Reliability / Probability in Structural Design
Reliability / Probability in Structural Design
(OP)
I've been reading a few papers on reliability and probability in structural design. They have all seemed pretty theoretical so far. I'm wondering if anyone is incorporating these methods in their design work and if you could suggest PRACTICAL books on the topic.
If you are using these ideas, where do you get information regarding the statistical parameters to use?
If you are using these ideas, where do you get information regarding the statistical parameters to use?
RE: Reliability / Probability in Structural Design
For instance, a concrete beam's flexural strength is affected by flange width, B; web width b; depth to reinforcing, d; concrete strength, f'c; reinforcing yield, fy, etc. Each of these has larger and smaller direct effects on strength....f'c not affecting the flexural strength as much as d does. So each parameter has a level of influence on predicted strength, and each parameter has a degree of variability in the field vs. estimated. (i.e. - d can be strictly calculated but in the field, it varies based on how accurate were the chairs, did they sit tight to the forms, were the forms cut/sized correctly, etc.)
One of the most prolific writers on reliability in structural engineerins is a professor named Bruce Ellingwood. Don't know where he currently works but an internet search could bring him up.
On paper I wrote some time ago for a graduate class dealt with taking a large number of tests of composite metal deck/concrete slabs and developing a phi factor (strength reduction factor) for the shear bond mode of failure. Again, lots of variables in the predictive equation for shear that had different amounts of influence on the shear strength (the weight of the variable) and different amounts of variability. These combined into a sort of statistical weighted average to develop a probability of failure for various phi factors (0.65, 0.75, 0.85) and these in turn were compared to other probabilities of failure of shear mode failures (i.e. concrete beam shear failure).
So the reliability that I'm aware of is focused on studying the statistical character of an estimate of strength and ends up being eventually published in a code somewhere.
RE: Reliability / Probability in Structural Design
Thanks for the note, you have obviously been reading some on this topic too and you hit upon my real question. How can we make use of these ideas in the real world for better design; a safer or a more rational design.
One of the papers I read had a practical example where pile groups under a building footing/pile cap were analized. The author showed how a small reduction (about 8%) in the allowable pile stress resulted in an increase in footing reliabilty (about 100%). This was based on the fact that the probabilty of failure/pile decreased as the number of piles under a footing increased. I suppose this should be known intuitively because you can't install partial piles, but the author showed a relatively straight forward, practical format for calculating the relative magnitude of the increase.
I thought that there might be more practical examples out in cyberland. Some of these research papers can be hard to apply to everyday design work, so we tend to do things the same familiar way.
RE: Reliability / Probability in Structural Design
www.ce.gatech.edu/~bellingw/
bruce.ellingwood@ce.gatech.edu
RE: Reliability / Probability in Structural Design
Where did you find the article on pile groups? I missed it -
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RE: Reliability / Probability in Structural Design
It is an old paper but, not having read it before, it is new to me. If you can't find a copy, I can try to scan it or fax a copy to you. Let me know at jheidt2543@aol.com
RE: Reliability / Probability in Structural Design
Those guys with Jackson & Moreland sure dominated the discussion...all five from the same firm?! I assume they were all Freudenthal's former students -
Jose Corso's discussion was quite good. It was actually quite readable while Freudenthal's really wasn't. An interesting read - thanks.
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RE: Reliability / Probability in Structural Design
I don't know how you found it and read it so fast, unless it was already in your library. I feel vindicated! I have now read 5 or 6 of Frudenthal's papers and while he is obviously a very smart man, he can't write in an understandable way for .... and I don't mean the math. Some of his sentences are longer than a paragraph, 8-9 lines! He must never have learned what a period was.
I wish I could find more examples like Jose Corso's discussion, I thought it was very practical and usable. Thanks for looking it up and the comments.
RE: Reliability / Probability in Structural Design
I am sure he might be gracious to pass a copy of his loads notes.
Good luck.
RE: Reliability / Probability in Structural Design
Well, I have a recent instance of it. Now LRFD says follow ASCE 7 for combinations. OK, as long as the ASCE are those of the date, as mandated, and "if equal" to those that were consistent or better than safety by ASD, as was touted was made for LRFD, OK. But really any change in the combinations of course affects overall safety, and its statistical forecast of safety as wanted to remain consistent with standing safety. Hence, I don't think remitting the combinations to ASCE was at all wise.
RE: Reliability / Probability in Structural Design
Second bookshelf, top row, 5th book - right in front of me. While I can speed read, I didn't read them in detail. In fact, I found most of the work too tedious to study. I hate those run-on sentences that are two paragraphs long - especially when they have partial differential equations and probability theory in them as well. Can you say, "My brain hurts!"?
I suspect that Freudenthal's ideas might have been widely accepted and adopted at the time if he had done a better job of communicating those ideas.
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RE: Reliability / Probability in Structural Design
I see quite a few citations of Ellingwood's papers. Could you tell us when the seminar was and who sponsored it? Perhaps the lecture notes might still be available through the organization? I have been to Ellingwood's webpage and looked at his list of publications, very impressive!
Sorry, I'd rather be a Badger. Good school, sometimes good sports, always a good marching band! GO BADGERS!
RE: Reliability / Probability in Structural Design
Although I am not a badger, I think it is a darn good school.
RE: Reliability / Probability in Structural Design
D'Appolonia, 24th Karl Terzaghi Lecture. "Monitored Decisions" Journal of Geotechnical Engineering , Vol 116, No. 1, January 1990.
Whitman, 17th Karl Terzaghi Lecture. "Evaluating Calculated Risk in Geotechnical Engineering" Journal of Geotechnical Engineering , VOl 110, No. 2, February 1984.
Peck, 9th Rankine Lecture. "Advantages and Limitations of the Observational Method in Applied Soil Mechanics", Geotechnique 19, No. 2, 1969.
Also I have a chapter in a book (compiled in India) as per:
Muir Wood, Control of Uncertainty in Geotechnical Engineering
I will get the proper citation of Muir Wood's chapter tomorrow and post it then. These references might contain some provoking thought for some.
RE: Reliability / Probability in Structural Design
RE: Reliability / Probability in Structural Design
Thanks, I'll do some more research...