Eng. practice of allowing 5% overstress 33

[radair]

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?

 

[frv]

That's what I'm saying.

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.

 

[JrStructuralEng]

FIRST OF ALL...

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.

 

[csd72]

DaveAtkins,

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.

 

[JrStructuralEng]

I just thought of what would actually happen in a lawsuit. The lawyers would bring in their expert witnesses. For every witness who said 5% overstressed is acceptable, there would be another who said it wasn't. I'm not sure what the courts would do. Probably a stale mate.

 

[cvg]

depends on what the jury thinks and can go either way. Remember how OJ got away with murder? If you don't, read his book...

 

[DaveAtkins]

csd72,

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

 

[JrStructuralEng]

I agree with Dave, I went to a seminar on seismic design and a 'world expert' said that if we take into account all the redundancy and conservatism in design, and add that to the increased actual strength of today's materials, we end up with a building that is about two times stronger than we would think. That is over and above the safety factors.

 

[JAE]

Way out in western Nebraska the Department of Roads had a bridge that was no longer used (they had re-routed the highway so it was a bridge to nowhere).

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.

 

[cap4000]

JaredS

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.

 

[civilperson]

Stresses are nebulous quantities and only strains are able to be measured. Wooten's Third Law is correct.

 

[JaredS]

cap4000

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.

 

[cap4000]

JaredS

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://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_563appendix_C.pdf

 

[JrStructuralEng]

You show me a design where 5% overstess will cause failure, and I will show you a cookie.

 

[Tomfh]

If the structure is at the failure threshold for whatever reason then by definition an extra 5% stress will cause failure.

No one is arguing the 5% stress will cause failure on its own.

 

[JrStructuralEng]

????????????????????? is any of this post talking about failure threshold???? were talking about design "overstress"

 

[tngv752]

I dont think the structure will fail when it's 5% overstress. Maybe, the worst thing is that the steel and concrete structure have some small yielding or minor cracking behaviour when the limit stress get to 5% over.

 

[Tomfh]

is any of this post talking about failure threshold

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.

 

[apsix]

"My point is that in failure situations the last thing you want is for the designer to have designed to 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?

 

[miecz]

To paraphrase apsix in terms of the statistical basis for limit state design:

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?

 

[miecz]

I meant, of course, increasing that probability.