Overstress in existing building evaluation
Overstress in existing building evaluation
(OP)
My firm's current steel design standard is to allow up to a 1.03 overstress on new designs. We are reviewing the design of an existing building, no new codes or loads but we are finding some isolated overstresses on the order of 1.10 or less but greater than the 1.03 that we usually allow. Mainly looks to be resulting from compression flange local buckling on OMF Rigid Frame columns and rafters. The building is being checked under ASD. Question is would anyone feel uncomfortable allowing a 1.10 overstress for this situation when considering the safety factor that you still have in place? This is a pretty straight-forward building, not high occupancy, no cranes, no second floors.






RE: Overstress in existing building evaluation
RE: Overstress in existing building evaluation
RE: Overstress in existing building evaluation
I question your 1.03 for new work. Does your contract with the client allow judgment or does it say you will comply with codes? it is unlikely to be a problem, but if you do have a problem somewhere, the bean counters will be all over that 1.03, not because it caused the problem but because it lowers your credibility.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Overstress in existing building evaluation
For an existing structure I'll use something around 1.05 as an upper bound.
RE: Overstress in existing building evaluation
Especially for an existing building, I tend to come out on the side of accepting it. Don't forget, you're really not overstressing the member; you're reducing the factor of safety. Document your reasoning in your report to the client.
RE: Overstress in existing building evaluation
If you are finding that the building is 10% overstressed, I would have another engineer in your firm independently review the structure and if they also establish a 10% overstress, I would tell the building owner. It is a duty of care to the community.
RE: Overstress in existing building evaluation
How many engineers here feel we can justify more than two significant figures for the loads (live and dead) being applied to a building? I was taught that if one of the input numbers were significant to only two figures than the answer could only be significant to two figures. So that 1.03 would need to the rounded to the nearest two significant figures of 1.0. Would any of you require the number to be rounded up to 1.1? Or do some (or most) feel that the loads are to three significant figures and that the answer should be too?
Garth Dreger PE
AZ Phoenix area
RE: Overstress in existing building evaluation
There was an interesting story that went along with the seminar in regards to an existing structure the speaker was asked to apply additional loads to. The speaker said that according to his calculations, the structure was overstressed and that no additional load could be applied and that the building in fact needed to be reinforced. However, the building had been standing for a long time without incident. The job never proceeded forward.
After a few years, the speaker came across a paper that described an old hand method to designing structures (this structure was quite old). Applied to the overstressed building previously and the structure worked.
Depending on how old your structure is, there may be more to the system than running it through computer program.
RE: Overstress in existing building evaluation
woodman88 - Well said.
I hear this all the time from my compatriots who go ballistic if anything over 1.00 is accepted (or whenever members turn red on the computer screen). I learned very early to be conservative on my load assumptions. Then knowing that when I get something that checks out just slightly over 1.0, I can still feel comfortable with it.
Your argument about significant figures is very pertinant. Collectively we seem to have lost some of the common sense thinking that was needed "when life and Codes were so much simpler".
gjc
RE: Overstress in existing building evaluation
When you model the actual structure the threedimensional restraints within the material come to work, and maximum stresses (at least the ones we use to worry about) come somewhat down.
RE: Overstress in existing building evaluation
RE: Overstress in existing building evaluation
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Overstress in existing building evaluation
RE: Overstress in existing building evaluation
I couldn't agree more with paddingtongreen. There is absolutely no excuse for allowing 3% overstress in new construction. The cost to increase your capacity by 3% should be rather small, but the cost to fix a problem or add new loads would be extremely large.
RE: Overstress in existing building evaluation
RE: Overstress in existing building evaluation
So do you have an allowable overstress figure? Such as 1.02, 1.005, etc?
What if a typical connection or member in the building is at two or three locations 1.005 overstressed? If you would not allow this, have you checked out each program you use to be sure it meets this requirement? Because some of the programs I have checked would give a 1.00 answer for a 1.005 calculation. Others will show the 1.00 as overstress.
I personally like to keep to a maximum of .90 stressed for connections and members. But will allow a 1.049 (my personal limit) to avoid going to a odd size, spacing or connection.
Garth Dreger PE
AZ Phoenix area
RE: Overstress in existing building evaluation
RE: Overstress in existing building evaluation
Where I first ran across this in the past, the people there had some old textbook that said that in soil analysis, 5% more or less didn't matter, so the design procedure there was to design every foundation for 105% of the bearing recommended by the geotech report.
As to the original post, I would also wonder if it wasn't a matter of how the analysis was done rather than just having an overstress.
RE: Overstress in existing building evaluation
In this view, the "old texts" are still the new texts on this: we use standard procedures to generate our structural design, but that doesn't mean that such procedures, stated wherever they be, represent in the most accurate form the actual behaviour, or the solicitations for the case; there maybe more accurate solutions elsewhere, or simply, the methods themselves maybe conventional or notional but never corresponding to accuracies in the quantificable measures in the order of a 3%, or even 10%. Try to predict with such accuracy the deflections in concrete, or believe the axial forces you are determining (commonly) for your columns are (or will be) the actual ones: such won't be the case.
And this is not to advocate for imprecision, or lazyness, but a necessary statement on that the art and science of designing structures has much ahead to progress.
RE: Overstress in existing building evaluation
A structure won't fail because it is 10 or 15 percent overstressed. It will fail because it is 100 percent overstressed, due to a major design or construction error.
Now, what an attorney thinks may be a different story...
DaveAtkins
RE: Overstress in existing building evaluation
The one that says we use soft numbers for design, including guesswork loads that are expected to be conservative, assume minimum strength material with little expectation that they will actually be minimum strength, use large safety factors; and then decide whether they are good enough based on a hard number.
The second is from working on the design of nuclear power plants, where everything is documented, and has needed to be documented for analysis of the problems that have developed.
I have allowed slight overstress in nukes, but always with a note giving a reason that was specific to that incidence. I carried that on in the rest of my work for other industries. Quite frankly, I wanted to know that any engineer working under my supervision actually considered what he or she was doing when they permitted an overstress. I see nothing wrong with that approach, I want everybody to think about every step they take in their design.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Overstress in existing building evaluation
I don't know that I have ever investigated a building failure, and probably not a significant functional problem either, which was caused by a 3 or 5% overstress as compared against code allowable stressed or interaction equations. We just really can't design, detail and analyze this closely, at least I can't, even with today's fancy computers and software. There are a bunch of suggestions above to remedy your calc. problem. As Paddington suggests, I would probably never tell you that 1.03 or 1.05 was O.K., on a blanket basis, and I certainly wouldn't write it down anyplace. It is just too easily misunderstood, misinterpreted or twisting when the whatcha-call-it hits the fan. By the same token, I have done exactly what he suggests in my calcs., many times, with some explanations of why it's O.K. in this particular instance.
How old is the bldg., under which code was it designed, did it check then and under that code? Look at the load history, has the bldg. seen some significant (maybe historic loads) wind, rain or snow storms or earthquakes? If it has, it has withstood the test of time. Is it still standing and functioning well under its intended usage? Are there any signs of distress, are there going to be significant new loads added? And, I don't mean the new hyped-up wind and earthquake loads, although I would ask how has it performed in past events, and how I think it might perform under today's more critical loadings?
What is the bldg's. condition? There are many reasons why the answers to this question might lead me to pass or fail the bldg. in my report. And, this issue is probably far more important than a few per cent overstress, determined by today's methods, in some discrete members. How do these members look in the actual structure? What do the connections look like? What redundancy exists to cover this overstress or your 'compression flange local buckling?' Is there some bracing which has actually been ignored or just spaced 5% to far apart?
We are called upon to design safe structures, to safeguard the users, and we certainly should meet the bldg. codes, if for no other reason, to cover our own butts. But, we are also called upon to use our engineering judgement and interpret the codes, and I don't mean in some stupid or reckless fashion. If you don't comply with the code, or are reckless, or negligent, you will be shot out of the saddle very easily, if anything goes wrong. On the other hand, if your interpretations and judgements are well reasoned and based on sound, well established, engineering and principles of mechanics, you can generally defend your actions. It's the old idea that bldgs. designed, years ago, using a slide rule, should most certainly fail by today's standards, because we certainly didn't produce numbers with 10 significant digits like today's computer printouts do.
Explain your findings and the pros and cons of each condition where you find an overstress or an anomaly in your report. Then, it is ultimately up to the owner, with your engineering judgement and guidance, to determine if he wants to spend a million dollars to reinforce or brace for a condition which is unlikely to ever occur.