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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: Thu, 17 Jun 1999 13:45:10 -0700
> [RT] ASD is not based on "empirical" or "arbitrary" stress limits. ASD is > [RT] based on a factor of safety with respect to first yield. LRFD is based > [RT] on a factor of safety with respect to a fully plastic (yielded) section. > [RT] The factor of safety for ASD comes from the same place that the factor of > [RT]safety for LRFD (load factors divided by phi for members in pure tension) > [RT]comes from: from testing of material. Yes, my point exactly. ASD *is* empirical, as it is based on "testing of material(s)." ASD *is* arbitrary, because it is based on a human interpretation and application of the results of those tests. ASD and LRFD are just like anything else short of pure mathematical pursuits. To defend ASD as being superior to LRFD because ASD is neither empirical nor arbitrary just doesn't work. > [RT]I can ask you the same questions about the different factors of safety > [RT]required in LRFD for tension on gross and effective net sections (1.57 and > [RT]1.87 respectively) that you asked about ASD, but I think that the answer is > [RT]obvious. My point was not to say that the choice of 2 and 1.67 are not based in the same logic as the choice of 1.57 and 1.87 are. They clearly are. But why not 2.2 in place of 2.0 and 1.87 in place of 1.67? That would be safer. Maybe 1.8 and 1.47 are better? That would be more economical. It's rather arbitrary. [RT]In the net section analysis, you have discontinuities, which cause [RT]stress concentrations, which create uncertainties, which require a greater [RT]factor of safety. Oh good, you'll be an LRFD backer in no time with this line of reasoning! This is exactly the line of reasoning that LRFD uses to place a greater factor on LL than on DL. You could say that live loads "create uncertainties, which require a greater factor of safety" to borrow your wording from above. You've said in numerous posts that service load is more correct than factored load, because it what the system really feels. If you just use the logic that you've laid out above for material discontinuities - uncertainties - and apply it to live loading - uncertainties - then you'll see why the load factors are a logical alternative to service loads in systems where the loads are not well established - such as almost any real world building the lets people crawl all over it and put stuff on it in places that weren't necessarily intended. Paul Crocker
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