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Exceeding allowable stresses
3

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.

RE: Exceeding allowable stresses

I'm sure the logic changes from person to person - for me, the amount of overstress I'm comfortable with depends on the component that I'm designing (floor beam, fall arrest anchor, lifting lug, etc...), the nature of the loading, and the assumptions made in the design, among other factors.

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

I was once told that PEMB often design to 1.039. and the reason given during the conference was that 1.04 was too high.

We laughed, but I did not take it was an exaggeration or lie.

RE: Exceeding allowable stresses

When it's new design, you should not be exceeding allowable stresses. However when checking an existing structural member, slight overages are generally acceptable. A few codes have sections dedicated to existing building elements and seem to indicated up to a 5% overage is typically acceptable.

RE: Exceeding allowable stresses

(OP)
Jayrod, any way you could point me to those code sections?
Thanks.

RE: Exceeding allowable stresses

pcbtmr2, if you're looking for something in code to allow you to exceed allowables, good luck. This is tribal knowledge, passed on from generation to generation. Every engineer does it, but no one would want to defend it in court.

RE: Exceeding allowable stresses

I think as engineers we know that allowables are not ultimates. 5% overload on a beam in not going to bring it down. You can always say the dead load is not right, who uses 150lbs/ft^3 for concrete still? Could it be 145, could it be 155?

RE: Exceeding allowable stresses

Simple logic. Despite our best efforts, we cannot possibly predict the loads a structure will see to within a 5% accuracy. Hence the reason for safety factors in ASD or load factors in LRFD. They are merely fudge factors to account for variations in design vs. actual loads, design vs. actual material properties, etc. and are based on statistical models and probability.

RE: Exceeding allowable stresses

(OP)
MotorCity, I have used your rationale before and "they" didn't accept it. If there was something code-based that says something similar and thus an over-stress of 5% is typical and acceptable, they would buy it.

RE: Exceeding allowable stresses

pcbtmr2,
Look in the IBC - chapter 34. Overstress allowances for existing structures are outlined there.

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RE: Exceeding allowable stresses

I think a lot of people are comfortable with it because in most cases we are talking a very short duration load causing the overstress. Steel (for example) shows a higher yield strength and buckling value (under some circumstances) under rapid loading than under long-term loading. There is also the probability issue (i.e. everything in the load combination happening at once).

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

I think part of what MotorCity is getting to is that you don't know many of the factors well enough to say that it is actually over stressed. So we develop reliable estimation methods. In cases that I have seen there was more than enough 'slop' in some of the load assumptions to make up for the supposed 'over stress'. When things get padded at every step you end up with lots of margin.
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

Various arguments are as follows:
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

You don't know anything about the building (except geometry) to a precision beyond 2 sig figs, if that even. How can you say it's 3% overstressed? That's my rational anyway, and load factors/resistance factors are only ever given with 2 digits.

RE: Exceeding allowable stresses

I like CANPRO'S approach to your question because as an engineer I feel if something is overstressed, then more analysis should be done, and load magnitudes looked at more in-depth. So his comment along the lines of - if I'm doing a quick analysis of something I generally take into account any assumptions and over 'calced' situation then I'll give it +5%.

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

We've all heard it, but its one of my favorites and further reinforces (no pun intended) why a slight overstress is ok....."structural engineering is the art of molding materials we do not wholly understand into shapes we cannot precisely analyze, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance.”

RE: Exceeding allowable stresses

So if it's a new member it can't go over 100% but if you check it a day after it was installed the existing building code (Chapter 34 as mentioned) let's you go up to 5% over without question...

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

You mentioned AISC ASD. If you check your design using LRFD you might be able to get the design to "work". ASD and LRFD will give the exact same answer when the average load factor is 1.5. If you calculate your factored loads and then divide by the service level, that will give you your average load factor ("ALF"). If your ALF is < 1.5 then you'll see a slightly more economical design using LRFD. (Wind loads are the wild card because they get a 1.6 factor, so hopefully you have a lot of dead load, and not very much live load and wind load.)

RE: Exceeding allowable stresses

I've never read IBC Chapter 34 to mean that you can exceed the capacity of a member per se. An increase in force of up to 5% might be under the assumption that the member was designed to 95% of its capacity originally. Of course there is nothing that prevents the original designer to utilize 100% of the original capacity, but if you are looking for a reason to not permit an overstress, that might be the language you need.

RE: Exceeding allowable stresses

Everyone does it, including me, but it really really annoys me. It's like the Spinal Tap amps which "go to eleven". Can't we just make the allowable stress a bit larger and call that 100%?

RE: Exceeding allowable stresses

Yup. Just a call by a committee of men. Arbitrary value based on test data.

Mike McCann, PE, SE (WA)


RE: Exceeding allowable stresses

A few things for me:
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

Pcbtmr2:
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

dhengr....finally some engineering logic and common sense.....the engineers that parse the ever-increasing code for every nit-picking change and nuance would do well to read the blurb at the beginning of the AISC code which essentially places the final responsibility for design in the hands of the engineer and engineering judgement...

RE: Exceeding allowable stresses

The problem of imposing precise limits on continuous variables occurs everywhere. The solution is not to second guess what the governing authorities will let you get away with (should they ever check). The solution is to decide a reasonable margin (say 5%) and then aim to be as close as possible to that margin below the limit. If you then hit 101% you either review the calcs to get that number below 100, or change the design.

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

Long ago- company had some old textbook that said "In allowable soil bearing, 5% really doesn't make much difference" or words to that effect- basically, some of the lines of reasoning as used above. Consequence: Every project, size foundations for 105% of the recommended maximum allowable soil bearing.

RE: Exceeding allowable stresses

I had a great boss/mentor straight out of school. He would always say that the code(s)are guides only, BUT you have better have a damn good reason for working outside of the recommendations of the code. And that goes both ways, you may follow the code to the letter and at the end feel like something is unecessarily large, or not as strong as it should be. I learned from him that after all the numbers are crunched and code checks are made, you still have to use your own judgement and ask if this makes sense.

RE: Exceeding allowable stresses

Speaking of the 500 year event, in the winter of 1996-97 Yakima, WA had a 500 year snow storm on Christmas Eve, then had a second 500 year snow storm on New Year's Eve one week later. So much for recurrence intervals, and yes, quite a number of buildings didn't survive for one reason or another although in my limited inspections most came through just fine if something had not either been missed during design, adjacent higher building or something changed after erection, "we don't need all these braces in the walls, they are just in the way of where we want to put the door."

RE: Exceeding allowable stresses

The German Steel design code allows 10% overstress in 'small' areas
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

All design is an analysis of probabilities. That's what you're paid for: to analyze the probabilities & tell someone what to do to make them work for them. Do you honestly think that any set of load calculations is actually 100% accurate? Do you honestly think that any analysis of capacity is 100% accurate? Who produced the material? Is it a tree that grew in an open field, a plantation, or a virgin forest? Is it steel produced in a new plant, an old plant, with scrap, with raw ore, with new equipment, on a cold day, or a hot day? Who mixed the concrete? Who produced the portland cement? Where did the aggregate come from? You have to be kidding if you think any number you produce is specifically and significantly more accurate than any other if it is based on values in a code or a set of tables; step back & look at what we're really doing here. Generally speaking, we know where the lines are drawn, we stay within them, and we crowd them or cross them when we feel comfortable about it.

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