ASD: 1.03 Allowable Overstress 6

[SmithJ]

I have been used to allowing a 3 percent overstress when working with the AISC code. However, I recently realized that I can not adequately identify the basis for this overstress allowance. Does anyone know the origin of this 1.03 or where I can find information that would explain it.

Thanks.

 

[SacreBleu]

The only instances I have observed are gross mistakes such as neglecting to check punching shear in concrete flat slabs, etc.
Another case, engineer significantly (50%) underestimated loads on beams.
Another case, engineer divided by 1.33 (this was checking 16" on center rebar spacing) instead of multiplying by 1.33 for required rebar area.

 

[mrMikee]

I was just looking for something in my "Steel Structures Design and Behavior" text by Salmon and Johnson (1971) and found an example problem allowing a 1.03 overstress (pg 657.) There was no comment other than the fact that 1.03 is approximately equal to 1. In another example they went to the next size when stress was slightly over 1.03.

I thought it interesting that a textbook published for the purpose of teaching engineers would use and apparently approve of this practice. Not that I'm strictly against it and in fact have done it on occasion myself. Sometimes it seems reasonable when the option is to go up another size in steel that would add another 10% to the weight and cost. It depends on the situation in my opinion.

Regards,
-Mike

 

[JoeTank]

Wow this thread has been goning on and on hasn't it?

Back in school our prof suggested that we not permit any calculated overstress at the design stage. This would allow a margin for stuff to happen during the design, fab and erect stages. I always follwed his advise and have never come to regret it. He was absolutely right... stuff happens.

I am also reminded that Abraham negotiated well with God about destroying Sodom. Eventhough the place was doomed, he got it down to just ten just men. Well, I've often wondered if the "3% rule" guys would someday get comfortable with 4%? How about 5%? Seemed like a slippery slope to me.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[DRC1]

I do a lot of design for ostruction which involves both structural & geotechnical engineering. Much of the geotechnical work does not have specific codes, so I guess I have a little more flexibility in my outlook.
being wed to a number with out feeling you can ask why or look at it in the overall scheme of things defeats some of the purpose of engineering. I have seen a lot of money spent to beef up memers that if they failed, would not threaten the stability of the structure, but it was code. I have also seen nonredundent pimary members subject to dynamic loads that are a guess at best designed up to the very limits of the code, sometimes even allowing some overstress. A part of engineering we forget is to stop and look at how our design fits the big picture in quite contemplation.