Eng. practice of allowing 5% overstress
Eng. practice of allowing 5% overstress
(OP)
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?
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?






RE: Eng. practice of allowing 5% overstress
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Overstress by definition violates code, hence I can't imagine you'll find a code reference deeming it acceptable practice.
RE: Eng. practice of allowing 5% overstress
I have never found it codified (someone correct me if I'm wrong). I doubt it ever will be codified, due to the "where do you draw the line" counter-argument.
In your letter, I'd vote for the significant figures argument as to code intent. Often the unity stress equations are given as <= 1.0 1.049 rounds down to 1.0 with 2 significant figures, and 2 sig. fig's is the code language (e.g. not 1) I might also write something about what a "calculated/predicted overstress" ACTUALLY means (to the lay person), with info about load approximations, safety factors and this minor relative encroachment, ductility, "failure" modes, yielding, and NOT "detrimental structural collapse jeopardizing public safety", ... stuff like that. It's true a lawyer might challenge a 3% overstress in court, but your lawyer could use the above as an industry-accepted defensible position.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Mike McCann
MMC Engineering
RE: Eng. practice of allowing 5% overstress
When being challenged in court,I heard an expert wittness refer to various textbooks that allowed an overstress of 5% in their examples which was used as evidence of "normal standard of care".
RE: Eng. practice of allowing 5% overstress
If this is a continuous beam in bending, then you may be able to claim the 5% under load redistribution.
RE: Eng. practice of allowing 5% overstress
I think you're right that a lawyer would have a field day. However, I read an article in the last year relating to liability in design, and it mentioned that the standard is something like "care that another reasonable engineer would take". Obviously, I'm paraphrasing.
There are many reasons for arguing that a 5% over stress is acceptable. Every engineer I know tends to be conservative in their dead load calculations. As abusemntpark mentioned, we frequently assume pinned when almost any connection has the capacity to transfer some moment. A structure is designed for extreme events. The loads at these extreme events (wind and earthquake, specifically) are not really known and are very conservative. You could use exposure C in your wind when in reality you have exposure B. Most materials have well in excess of 5% more yield strength than specified.
At the end of the day, I think allowing a 5% increase in stress would fall under "engineering judgment", which is why we get paid the "big bucks" :)
Having said all that, I too fall on the side of designing for 5% less stress than allowable.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
This whole idea goes against the grain with me. I wonder, though, had those same equations been written to show "<1" instead of "<1.0", would everyone be figuring that a 49.999% overstress was okay? That just seems an unreasonable way to interpret a code unless there is something that specifically says to do it this way (which there may be- I think plate thickness tolerances are done this way per ASTM A6). But I note that in the Numerical Values tables in AISC, that they do NOT follow this significant digit usage- 0.60 Fy is always shown to three significant digits, not two. Even in the older book that showed 24 ksi, it is shown as 24.0 ksi, not 24 ksi.
I also find it odd that if there was a requirement that Quantity X be > 1.0, then everyone would agree that 1.049 was definitely greater than 1.0. If the code intent is to round it off, it ought to be done all through, not just where convenient.
RE: Eng. practice of allowing 5% overstress
No one has mentioned the statistical origin of design loads prescribed by Codes. Or the idea that Code compliance alone is a valid criteria in judging the adequacy of a design.
RE: Eng. practice of allowing 5% overstress
In most all my new designs I don't ever think of using a 5% overstress because my unity is usually between 0.7 and 0.9.
For existing conditions where I'm asked to check a beam or steel joist or wood joist, etc. I do allow for a 5% overstress simply because I know and understand the reliability methods that went into setting the inherent safety factors and given a structure that is "older" I have more confidence in its design and compentence.
Also, the 5% overstress is usually confined to a discreet area within the span, not across the entire member so the overstress doesn't present a comprehensive violation of the "required" level of safety.
The concept of overstress should also be remembered as a diminishment of the required level of safety factor and not a condition where the loads are 5% beyond a theoretical failure limit.
RE: Eng. practice of allowing 5% overstress
you make a good point. I must admit that on occassion I have used the rounding of numbers to make things work.
I believe that the code prescribed safety factors allow for this.
I do think, however that stating that you accept 5% overstress is a red flag for lawyers if anything goes wrong.
frv,
You are correct in a way, you are compared to what your peers would do in the same situation, but only when it exceeds the code requirements. Once it is less safe than the code, this does not apply. As I said, a code violation is a free kick for the other side, but if it meets the code and there still is a problem then the other side needs to prove that a reasonable engineer would have done differently in the same situation.
RE: Eng. practice of allowing 5% overstress
My opinion--it is ridiculous to think a structure is dangerous simply because something is found to be 5 percent overstressed. hokie66 hit it on the head--structures fail due to gross miscalculation or gross misunderstanding. And all the others who mentioned how conservative we are in our assumptions are correct as well. I think a fair amount of overstress--even more than 5%--can be justified by relaxing our conservative assumptions.
DaveAtkins
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
However, that being said, as we have an obvious disagreement here among competent and experienced practicing engineers in the use of the 5% overstress, it is blatantly obvious to me that it is not a uniformly recognized "standard of care", and, thus, subject to legal scrutiny.
To each his own...
Mike McCann
MMC Engineering
RE: Eng. practice of allowing 5% overstress
It is ludicrous to believe we know the true state of stress in any member, mush less to believe that we know that a member will be "dangerous" because it is 5% over stressed based on our assumed loads. Again, these loads are almost always conservative (even live loads). If I have a member 5% "overstressed" in design, I would sleep very well at night knowing that my assumptions have been conservative.
Think about it. Your typical metal roof deck load is usually taken as 20PSF. In reality, it is closer to 15PSF (including MEP). A 5% increase in dead load is 1 PSF!. Same applies to Code LL for roof loads. If any building fails because you failed to take into account 2 PSF, there is something fundamentally wrong with your design.
RE: Eng. practice of allowing 5% overstress
Of course I agree that the aim is to try to stay out of court. Sometimes thats not within our control and out of the blue so to speak you get a writ in the mail to appear in a court of law.
A building will not fail by being 5% overstressed, but just imagine being cross examined under oath by a QC (Queens Council) who finds out that technically you the responsible designer have deliberately and knowingly overstressed a member and gone against the Code of practice and therefore against the building act. Believe me its a minefield for a Barrister at law.
They use everything and anything to win the case. I would keep everything strictly within the code of practice.
RE: Eng. practice of allowing 5% overstress
Take the recent bridge collapse as an example.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Exactly!
Didn't they find out that a gusset plate (a critical component in a statically determinate bridge) was half the size it was supposed to be? And it still took thirty-some odd years for the bridge to collapse! And remember that for years the bridge was also seeing much higher traffic loads than the original designers had anticipated.
And people are worried about 5%?
RE: Eng. practice of allowing 5% overstress
I have been in a 6 month court case (not engineering related) and it was pure hell.
RE: Eng. practice of allowing 5% overstress
It seems odd to me to say, on the one hand, that this 5% is so trivial that it will never make a difference. And on the other hand, to say that it is so vitally important that we must do it to stay in business. It can't very well be both.
It seems to me that most of the arguments that you could make for allowing a 3% overstress across-the-board could also be used to justify a 5%, 10%, 15% or 25% overstress as well- sort of a slippery slope once you get on it.
RE: Eng. practice of allowing 5% overstress
There is a difference between 5 and 25%. Where you draw the line is a matter of engineering judgment. I feel like I could defend myself in court for allowing a 5% over-stress. Don't know about 25%.
RE: Eng. practice of allowing 5% overstress
You could use a similar argument for concrete, as concrete strengths will always be higher.
For other materials, I think most allowable stregth/stress values have been determined by testing, and safety factors are applied to the values given, so a small overstress is ok.
I typically limit it to 3%, but 5% would be an engnineering judgement.
RE: Eng. practice of allowing 5% overstress
Where you should draw the line is specified by the code!
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Nearly every professor I've had (in engineering anyway) is directly involved in code writing and/or research relating to the codes.
Invariably, they all stressed using our judgment when it came to things like this. If you understand the statistical nature of loads and resistances, you understand the magnitude of 5% (please see my post on DL and LL for metal roof deck), understand that materials will have a significantly higher resistance than specified (much more than 5%), that your assumptions are conservative to begin with, then you can justify, IF YOU SO FEEL INCLINED, a 5% increase in stress without having to go back and revise your calcs. If ever it came back to court, you could then annotate your calcs to prove that whatever went wrong, it was not your fault.
So no, my judgment doesn't supersede that of the code writers. It understands the intent of the code writers.
And no, I'm not a better man/woman than you.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
I've been away from school for so long, I haven't spoken to any code writers for years. It what you write is the general thinking, then it should be easy to get someone to write it down. If I were the original poster, I would simply write to AISC or ACI and request such a statement.
civilperson-
We're required to submit our calculations with the final plans.
RE: Eng. practice of allowing 5% overstress
This instance involves analysis of existing telecommunication towers regarding their capability of supporting additional antennas. I have a number of examples of other engineers using a 105% maximum.
I am going to decline providing them with a signed and sealed letter, as this question does not involve a project of mine. I will likely give them a response by email and incorporate some of your comments.
Thanks for the help.
RE: Eng. practice of allowing 5% overstress
synopsis - Code allows a 5% overstress for existing structures. For new construction, this is a no-no except for as-built pile cap designs.
RE: Eng. practice of allowing 5% overstress
The IBC has decreased the LL in garages 20% (50 psf to 40 psf) since the 2000 edition but it is considering going back up to 50 psf. Does this mean they are going to close all of the garages that were built in this time frame? No way owners and lawyers would allow that.
Does that mean that the garages will be 25% overstressed? Probably not.
There is so much "play" in the loads and actual stresses that who knows what the actual stress in the member is and if it actually 5% overstressed. That being said, to design per the code you have to keep the ratio less than 1.0.
RE: Eng. practice of allowing 5% overstress
synopsis - Code allows a 5% overstress for existing structures. For new construction, this is a no-no except for as-built pile cap designs."
No, I don't believe this can be interpreted this way at all. The Code states that "...additions or alterations to an existing structure shall not increase the force in any structural element by more than 5%". This is not addressing overstress, but forces on structural elements. Unless your members were designed to 100% of their allowable stresses this does not apply to my intial overstress question.
RE: Eng. practice of allowing 5% overstress
Who mandates the calculations are submitted with the plans? If I state OK by inspection or model with a simplified analysis, (i.e. simple hinge connection with shear tab between beam and column or pinned base plate when four anchor bolts exist), who is the naysayer to my judgement? If a PE is hired to check the design/calculations, does the rounding and arithmentic become the focus or an independent method used producing similar member sizes and connection reactions? How many load combinations are modeled or can just the critical ones suffice? Are serviceability criteria checked as to vibration, deflection, drift limits? Probably not and definitely not if not included in calculation package.
RE: Eng. practice of allowing 5% overstress
Nothing that went into that calculation to begin with is known to that close a degree. The stresses aren't that well known (other than dead loads, they're not even calculated but standardized), the various load factors are committee compromise based more on "feel" than anything else, the material properties themselves are based on minimum requirements rather than what is likely to exist in the actual material, etc. It's all, to steal a phrase from somewhere else, a wild guess carried out to two decimal places.
Hg
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RE: Eng. practice of allowing 5% overstress
If the code was suggesting that it was OK to add a 5% load to an existing member that is currently at 95% of its allowable capacity then this section is unnecessary.
Answer the following questions:
1. If an existing member is loaded to only 50% of its allowable is this section saying that you can only add 5%?
2. If this section does not give you the ability, per code, to go to 105% of allowable, what is this section saying?
As for going the final step, you pointed to it in your response. If the original structure was designed to 100% capacity, then this section says that, per the code, you can go to 105%.
I understand that doubling the loads on an existing structure so that the structure is at 105% overstress is not exactly the correct interpretation but a structure (such as a transmission tower) is usually pretty close to 100% utilized before adding todays cell phone receivers.
RE: Eng. practice of allowing 5% overstress
That's a great line.. Hadn't heard it before, but it is perfectly applicable.
I had had to contain my laughter when someone on this thread earlier stated that allowing a 5% over stress is "bad engineering". I think we sometimes forget our own job description and can't see the forest for the trees.
If I have a beam with 100PSF live load being over stressed by 5%, I'll sleep soundly at night.
RE: Eng. practice of allowing 5% overstress
I was about to point out the same logical inconsistency in radair's post. I was even going to use a similar example.
RE: Eng. practice of allowing 5% overstress
ConnDOT and some municipalities.
RE: Eng. practice of allowing 5% overstress
Should the increase exceed 5%, calculations would be required showing the structural members meet Code requirements for new structures.
As I understand it, the purpose of this provision is to allow minor loads to be added to existing structures which may have been built to previous revisions of the building code (i.e. less stringent versions). Without this provision, older buildings would have to be retrofitted to current standards.
Again, this rule would allow 5% overstress only if members were designed to 100% of their capacity, however I don't believe that is the intent. Your view may differ.
RE: Eng. practice of allowing 5% overstress
I don't agree with starting out with the intention of designing every component with 5% overload, as JStephen said his former employer did with footing bearing pressures, but what is wrong with an acceptance criteria with some tolerance when an experienced engineer judges that the component in question is satisfactory?
RE: Eng. practice of allowing 5% overstress
As engineers we must work very hard to not be arrogant and to work within the limits that the public have set us. If we have to sharpen our pencils to get the loads under 100% allowable, then we need to do that simply because we have agreed to.
An experienced engineer has the option of finding a different load path to carry the extra load. He does not have the option of changing the agreed upon building code criteria.
Remember Babel
RE: Eng. practice of allowing 5% overstress
Acceptable fabrication and construction tolerances are specified in the contract documents. Industry Standard tolerances are written into published documents, such as ACI 117. Soon as a code committee writes a standard design tolerance, I'll follow it.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Its PT though so it still has a load path of sorts.
RE: Eng. practice of allowing 5% overstress
Star for that one!
Indeed, a picture is worth 1000 words.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
However, I was somewhat alarmed when I read the argument that using conservative loading estimates allows for exceeding limits. I see the acceptable stress as just that, a limit. If you are going to exceed that, you should not say that it was the limit. You need to set a limit and justify it, not set the limit and then say, "Well, we are only a little past acceptable, so close enough." It is acceptable or it is not. No gray area. Additionally, if your saying that conservative loading estimates allow exceeding the limit, then why use the conservative estimate? Why not refine your loading calculation and reduce the design load?
If I was having a second story balcony put on my house, I'd much rather hear, "Originally we were going to rate it so it could hold a heavy-duty truck, but based on design we can only rate it for a mid-size car." than having the engineer tell me it is rated for a heavy-duty truck, but is designed at over-stressed values.
I guess I just feel that a limit is a limit. If you don't like the limit, justify changing it (a little tricky if it is specified by code). If you don't agree with the calculations that caused the limit to be exceeded, then change the calculations and justify them. I completely disagree with calculating values and then disregarding them or diminishing their significance.
I mean no offence to anybody. These are just my opinions. As I said, I am not a structural engineer. I am sure many who have designed for 5% overstess are much more qualified than I. I'm also sure that if I were a prosecuting attorney that many of you would put me to shame in court when you justified your calculations. Just wanted to add my two cents...
-- MechEng2005
RE: Eng. practice of allowing 5% overstress
The issue is that the difference isn't between a full-sized SUV ad a mid-size sedan.
It's that we designed the balcony for a full sized SUV, we actually have a mid-sized car and someone asks, is it OK if we fill it with gas?
RE: Eng. practice of allowing 5% overstress
In all of these, there is an element of uncertainty and usually for we structural engineers, uncertainty tends to send us to higher levels of conservatism rather than lower.
However, we are also professional who understand structural behavior and how dead loads and code prescribed live loads are developed. We aren't (and shouldn't be) blind slaves to code provisions.
For new construction - forget about any 5% overstress. It usually isn't justified or needed.
For existing circumstances there is nothing wrong with initially and conservatively estimating loads, finding out it isn't going to work, refining the assumptions to a tighter (more time consuming) estimate, and re-calculating.
I have zero qualms about facing a lawyer and justifying my calculation where I write "Unity = 1.03 - call OK". There is nothing unsafe about this since I know the history of code provisions and understand that even the base forumlae for strength has uncertainties greater than 5%.
If you read anything about reliability concepts you know this. Concrete beam shear capacities, for example (ΦVc = Φ2(sqrt(f'c)bd is set off of a very scattered set of lab results with highs and lows beyond the 5% either way.
MechEng2004 - I understand you saying that the idea of first calculating something and then "disregarding it" is wrong. But I'd counter that many initial calculations are purposely conservative to help speed up the design. After struggling with a result, it is common to go back and refine the numbers to tighter estimates to get something to work.
RE: Eng. practice of allowing 5% overstress
We will have to agree to disagree on this one. I fail to see the distinction between applying your "Unity=1.03-call OK" to new or existing structures. But I will give you a star for arguing both sides.
RE: Eng. practice of allowing 5% overstress
The distinction between new and existing is that with new, we aren't usually compelled to "stretch" our design a bit since we have command of most design outcomes - simply size the member for what you need.
For existing conditions, we are given a fixed structure with presumed applied loads. Under this situation, we sometimes get faced with a structure that doesn't work under our initial assumptions.
My argument then is that when faced with this constraint, we should keep in mind that we do have some wiggle room in that:
1. The theoretical capacity of a structural element is based upon formulae with variable accuracy.
2. Our assumed dead loads are sometimes a bit on the conservative side.
3. The live loads are also global code-mandated loads that in reality do not normally get up to the supposed amounts (i.e. office live loads are typically in reality about 10 to 20 psf vs. the 50 psf mandated).
4. The safety factors were not developed as some kind of hard limit. For example, a 1.6 factor on loads does not presume to suggest that at a unity of 1.61 means it will collapse. They are based on reliabilty methods that tried to match load factors and resistance factors combined with the current ASD allowable stress levels of safety.
Finally - look at the relative designs between using ASD and LRFD and we see that doing the design under each of these results in different member sizes. Sometimes these differences are more than 5%.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Transmission towers (I'm thinking cellphone towers, here) are unique structures;
1. The antennas are frequently changed.
2. There is a Design Standard that specifically addresses these (TIA/EIA-222).
In my mind, when you have a Specific Design Standard for a type of construction, that standard overrides the generic Building Code. We wouldn't want builders of Nuclear Generating stations to be following IBC. Article 15 and Appendix F of TIA/EIA-222 covers criteria for the analysis of existing structures. There is nothing in Appendix F that allows (or disallows) an overstress. Because antennas are changed so frequently on these towers, I believe an overstress allowance would be mentioned in Appendix F, if it were applicable. I believe it is standard in the tower industry to recheck a tower each time an antenna group is added, with no allowable overstress. After all, many separate additions could add up.
RE: Eng. practice of allowing 5% overstress
In the example, if the load of the car and fuel were less than the SUV that it was designed for, then this wouldn't cause over-stress. If the structure was designed to hold exactly the weight of the car and somebody asked, "can we put gas in it?" in my opinion the answer is no.
JAE -
I understand the using conservative estimates for a quick calculation. However, I feel that if it is worth enough to not have to increase the size of a beam or somehow create a more heavy-duty design, then the extra time spent on a calculation to re-evalaluate the calculations is the cost of that. If it is a minor change in load, it shouldn't be hard to "tweak" the calculation to get the result you desire.
RE: Eng. practice of allowing 5% overstress
I suppose one could bury that 1% (or 5% or whatever) by playing games with rounding and significant figures along the way. But the only reason to do that would be to make the lawyers happy, not for any real structural reason. This is NOT a safety issue. It's not even an ethical issue. It's purely a legal issue. (Moral<>ethical<>legal, though there's overlap.)
Hg
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RE: Eng. practice of allowing 5% overstress
I've attempted (and failed) to explain to our piers that we are not simple number crunchers.
The reason we go to school and the reason they teach us the theory of structural and material behavior is to give us the tools with which to exercise judgment. Our classes are intended to teach us why code provisions are written the way they are. This in and of itself arms us with the knowledge to determine when and why exceptions can be made.
If the idea is to rigorously follow every code provision without the possibility of using judgment, then I'm sure a technical Associates degree would suffice to practice engineering.
Read the disclaimers at the beginning of all the codes; invariably they state something to the effect that the information is though to be suitable, but that it should not be used without "competent professional examination and verification of its accuracy, suitability, and applicability..." (from AISC). In other words, USE YOUR JUDGMENT
RE: Eng. practice of allowing 5% overstress
It is true that if there is loading greater than anticipated on a structure that the supporting elements are overstressed. However, it is not true that the supporting elements will be stressed to failure. Probabilistically, the overlapping regions between the statistical capacity and the statistical loading (assuming normal distribution for both) is typically outside of 5 standard deviations. This indicates a confidence interval for the design on the order of 99.9999994%. In other words the design load would need to be significantly greater than considered, such as replacing that SUV with 3 or 4 SUVs, or the element would need to be critically weakened or crippled, in order to cause failure. Because of this it is conservative to allow loading greater than 5% of capacity.
Jared Stewart
RE: Eng. practice of allowing 5% overstress
"And I think it is spurious to argue that old designs are more conservative than new ones because the codes have changed"
I have to take issue with this statement in one particular, major, and far reaching case here. I am running into a lot of problems in the foundation loads on remodels where the allowable soil pressure without a soils test was 2000 psf. It is now 1500 psf without a soils test. In this case, based on the new IBC, the old UBC value is NOT conservative, but a real problem for extensive remodels, particularly where a flat, stick framed roof is changed to a trussed roof.
I don't think that you can make that blanket statement JAE. I'm sure there are other code examples too.
Mike McCann
MMC Engineering
RE: Eng. practice of allowing 5% overstress
I don't think that's so much of a code issue as it is a "standard of care" issue.
I've always required a soils report. This touches precisely on what many of us have been arguing, as this is a perfect example of when you should be hesitant to allow "over stress". In this case you DO NOT have the information required to make an intelligent decision.
Quite honestly, if I were asked to design something without a soils report, I would be more conservative than the IBC in the design of the foundation system.
RE: Eng. practice of allowing 5% overstress
The notion to "allow" up to 5% overstress is an incorrect one. Final design, even with 5.1% or 10%, may satisfy the performance requirement without failure. The safety of the end user depends on the expertise of the engineer making the judgment, not whether a member or a connection is numerically determined to be overstressed.
Though calculations are typically not a part of the contract document, any indication of accepting member that is overstressed can be used against you as you have no control over how many "copies" of calculations are floating around. To be on the safe side, legally, avoid ANY overstress in the final calculations that leave your office. If this can not be avoided, where acceptable amount of overstress is shown on the calculations, state the reason for accepting the overstress without increase in size.
RE: Eng. practice of allowing 5% overstress
Well, I didn't provide that quote - hokie did.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
good answer and I agree with almost everything - except - I require (and my clients generally do also) calculations be submitted for all projects. Design requirements should be stated on the calcs. Anything exceeding the requirements is not allowed and I check it myself. Any overstress would need to be thoroughly justified before the plans are considered final. Even though I am not sealing the structural calcs, I am sealing the plans for which the structural subconsultant has prepared those calcs.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Don Phillips
http://worthingtonengineering.com
RE: Eng. practice of allowing 5% overstress
To those that give the "slippery slope" argument, that's hogwash. Engineering judgement will tell you that a tiny bit is OK by inspection without even looking at it - anything more requires closer inspection. There is no slippery slope - the wall of engineering judgement is very near the top of the slope to stop you from falling further. Those that don't have a wall, or if the wall is too far down simply don't have good engineering judgement and those folks stand to get burned from it...
If you "heard" it on the internet, it's guilty until proven innocent. - DCS
RE: Eng. practice of allowing 5% overstress
It's not clear to me what you meant by Does that mean that you design to a 5% "overstress"?
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
They are notorious for using any and every code "loophole" to get loads down, even if the intent of the code is not there.
This is an example of where you know they are not using conservative load estimates, they probably always use exposure B for wind, the connections are done as cheaply as possible, nearly every member is built-up with no "fat".
I think this is a perfect example of not exercising engineering judgment in the other direction. I'm willing to bet that their buildings do, technically, meet code (i.e. they are not overstressed); however, every time there is a big wind storm one or two of those things fall down.
RE: Eng. practice of allowing 5% overstress
Yea... you're right. I guess I was 180 degrees out of phase with the conversation. Cheers...
Mike McCann
MMC Engineering
RE: Eng. practice of allowing 5% overstress
I understand that "overstessed" does not necessarily mean failure. However, the values calculated give a certain confidence interval. You stated this is very high. My issue is that there is a reason for the high confidence interval. I don't think you can ever completely prevent failure. Even if you design with a factor of safety of 200, maybe an ice age comes along and a glacier sits on the house. It will most likely fail. The point is that the confidence interval is supposed to be very high, and I question when it is ok to reduce it. What is an acceptable failure rate? 1 in a million? 1 in a billion? By using the calculated values as a limit, you have a well defined value. I agree that engineering judgement and is important. I also agree with Swearingen that the "slippery slope" argument is, to some degree "hogwash" for the reasons he stated. However, engineering judgement leaves a gray area in what the limit is, and I would much rather be safe and have a concrete value. I completely understand that this is more a personal preference than something that is preventing failure.
Swearingen-
In the case of repeated members, I agree that an overstress in a single member can be accepted because multiple members will support the load. However, I think that to completely justify that assumption there should be a calculation. Perhaps the max stress of the members was determined to be 15, when it has been determined that actual values that beam will allow a stress in the range of 14.9-20. It is very likely that repeated memebers with an stress of 16 (overstressed) will be less likely to fail than a single member with a stress of 15 ("acceptable"). However, I feel that this method should be justified. Experience is certainly an option as a way to justify it, if the engineer is very confident that it will not be a problem. However, I think that when the overstess is calculated, it should at the least be noted why it was determined acceptable. In an ideal world, the load would be calculated and statistical analysis of the beams could be performed to show that the many overstessed beams has a greater confidence interval than a single "acceptable" beam. I understand this isn't always possible. Engineering judgement does have a role.
I guess my comments about not accepting overstress were more based on an ideal world, in which every possible item can be calculated (with statistical confidence values where appropriate). I understand that this isn't possible or reasonable for all cases. However, I do think that a reasonable effort should be made to shoot for the ideal case when possible and reasonable. This is where engineering judgement comes into play, What needs to be calculated, what needs to be estimated, and what can be shrugged off as acceptable by experience without even needing anything written down.
-MechEng2005
RE: Eng. practice of allowing 5% overstress
Building design and consumer product design are different in many aspects. One is that each building is considered unique (even identical buildings side by side) whereas consumer products are not. The other is that manufacturers set the acceptable failure rate for their product (driven by finance) whereas in building design, the building codes establish factors (determined based on probability of occurance, variability in material properties, system's ability to dissipate energy, etc) to prevent failure.
RE: Eng. practice of allowing 5% overstress
The problem is that the code was written by men, not some higher authority. The problem is that the code is only good for ideal cases. The problem is that there is nothing ideal in or about our world. Just as you said a glacier !surprise! could come out of nowhere and squish that house. The solution is laying your foundation on the code but building out with knowledge. I may not have a lot of experience, I only just graduated, but I did learn a lot and I can tell you that the code is not just conservative. It is very conservative. I think it is too conservative in fact, however, because the code takes for granted a few things, like loads, sizes, connection fixities etc, it is a good thing that the code is so conservative. However, because the code is so conservative it is very safe to design inside 5% overburden provided that the design is not too wild.
For really wild things like this
h
the code is of really limited use. It is in these applications where I would base my design off a yield theory, and not simply what is stated in the code, where I would be wary of using a 5% overburden. However, this level of design is not required for things that are in the code and fortunately I am comfortable with allowing to slip a little past what the code deems as safe.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
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
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
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 !
RE: Eng. practice of allowing 5% overstress
Wooten perhaps?
RE: Eng. practice of allowing 5% overstress
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...
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Excellent post YS, summarises the situation beautifully.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
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.
RE: Eng. practice of allowing 5% overstress
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...
RE: Eng. practice of allowing 5% overstress
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...
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
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.
RE: Eng. practice of allowing 5% overstress
Unfortunately, and especially for those who are afraid of lawyers, everyone is not as eloquent as Mr. Wooten.
RE: Eng. practice of allowing 5% overstress
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!
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Anyway amazing how popular this topic is and how touchy legal issue are.
Cheers!
RE: Eng. practice of allowing 5% overstress
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.
RE: Eng. practice of allowing 5% overstress
Are you refering to the article posted by Tomfh above?
If its thats good I'll download it !
Cheers!
RE: Eng. practice of allowing 5% overstress
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! :)
RE: Eng. practice of allowing 5% overstress
It's very well written, but I'll reserve that adjective for other forms of literature.
RE: Eng. practice of allowing 5% overstress
I have spent most of my career designing bridges. Before you decide 5% overstress is OK, go out and look at the condition of some of the structures in service near the end of their design life. The corrosion loss, damaged members, frozen expansion bearings, ... might shock you. On the other hand, I believe I heard early in my career that any structure that doesn't fail in the first five years of service is considered an adequate design. I wish I had asked for a citation on that assertion back then.
I still fear the consequences of the slippery slope. One guy decides 105% is OK. Another fellow later decides 110% is OK. Another fellow thinks he will let 112% go on a member. Where does it end?
I have enjoyed reading this discussion. Thanks.
RE: Eng. practice of allowing 5% overstress
I think Ive learnt alot from this post.
Cheers!
RE: Eng. practice of allowing 5% overstress
It looks to me like there are three separate questions that get combined, although they don't have the same answers.
1) Will it fail?
2) If it somehow failed, would I be considered negligent in a court of law?
3) Does it meet the code?
Now, it seems that everyone agrees pretty much on #1. That a 3% or 5% increase, by itself, with current codes, isn't going to cause a failure. And this seems to be the point that the Wooten article is getting at.
Question #2 seems murkier, with conflicting opinions given above.
Question #3 seems to me to be the sticker point, though. When you have a set of drawings that purports to meet a particular code, and seal those drawings, are you certifying ONLY that the item will not fail?
Let me give you an example from my field. In pressure vessel codes, you might have an equation given for wall thickness along the lines of t=PR/S, with P, R, and S being pressure, radius, and allowable stress. But the exact equation to be used will be specified. And somewhere in that same code, you'll find tolerances for allowable plate underrun. So when you calculate the thickness as 0.52", can you use 1/2" plate? Generally, not.
Now, if you look at the failure of that 0.52" plate, you'll find that they recently changed the code safety factor from 4 to 3.5, and that other codes use different safety factors where perhaps that same plate could be 3/8" plate. So the issue of it failing for lack of that 0.02" is pretty much a non-issue. But the question of whether you've met the code still remains.
Switch from this to say, a building, where you have very involved loads and analysis, and could approach it quite a few different ways. Suppose you calculate the beam stress as 25,000 psi and the code calls out 23,760 psi maximum. Can you say you met the code requirement simply because the problem got more complicated? Or that you meet the code because the loads are more approximate? Or that you meet the code because the code failed to specify a tolerance, or specified it in an ambiguous manner? Maybe you suppose that you could go back and re-work your loads and analysis to make the numbers work, but you haven't actually done so, either. What then?
RE: Eng. practice of allowing 5% overstress
Why risk your license in order to save a few pounds of materials. I really can not believe this is being debated by so many professionals. Design to fail should not be an option. If you want a reason why you should not design to a stress level of 1 just look at how many Pre Engineered Metal Buildings fail every time it snows or the wind blows hard.
RE: Eng. practice of allowing 5% overstress
The term 'over-stress' is also misleading. Using a limit of 1.05 reduces the factor of safety written into the codes, as noted by JStephen. It does not lead to over-stress of the member.
However, I would very rarely design at or beyond member capacity. In the industrial field no one will thank me for saving just a few dollars at the risk of serviceability issues. Having the robustness to cope of some corrosion, an unforeseen vehicle impact or additional loading is part of good design in my book.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
"Designed to fail" has got to be one of the most idiotic statements on this thread.
There is certainly room for disagreement, but your statement is disingenuous at best, or negligently uninformed at worst.
RE: Eng. practice of allowing 5% overstress
We must, above all other things, be practical. In my view you are little better than an "Applied Scientist" with arguments like that...
To purposefully paraphrase a famous quote, I think an apt saying for this situation might be "Engineering without art is science". Personally my degree and job titles have all contain the word "Engineering"; Not one contains "Science", nor would I ever want them to...
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
I think BRGENG was just imitating the Lawyer cross examining the engineer in a legal suite. Believe me they could very well ask the question in that way. Remember the Jury members are not engineers nor is the Judge.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
It does seem to be that way in Canada, despite our much vaunted standardized engineering education system.
If I have insulted you at all, I appologize profusely. I really hold you in great estime; Please understand this is my prejudice, and not my honest view.
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Eng. practice of allowing 5% overstress
I know no offence was intended, and none was taken. It is just that in the US, a lot of the major universities call their engineering degrees Bachelor of Science in Engineering, etc. I've always thought it a bit strange myself.
RE: Eng. practice of allowing 5% overstress
I've heard that saying as "Engineering without art is calculating, Art without engineering is dreaming"
RE: Eng. practice of allowing 5% overstress
(I just thought it bore repeating.)
Hg
Eng-Tips policies: FAQ731-376: Eng-Tips.com Forum Policies
RE: Eng. practice of allowing 5% overstress
Joe Tank
RE: Eng. practice of allowing 5% overstress
We do however use 1.05 for existing structures when required.
RE: Eng. practice of allowing 5% overstress
I completely understand that the code is very conservative. However, it is written that way for a reason. I question who/when gets to decide when it is acceptable to reduce the "factor of safety" (in quotes since I think it has been agreed that a structural member at 1.05 is not necessarily less safe than a member at 0.95, depending on the case). I question why a code written by engineers should be able to be violated based on engineering judgement. Isn't it then a case of the judgement of the engineers writing the code versus the engineer designing a specific structure? I believe that if something is designed "to code," it should meet ALL the code requirements. The amount of engineering judgement used should be specified by the code. If the code recommends a range of values (i.e. safety factor of 3-4) and the design engineer accepts the lower end of the range, fine. However, if the code says greater than 4, then I don't think "3" is an acceptable option.
I'm not a structural engineer and am not familiar with the code. I do not doubt that they are over-conservative and a hassle. I just disagree with the "theory" of exceeding code limits. I do not have the experience and knowledge to determine the implications (and thus if acceptable) in practice. I understand most of those posting here are practicing this type of work, and therefore have more credability on this topic than I do. However, I hope that my opinion is considered and can provide some insight into an opposing view point. I'm just saying take it for what it's worth (which may be nothing to some reading this, and that's fine by me).
JStephen-
Couldn't agree more! Star for you!
-- MechEng2005
RE: Eng. practice of allowing 5% overstress
Perhaps this discussion gives the impression that 1.05 is the basis for design. I don't think that is what I and others are arguing.
Quite frankly, most of my designs are well below .95. What we are arguing is that for any design, if a few members come out at 1.05, there is really nothing to be concerned about and it is perfectly acceptable to allow it if you know (and why shouldn't you) the full extent of your design- where you were conservative, how realistic your loads are, etc.
If, however, your entire design is right at 1.0, then common sense would tell you that perhaps it's a better idea to beef up some of your more critical members.
RE: Eng. practice of allowing 5% overstress
It has been interesting, and I do believe the majority agree that 1.05 is nothing to stress about, while NOT being something anyone shoots for...
I honestly think we've exhausted the issue. See you all in the next thread.
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Eng. practice of allowing 5% overstress
I never claimed the code was a hassle, it isn't, the code is probably the most valuable tool a structural engineer owns. It is just that the code is so limited. The code cannot possibly cover all the situations that an engineer is bound to encounter, however, because of broad generalizations made by the code, it covers a great many of cases. Because these conditions are so generalized though it makes the code in particular very conservative.
Furthermore, most structures, not bridges, have great degree of redundancy. These structures, should they have one element fail, will simply redistribute the load. Also, this "failure", unless the engineer responsible for designing the structure exhibits gross negligence, is not the kind of failure that results in building collapse. In fact one specific code reasons that because of redistribution effects that members undergoing torsion do not need to be designed to resist the applied torsion and that they only need to be designed to control cracking.
RE: Eng. practice of allowing 5% overstress
However if one of your buildings did have a failure and they see a calculation that the allowable load on a column in 100 kips and the actual load on the column is 105 kips. Guess what, you probably just lost your case, your license and a bunch of money.
When doing a code review for one of our clients we will kick back every design we see that has a 5 percent overstressed member. Unless you can show me in the Code where it is allowed. Besides if you are going to bend this rule what else are you bending?
Apsix and frv, I can only assume your experience is lacking or you have been mislead at some point. Long ago I remember the old timers using the 5 percent factor and wonder what the hell are they thinking.
Unfortunately 1/2 of engineering is avoiding litigation. Breaking the code is not a good start in doing that.
Your arguments that if everyone else is doing it then it is standard practice is weak and negligent. Per the hundreds of review we do each year many engineers don't use the new requirements for splice lengths in masonry or development lengths for anchor bolts. You would be amazed at how many don't even look at seismic loads and just put " by inspection wind controls". Even worse is the use of the ASD 9th edition when it has now been removed from the Codes in most cases. People still use old version on Enercalc that are based on the 1997 UBC code and not IBC.
While the building will probably still stand if you don't design per code you are wrong. It is that simple. As I said before, if you think the 5 percent rule is okay go inspect PEMB after a big snow or ice storm.
RE: Eng. practice of allowing 5% overstress
Are you refering to "It does not lead to over-stress of the member"?
It depends on your definition of over-stress. I am equating over-stress with failure, eg. going into yield. Obviously that doesn't happen when you design to 1.05 instead of 1.0.
The point I was trying to make is that it's a term which can mislead people and lead to statements like "Designed to fail".
I thank you for your concern regarding my experience/competence, but assure you that you have no need to worry.
PEMB; a different kettle of fish entirely, not to be confused with a robust structure.
RE: Eng. practice of allowing 5% overstress
I, too, am getting bored of this thread.
It seems obvious that you skipped over large parts of this discussion.
But I can say, beyond a shadow of a doubt, that if any building fails, it is not because there is a 5% "over stress" in any one member. There is some huge mistake somewhere else.
As others and I have repeated ad nauseum, as engineers we are not simple number crunchers. We are allowed to use the knowledge knowledge and experience we've gained to make INFORMED judgment calls.
Misled? EVERY ONE of my (engineering) professors alluded to this in one way or another. And before you go off discrediting them, don't; trust me, you won't win that battle.
Anyway, I'll join youngstuctural in abandoning this thread.
RE: Eng. practice of allowing 5% overstress
I would suggest any engineer worth calling himself one can recognise an indivudual member that in theory is 'ovestressed' by 5 % can be, if you feel inclined, be proved from first pronciples to work as part of a structural frame.
RE: Eng. practice of allowing 5% overstress
Dont take offence, its what your post implies. Not what your like i'm sure!
RE: Eng. practice of allowing 5% overstress
r.e: "ticking every minor code clause as they are the only ones who have time to do it. "
To be fair, engineering firms should quote with enough in the budget to check full compliance with the code. This is a failing of the engineering industry, not the checking engineer. It is amazing how much better the checks and measures are in accounting than they are in structural engineering. It does annoy me too though, when I have a pedantic checker.
As per the 5%, what if the original designer allows 5% and then the engineer for the renovations also allows 5% (as the code implies they can), the building is now taking more than 10% additional load above what the code says it should - are you going to say this is negligible?
RE: Eng. practice of allowing 5% overstress
If you "heard" it on the internet, it's guilty until proven innocent. - DCS
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Hg
Eng-Tips policies: FAQ731-376: Eng-Tips.com Forum Policies
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Look at it this way--failure is not caused by using a W18X35 beam where you should have used a W21X44. It is caused by using a 2X12 where you should have used a W21X44.
Design to Code, yes--but use engineering judgment as well. How many of you, if you find a mistake in your work after something is built (a mistake that results in, say, a 10% overstress), will call the Owner, tell him his structure is in danger, pay for repairs, etc.? No--you sharpen your pencil, try to justify what you have done, and use your engineering judgment to determine if there is really a problem.
DaveAtkins
RE: Eng. practice of allowing 5% overstress
if i was given a building I would reanalyze it whatever, how do you know the original engineer didnt make an error? Fo instance the bridge collapse over there. You in theory would have told them they could increse load by 5% on an already incorrectly detailed connection. The japanese shopping mall collapse (forget the name), increased loading combined with poor construction led to collapse. I cant believe anyone would advise a structure is ok just by inspecting the load and 'assuming' it was originally correct. You would definetly end up in court in it failed justifying. I find it kind of shocking actually.
RE: Eng. practice of allowing 5% overstress
Here's how I see it. Say there are two buildings standing near each other, and an unusually strong hurricane is approaching. One building has been standing for 40 years, and the other was completed yesterday. If you had to bet which one would be standing after the event, which would you chose? I'd go with the old building, as it has proven itself over time.
I believe this reasoning applies to a 5% overload. We know something about a building that has stood for 40 years that we don't know about a building under construction. It has proved itself time and again under load, the only true test. The probability that a 5% additional load is going to fail an old building is less than for a new building.
RE: Eng. practice of allowing 5% overstress
If it is underdesigned from the start a further increase may cause problems. You wouldn't know that it was underdesigned unless you check. I would check personally.
RE: Eng. practice of allowing 5% overstress
For concrete it would be similar, since test reports show that the in-place 28-day strenghts are higher than what is normally specified. I would think that other materials would be similar.
Based on this engineering judgement, along with the embedded code safety factors, I feel comfortable with this design practice, and feel that the I steel meet the intent of the code.
RE: Eng. practice of allowing 5% overstress
strguy11-
Although I agree with you, higher compressive strength doesn't do much for flexural capacity. I think here you rely more on conservative assumptions about loading and the fact that you can get a lot of moment redistribution than on the fact that you get higher strength concrete.
RE: Eng. practice of allowing 5% overstress
True, but with concrete, you typically get about 3500 psi, even when specifying 3000, which represents a 16% increase. Looking at the design equations for concrete beams, (A = T/.85 * F'c) The higher F'c would actually give you a smaller Area of concrete tequired to resist the compressive force from bending. This smaller block, would actually increase he effective depth to rebar. The rebar usually has a greater Fy than 60 also. (More like 62 or 63 based on mill certs) Therefore a higher F'c would help in bending. With all of these factors, I feel comfortable since we are only talking about 5% anyway.
RE: Eng. practice of allowing 5% overstress
It will also help with shear strengths, which can be an issue in concrete (and typically is not in steel).
RE: Eng. practice of allowing 5% overstress
I'm well aware of the mechanics behind concrete flexural members. The increase in compressive strength has a negligible impact in flexural capacity. I know there is some, but increasing d from say 20 in. to 20.5 in isn't going to give you a tremendous amount of additional flexural capacity.
RE: Eng. practice of allowing 5% overstress
If you are only 5% overstressed your building will not fall down, your member will not fail, no one will notice, and you will not have a lawsuit. So this is somewhat of a rhetorical question.
However, if for some reason you make a mistake and a structural panel reviews your entire design for structural adequacy. A member that is 5% overstressed member would not hold any merit in a lawsuit case.
RE: Eng. practice of allowing 5% overstress
re: "And 10% overstress won't cause failure either."
How do you know this for sure, have you ever done the numbers on a building that fell down?
You could have:
Loading at the high end of the use category
Allowable stresses at the bottom end of the allowable.
Beam out of straightness at its maximum
e.t.c.
All eating up your safety factor.
Dont forget that working stress wind loads are only 1 in 50 years so a good part of the safety factor allows for the possibility of a 1 in 500 year (or whatever) storm.
If there is a storm and you designed the only building that fell down then you will get legal issues if it is 5% over. Even if the storm was above and beyond what is designed for the lawyers will claim "It would have stood up if it was designed to code".
I personally dont think it is worth the risk.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
The point I (and several others) have been trying to make is that failures result from "elephant size" problems. At the Hyatt Regency, I was told, the walkway supports should not have been able to support even the dead load alone (by calculation). At the bridge in Minneapolis that failed last year, weren't the gusset plates undersized by a tremendous amount?
You are correct in saying many things can eat into a safety factor. But in general, there is a lot of redundancy and conservatism in structures that we ignore.
DaveAtkins
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
They decided to partner with the university and load test the bridge to see how much true strength it had. The bridge was, I think, about 20 years old and was a three span slab bridge with cast-in-place concrete rails on each side.
They set up a load test that replicated a standard HS20 truck loading.
It didn't fail until it was about 8 to 10 times the HS20 loading.
RE: Eng. practice of allowing 5% overstress
Your July 2nd response is scary. I think you are mixing up a statistical probabilty and the incipent failure of a single beam using the LRFD method. Using 1.6 as the inflated live load factor, anything above that psf loading may cause a plastic failure of the beam. Your 3 to 4 SUV's situation noted must of been a typo on your part. Based on the LRFD method, the chance of a beam failure is statistically around 1 in 500 to 1 in 1000 under typical everyday loading conditions. I would think this is on the order of 3 to 4 standard deviations of the mean and not 5 as you state.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
your statistic is far from clear. Is it that 1 in 500 to 1 in 1000 beams ultimately fail under the LRFD design regime, or is it that 1 in 500 to 1 in 1000 failed beams under the LRFD design regime fail due to typical loading conditions.
I am not mistaking an incipient failure for statistical probability. One of my friends has, conveniently enough, been researching probabilistic engineering. What he does is compare the actual capacities of the beams with the design capacity of the beams. These capacities are represented by a normally distributed bell curve. When the capacity curve is lower than the design capacity curve, a failure happens, this happens to occur in a very very small part of the curve, typically outside of 5 standard deviations, any less that say 3 standard deviations, 99.73%, and the code would consistently design members that failed. I am not suggesting that we should design to capacity, which may well be the 3 or 4 SUVs that I mentioned above, the structure in all likelihood has broken a serviceability limit state at this point and despite the fact that the bridge can hold 3 or 4 SUVs, the SUVs cannot possibly drive over it. However, this bridge that we designed for a single compact car can in fact hold up 3 or 4 SUVs and therefore it is acceptable to allow 5% wiggle room beyond the code.
However, I don't need statistical evidence to show you that the code is conservative enough to allow for 5% beyond design capacity. Consider LRFD.
1st the design values for determining the available strength in the first chapter of AISC 13 you may notice that there are always two values, this is because the values that the engineer is supposed to design with are the smallest recorded values for each member. The other values are the values that the product has been specced for and they are the values that the detailer should use in design.
2nd consider that A36 doesn't yield at 36 ksi like the code says it does it is expected to yield at 39.6 ksi instead. In this step alone you gain ten percent strength for all things A36, I'm not sure what the expected yield factor, Ry, is for A992 but I'd believe that it is somewhere in the 1.05 to 1.15 range.
3rd you magnify your already conservative estimates for your loading. Although I do agree with this step, mostly because I can't see into the future to tell what the actual loading will be.
4th you aren't allowed to take the full strength of your member, it is reduced discriminatorily based on what failure mode governs the design. Although I agree with this part too, mostly because it gives credence to the age old term "run time".
Ultimately its pretty clear that your members are designed much stronger than they need to be. I am therefore comfortable saying that 5% beyond design capacity is ok.
RE: Eng. practice of allowing 5% overstress
I think I mixed up a classic textbook design problem with your real life bridge design condition. As far the LRFD failure rate AISC is very silent on this issue. However books that I have put it around the numbers I suggested above. This is based on typical everyday loading conditions and not an overloaded or an abuse of the design. Take a look at the last page from the link below done by T. Galambos. It gives some failure rates of a bridge beam he calibrated.
http://onl
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
No one is arguing the 5% stress will cause failure on its own.
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
No, most people are thinking of normal everyday situations, in which case 5% design overstress obviously makes no difference.
My point is that in failure situations the last thing you want is for the designer to have designed to 5% overstress.
RE: Eng. practice of allowing 5% overstress
Exactly.
Design codes and standards had been written to provide a Factor of Safety before collapse.
The question is; are we as individual engineers justified in reducing that FoS when it suits us?
RE: Eng. practice of allowing 5% overstress
Design codes and standards had been written to provide an agreed upon probability of collapse.
The question is; are we as individual engineers justified in reducing that probability when it suits us?
RE: Eng. practice of allowing 5% overstress
RE: Eng. practice of allowing 5% overstress
Nor do I see the justification to make such an allowance. All codes are clear in not permitting overstress in general and also clear in the specific cases that they might. What I don't understand most about this whole discussion is why an engineer would even think of exposing himself to such an excessive legal risk, if not an actual risk of failure. Do they just like laying down cash at the lawyer feeding trough? Would they like seeing two lawyers argue with each other at $1000/hr knowing that they're going to have to pay for both of them, all the while with this little voice going around in their head, "Why, why, why did I use 1.05?". IMO, if you have a case where "obviously a 5% overstress does not cause failure", from something like a 40 psf live load, simply state in the calculations your specific considered engineering judgement why the live load should be decreased to 38 psf rather than use the customary 40 and revise your calculations to show 1.00. To do anything else is accepting extrodinary legal risk, if nothing else, which virtually costs nothing to avoid. I see no sense in any alternative, unless Russian roulette with 6 silver bullets is your favorite game.
If I caught an engineer of mine doing that, I'm afraid he'd be DOA.
No need to respond.
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
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