD solutions.  Has been quite a few years and I can't remember where I saw it.  I just thought I would throw it out there as a possible lead for you.

RE: ASD: 1.03 Allowable Overstress

Whenever our firm reviews pre-engineered metal building calculations, it seems like the pre-engineered metal building manufacturers always allow a 3% overstress.  Not sure if it comes from a code they use or not.

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

It has to do with significant figures.  1.03 (and 1.049!) rounds down to 1.0.  None of our calculations are accurate to two decimal places.

RE: ASD: 1.03 Allowable Overstress

Some calculations are more accurate than others. When you have huge steel beam to design, it's your PC - duty to save steel and allow a 3% overstress. When designing a wood beam with a complex, hard to exactly calculate load in a custom home, you would be wise to bump up to the next larger beam size even if your stress or deflection ratio is only 90%. It's all a matter of personal druthers.

RE: ASD: 1.03 Allowable Overstress

Just got back from an AISI spec committee meeting.  The concept of 1.03 allowable came up in passing.  While it is agreed that most everyone does it at one time or another, the committee wouldn't think to codify it in any way.  We've all faced the expert who thinks 1.0001 is the end of the world.

RE: ASD: 1.03 Allowable Overstress

This is an interesting question for those of us who write software and have to decide whether something is acceptable or not. For example the AISC code refers to 0.66Fy = 23.76 for A36 steel, whilst most textbooks use a value of 24. The difference is just over 1% which, as already stated by several posters, is really irrelevant but when it comes to comparing numbers computers do what they are told. If you work to 3 figures Fb is 23.8. So if fb is 23.9 should this be flagged as failing? I would rather show the exact numbers and allow a designer to use his discretion if it 'fails' but on the stand the lawyer is going to say "if they meant to say 0.67 don't you think they would have done so?".

RE: ASD: 1.03 Allowable Overstress

tonyuk - it says in the AISC manual that 24 ksi is common practice, and acceptable. And regarding the 1.03 "say OK", issue, that is common practice, within the usual standards of care. I wouldn't let a lawyer bully me around with his interpretation of common sense.
What kind of software do you write?

RE: ASD: 1.03 Allowable Overstress

We produce several programs - mainly for the UK - that handle simple beam and column design: the USA version info can be found at www.superbeam.com . I think most of us aged 30+ appreciate that any structural calculations will, at best, only be accurate to a few percent if for no other reason than there is some tolerance on member sizing and quibbling over decimal places is ludicrous. But when you write a program, even simple (in engineering terms) ones like ours you have to decide where you draw the line.

RE: ASD: 1.03 Allowable Overstress

ton,
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

2

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

JPJ

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

As a metal building engineer I can confirm that the industry uses the 1.03 allowable stress ratio as the staple of our design diet.  There are probably two underlying factors why this is the case:

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

ChuckerD, just a question. If it is all about being competitive, why not use 1.05? 1.07? Why stop at 1.03?  Or is 1.03 just one of those unwritten industry standards, i.e. "the way we have always done it"?

RE: ASD: 1.03 Allowable Overstress

broekie has a good point in my opinion.  You have to draw the line some where.  Why not 1.04?  That's only 1% over the "allowable" 3% and 1% doesn't matter right?  You have to draw the line some where, again.  The only time I go over is when I know I have been conservative in my estimations.  How often are we that close, who knows?  How many times have we approximated point loads as uniform?  That will give you a similar moment but only for many point loads.  Let's say I am designing a beam supporting 4 joists evenly spaced.  The moment considering the joists as point loads is 20% higher than that used by assuming the loads are uniformly distributed.  Now I started off 20% over my limit though the calcs won't reflect that because of the assumptions made.  This is just an example to illustrate how we make simplifying assumptions that may have already used up any allowance we may have.  It isn't hard to assume your way out of your 3% simply in the model you use without even touching on sig figs and accuracy and so forth.  I think if you take all this into proper account and have everything dead on then 3% isn't going to hurt.  You still have to draw the line somewhere though.  It also depends on what it is that I am overstressing.  I'm not worried much about overstressing a wide flange in flexure by 3%, but wouldn't consider it for, say, tapcons or expansion bolts.  Those never go in right in the first place.

RE: ASD: 1.03 Allowable Overstress

Very good question broekie.  As a new engineer fresh out of school a 1.03 allowable stress ratio is the "rule" that I was taught to be a self-evident truth.  It's definitely an unwritten rule - doesn't appear in the MBMA industry bible (Metal Building Systems Manual) in any fashion.

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

From Preface to Part 5 of the ASD spec (top of Pg 5-13)

"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

I don't even work for prefab building manufacturers, and I agree with their 3% "overstress" tolerance. After all, their procedures and documentation are very reliable, unlike the calculations of someone outside of that field of expertise.
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

Of course, in steel design, failure is not brittle in nature.  If you were designing a glass structure, you might do it differently.  Steel also age hardens, has tolerances on thickness, composition, hardness, ductility, erection, etc.  My goal is to design to 1.0 of the allowable.  If I am 3% over and there is some redundancy in the structure or conservativeness in my assumptions of boundary conditions, then I figure that if push came to shove I could re-calc and get below the magic 1.0 number, for the benefit of some idiot non-engineer who does not believe in "good engineering judgement".  I have to remind myself that designing to 1.0 does not relieve me of the need to still exersize "good engineering judgement".

RE: ASD: 1.03 Allowable Overstress

IFR- I totally agree. There is a recent-grad engineer who makes sure that he doesn't exceed 1.00, however, he is totally unaware that using a uniform load on a girder beam produces much less bending moment than modeling as point loads (girders with 2,3, or 4 equally spaced point loads). I brought this to his attention, and his attitude was, his method is more productive. The funny thing is, he is really an advocate of LRFD, yet doesn't understand basic common sense.

RE: ASD: 1.03 Allowable Overstress

I'm going to weigh in with a different perspective.  The company I work for is an OEM of industrial equipment and we perform EPC projects internationally.  We also get involved in upgrades and modifications of existing facilities where our equipment is installed.

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

I have comments on both sides of the issue. First in construction 10% design overstress is not uncommon. I know guys who go 20%. Second, It really is not an overstress. When Fb=Fy the steel at the outer fiber goes plastic (in theory). The rest of the section can still accept  moment until the entire section has gone plastic. So even at fb=Fy, the section can still take additional moment. Third the definition of Fy is somewhat arbitrary, the yield stress of steel varies through out the cross section and the certified test reports generally show yields consistently higher than the required Fy. Fourth, It depends on what loads are causing the overstress, how well you know the loads, how likely is an overstressing load, the duration of the load, what happens if the member yields. I have spent a lot of time explaining engineering to oposing attorneys. As long as you know what you did and why, you will be okay. I once explained that engineering is a human process and that if all we did was to blindly follow a code, we would not need inteligent trained engineers. The arbitrators seemed to agree.
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

This 3% "rule" should not be confused with building addition work or construction condition work.  

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

jmiec-
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

Sacrebleu,

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

jmiec,
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

Modern Steel Construction had a few articles a while ago about this topic.  One in October 2003 and other in October 2004.  Here are the links:

http://www.aisc.org/MSCTemplate.cfm?Section=Back_Issues1&template=/ContentManagement/ContentDisplay.cfm&ContentID=24932

http://www.aisc.org/MSCTemplate.cfm?Section=Back_Issues1&template=/ContentManagement/ContentDisplay.cfm&ContentID=26812

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

I think that's a different topic.

Hg

Eng-Tips guidelines:  FAQ731-376

RE: ASD: 1.03 Allowable Overstress

HgTX, You are right.  It's been a while since I read this entire thread.

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

SacreBleu,

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

So how many of you have actually had to defend your calculations in court?  On top of that, when did your design fail because of a 3% over-stress and not a bad connection, or construction fault, etc.?  In defense of a little approximation for sig. fig.s and conservative uncertainties, who can easily determine when something fails in the field because it was load at 103 k, when it was designed to carry only 100 k?  

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

The only instances I have observed are gross mistakes such as neglecting to check punching shear in concrete flat slabs, etc.
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 was just looking for something in my "Steel Structures Design and Behavior" text by Salmon and Johnson (1971) and found an example problem allowing a 1.03 overstress (pg 657.)  There was no comment other than the fact that 1.03 is approximately equal to 1.  In another example they went to the next size when stress was slightly over 1.03.  

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

Wow this thread has been goning on and on hasn't it?

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

I do a lot of design for ostruction which involves both structural & geotechnical engineering. Much of the geotechnical work does not have specific codes, so I  guess I have a little more flexibility in my outlook.
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

White Paper – Data Security and Know-How Protection
Our data is constantly exposed to the danger of being intercepted or stolen as it wends its way over global data networks. Data security measures and measures for protecting intellectual property should not, however, first be implemented when data is exchanged – companies must lay the foundation for these measures within their own organization. Download Now
White Paper – Collaboration in the PLM Context
The influence exerted by the Internet of Things (IoT) means that there is a steadily growing need for collaboration in industry. Partners from new industries and areas of application need to be integrated in cross-company business processes to ensure that the lifecycle of smart, connected products can be managed from end to end. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close