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Re: ASD vs LRFD
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- Subject: Re: ASD vs LRFD
- From: Paul Crocker <PaulC(--nospam--at)ckcps.com>
- Date: Wed, 16 Jun 1999 08:29:34 -0700
> 5. Probability is for gamblers and abstract mathematicians. It is based > on, "You win some, you lose some." If the probability is such that you win > more than you lose (a la the "house" at Las Vegas), that is fine for > gamblers, but I, for one, don't want to lose any! While gamblers gamble with > money, I don't want to gamble with lives. Unless you are using factors of safety _well_ beyond those used in ASD (approaching infinity?) and personally insuring that everything is built to your specification, you are still "gambling" with ASD and factors of safety. Being unaware of the odds does not mean that you aren't taking a chance. In "Steel Structures: Design and Behavior" by Charles Salmon and John Johnston, a graphic comparison of ASD and LRFD tension member calculations is made. The LRFD As value is any from 17% less than the ASD value to 3% more depending on the LL/DL ratio. These look pretty similar, and I don't see how this variation translates to "safe" versus "gambling." > That LRFD is set up for a 97 > percent "reliability" means that there is a chance of 3 percent of the > members failing: that is 3 out of every 100 members! I don't like those > odds! I have never heard it quoted anywhere near 3% for failure before. How did you arrive at this value? > The glass industry at one time produced charts that were based on > probability. (Probability of failure: 8 lights per 1000, or a reliability of > 99.2 percent!) So many lights failed that the glass industry revised their > charts so they were based on engineering principles with a factor of safety > instead of probability. You could just as easily say that glass makers decreased the probability of failure as increased the factor of safety, the two are just different sides of the same coin. Maybe I am misunderstanding the point you are trying to make, but I don't see where "factor of safety" is somehow inherently safer than "probability". I would rather accept a 1% probability of failure than a factor of safety of 3 in a system with a 50% chance of exceeding the design load by a factor of 4. A "factor of safety" is only as good as the odds that it will be exceeded. Without having some idea of the probability, setting a factor of safety is not too meaningful. Paul Crocker
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