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?

 

[BigInch]

First I'll say, yes I'm practical enough to understand the problem, but disagree with the overstress method of handling it. And I can tell you this 5% overstress allowance is in no way an "industry" standard, nor even a universally accepted structural engineering convention, for new or existing structures.

Nor do I see the justification to make such an allowance. All codes are clear in not permitting overstress in general and also clear in the specific cases that they might. What I don't understand most about this whole discussion is why an engineer would even think of exposing himself to such an excessive legal risk, if not an actual risk of failure. Do they just like laying down cash at the lawyer feeding trough? Would they like seeing two lawyers argue with each other at $1000/hr knowing that they're going to have to pay for both of them, all the while with this little voice going around in their head, "Why, why, why did I use 1.05?". IMO, if you have a case where "obviously a 5% overstress does not cause failure", from something like a 40 psf live load, simply state in the calculations your specific considered engineering judgement why the live load should be decreased to 38 psf rather than use the customary 40 and revise your calculations to show 1.00. To do anything else is accepting extrodinary legal risk, if nothing else, which virtually costs nothing to avoid. I see no sense in any alternative, unless Russian roulette with 6 silver bullets is your favorite game.

If I caught an engineer of mine doing that, I'm afraid he'd be DOA.

No need to respond.

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- Mario Andretti- When asked about transient hydraulics

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