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RE: [used to be] Channels - Weak-axis bending [now it's LRFD and ASD][Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: [used to be] Channels - Weak-axis bending [now it's LRFD and ASD]
- From: carter(--nospam--at)aiscmail.com
- Date: Mon, 27 Sep 1999 09:49:54 -0500
>Christopher Wright P.E. >Now I'm wondering how many other LRFD provisions are more-or-less ASD values factored upward. >Could be that LRFD isn't so obscure as the revised terminology seems to imply. You're onto something here, Christopher. In fact, LRFD and ASD are very similar; much more so than most people realize at first glance. Both methods start with a nominal strength -- the point at which we would expect a system, component or connection to fail. Sometimes, LRFD and ASD are based upon the same model to predict the nominal strength, such as for tension yielding and tension fracture. Sometimes, the models are slightly different, such as for flexural yielding (which we already discussed in the last round of postings). In other case, the models are completely different, such as for composite beam design. Incidentally, when the design model differs, you can be sure it's because the LRFD model is a more accurate prediction of the structural behavior being modeled. In LRFD, you multiply this nominal strength by a resistance factor and compare to the factored loads. The resistance factor takes care of material variability and the load factors/combinations take care of loading variability. In ASD, the factor of safety tries to address both variabilities at the same time. If you make some assumptions of loads and load combinations as I implicitly did in my last posting, you find that the LRFD resistance factor, LRFD effective load factor and ASD factor of safety are interrelated. For example, if the effective LRFD load factor is 1.5, dividing it by the LRFD resistance factor will tell you what ASD factor of safety is burried in the ASD formulation of the strength. What does this general "yada, yada, yada" mean? To me, it means that LRFD is really a lot easier to pick up than most people think because everything you already know in ASD translates over. I can tell you it helped me to make the switch when I figured out how similar all this stuff really is. MAkes sense, though, right? After all , the steel doesn't know what method it was designed under. (-. Charlie P.S., I think your email trailer quotation of General Sedgwick is hilarious. Did you know it was popularized a few years back by David Brinkley in his book "Everyone is Entitled to My Opinion"
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