Overstress in existing building evaluation 3

[boileralum]

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.

 

[DaveAtkins]

I would allow up to 15% overstress if it meant avoiding elaborate reinforcement, etc. Loads and material strengths are generally conservative, and on top of that we use factors of safety.

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

 

[paddingtongreen]

I really am approaching this from two points of view.

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.

 

[dhengr]

Is it still standing? Then it probably hasn’t been overloaded or over stressed too much.

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.