Exceeding allowable stresses
Exceeding allowable stresses
(OP)
Can someone offer a logical explanation for the common practice of slightly exceeding allowable stresses. I am having to work against the argument, "The code allows 100% of allowable stress, not 100+%". Forces are generated by wind and gravity in steel (TIA-222, AISC ASD).
Many Thanks.
Many Thanks.






RE: Exceeding allowable stresses
If I'm running a quick calculation and making a lot of conservative assumptions along the way and I end up with 5% overstress, I will recognize that I could re-visit my assumptions and get the allowable stress back to 100%. Again, depending on the nature of the design and the assumptions I have made, I might say 5% over is ok, or I might re-run the analysis with more accurate info. If its a life safety issue (fall arrest anchor or lifting lug for example) I'm likely going to be very rigid at 100%. If I can bump up my beam size, plate thickness, etc... without any issues then there is no reason to exceed 100%.
RE: Exceeding allowable stresses
We laughed, but I did not take it was an exaggeration or lie.
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
Thanks.
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
Look in the IBC - chapter 34. Overstress allowances for existing structures are outlined there.
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RE: Exceeding allowable stresses
Speaking of that, the fact is: some of this is historical: in the older codes, you use to be able to take the 1/3 stress increase for load combinations involving wind and seismic. The West coast guys (among others) fought against that for seismic......and eventually it went away. So some older guys still have that in the back of their minds and try to justify it that way. The 5% has been a standard in a lot of offices I have worked in. (Depending on the situation.)
If you have access to AISC's Journal (archives) there is an interesting article that develops the history of the 1/3 increase and mentions some of the points I make here. The article is:
'The Mysterious 1/3 Stress Increase', by: Duane S. Ellifritt, 4th Quarter 1977
RE: Exceeding allowable stresses
Now if you are dealing with people that believe that design, fabrication, and use are fully deterministic then you have another problem.
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
RE: Exceeding allowable stresses
1) 3% is all the closer a slide rule was ever expected to provide accuracy, so anything tighter than that is not required.
2) We really don't know actual loadings more precisely than 3% or so.
3) CSR equation (AISC H1-1) is expressed as less than 1.0. Given normal rounding rules anything less than 1.05 would round to 1.0. (I find this rationale stupid at best, but I've heard it a number of times.)
4) Common industry practice, probably not only for the metal building industry but in general. Our certification auditors have never had a problem with the 1.03. I think Jim Fisher is comfortable with it as well and he is a widely recognized expert.
5) A number of textbooks in their examples allow slight overstresses (say OK), although I don't know that anyone has explicitly stated that 3% is okay, but 4% is not, etc.
6) Our steel will generally have higher yield and tensile values than the minimum allowables That is not a particularly good rationale for initial design, however can be of some use in litigation and failure analysis.
7) Depending on the structure, what is the range or pattern of the overstress. If it is at only one point, it is not really a problem from a practical point of view as the forces will redistribute somewhat if necessary. This is particularly true if it is at a point say for example directly over an interior column. While the mathematical model might generate an overstress, when the physical structure is examined and the depths of members and connection areas are taken into account it is unlikely that the mathematical stick model will truly represent the physical at that location. A point out at the middle of a span would be interpreted differently. If one point is 1.03, and the points 5' to either side are in the 0.80 range (as an example) it is unlikely that there will be a long-term problem. Other items like partial base fixity, bearing lengths on purlins, etc. could show that the overstress doesn't really exist if you are willing to do some heavy duty analysis and model the structure in a higher level of detail.
8) Evaluate the load combination causing the overstress, is the combination one that is likely to happen, for example 12 psf LL in a no-snow county. Pretty unlikely that the roof is going to see a uniform 12 psf over its entirety once construction is complete. On the other hand, I get real conservative in the area of a step snow drift, where there is a history of design or higher loads in actual occurrences.
Bottom line:
There is no specific allowance for using 1.03, but most everyone does it. The checker is technically within his rights to criticize a 1.029 ratio, although in general I would say he could find something better to be uncomfortable with.
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
Instead of that, I'll put more time into honing down my analyses, not particularly to try to fudge the numbers but, looking at more exact data, trying other combinations such as LFRD.
One job in particular I found wind bracing failing, eventually I learned that if the braces are connected in the middle, then you can take half the L value for critical buckling stress formulas... there's a lot of these little hidden gems in the steel book that took me a while to learn.
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
Sometimes I look at the design and upsize certain members and/or connections just because they don't look right to me, especially in industrial facilities where any member might live a hard life and we just don't know what forces it might be subjected to.
I guess that's why it's called engineering judgment and why I like to be the EOR on projects.
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
Mike McCann, PE, SE (WA)
RE: Exceeding allowable stresses
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RE: Exceeding allowable stresses
1. I've typically made at least a half dozen conservative assumptions in my design by the time I get to the end. If I go back and revisit even one of them I could likely bring that number down under 1. We're talking a couple percent here. If I rounded 9.7 up to 10 in my calculations anywhere then I can bring 1.03 down to 1.0.
2. We typically carry our design checks to a fair bit more significant digits than is really warranted given all the assumptions we make. There's a reason most of our inputs in the code stop at two. Some even stop at one (R factors, some live loads). If we're doing things as we were taught way back in middle/high school, we should be doing all of our calculations to the lowest amount of significant digits for any of our inputs. If going to two significant digits, 1.03 = 1.0.
3. Material overstrength. At least with steel, I don't remember the last time I had A36 steel yield strength come in anywhere near 38 ksi (~5% overstrength), for instance. Typically mill reports come back in the 40s. Similarly for A992, don't recall the last time I had anything come in at 53 ksi (again ~5% overstrength). Typically it's at least in the high 50s. Concrete I see come in low all the time so maybe I'd be more likely to not take the 5% there, though rebar is similar to steel in that I don't recall the last 60 ksi mill report that came in at 63 ksi or lower.
4. At least with wind/earthquakes, keep in mind the magnitude and rarity of the event we're designing for. The design wind event in US codes is a 700 year return period, almost three times the age of the United States. Design basis earthquake is roughly a 500 year return, about twice the age of the United States. If you're going further and designing for full MCE the return period is closer to 2500 years, literally a biblical event. If our behavior during such rare events is actually within 5% of what we predict (up or down), I'd be pretty ecstatic.
RE: Exceeding allowable stresses
It seems to me that this particular topic needs much more common sense and engineering judgement, and much less code b.s. and pseudo justification and sometimes permitting. Certainly no one suggests you can do this 5% thing all the time, on every member. The idea that a group of people, however knowledgeable, or not, sets an allowable stress, not to be exceeded, except by 5%, doesn’t make much sense. Why not just set the allowable stress at 1.05(the former value), and be done with it, as others have suggested?
I’ve been at this Structural Engineering thing for many years, I’ve done a lot of buildings, non-building structures, equipment, machines, etc., and a lot of forensic engineering on problems and failures on all of these. And, I’m not sure that I can really point to an instance where a 5 or 10% overstress, w.r.t. normal design loads, was the cause of a failure, or I dare say even a major problem cause, when all other things were o.k. We are fooling ourselves if we think we know most structures that well, their loadings, the way they actually act under all the loadings, etc. And then, the idea that if it doesn’t check using ASD..., use LRFD and it’ll be o.k., or visa-versa, when the structure hasn’t actually changed one iota, is absolutely crazy. Who do we think writes these codes, God his/herself, following perfect logic and all the laws of physics and nature, infallibly? The only bigger fools are the reviewers or plan checkers, who know nothing except the code verbiage, verbatim. They don’t know the intent or history of the code or that paragraph, but they can recite the whole damn code section word for word. But then, you see hundreds of details and conditions where it is obvious that the detailer/designer/engineers hasn’t the foggiest idea how that detail really acts or works, how it is fabricated or how well, and why it is an inferior detail for that condition.
The way they are complexifying, finessing and continuously changing the codes these days, all for the publishing dollars and to keep a bunch of non-productive people employed as code diddlers is stupid. What they should be doing is making the codes cleaner and clearer, pounding home the intent of the codes, working to make them cleaner and simpler (as best they can); this, instead of more complexity to try to cover ever condition and detail in the universe, and pretending that we are being more exact, and complete, and perfect, in our approach.
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
The reason for doing that is not to reduce the probability of a major problem (it will do that, but only by a very small amount). It is because when a major problem does occur (perhaps because the contractor left out half the reinforcement), the design will be examined in minute detail, and if the contractor's defence team finds a stress of 101% of the specified limit, they will find that much more useful than finding a maximum stress of 100%.
Regarding the probability of extreme events; if you design 1000 structures over a 50 year career, then the probability that at least one of them will be subject to a "1 in 500" year event is close to 100%. Working on the basis that extreme events are so rare that you don't need to worry about them makes no sense.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
RE: Exceeding allowable stresses
Whatever small areas are...usually local areas like connections ....
But at the end it is the responsibility of the engineer...
best regards
Klaus
RE: Exceeding allowable stresses