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RE: [used to be] Channels - Weak-axis bending [now it's LRFD and ASD]

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>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

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. (-.


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"