Comments: Authenticated sender is <mtv(--nospam--at)linux.skilling.com>
> Why not use the same factor of safety (load factor) as the material
> recognizes only the total load applied to it doesn't care whether it
> is composed of dead, live, wind, or seismic loads or a combination
> of them.
You are missing the point.
Yes, the material in a member or structure "recognizes only the total
load applied." But no matter how many calculations you perform, you
don't know exactly what that load is! In ASD, we take loads that are
slightly wrong, add them to loads that are very wrong, and use a
single factor to cover the fact that we don't know the exact load.
Like a good ASD designer, the LRFD method tries to take into account
how accurate our estimates are likely to be.
To illustrate, if an owner asks you to design steel roof framing to
support mechanical equipment and they *guess* it will weigh 10000
lbf, you will probably apply some engineering judgement (a load
factor) when you calculate your ASD demands because your experience
tells you that their guess may not be correct. The purpose of the
LRFD method is to standardize similar decisions regarding commonly
occuring loads and to have a framework in place that can be applied
to strange loads.
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Michael Valley E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc. Tel:(206)292-1200
1301 Fifth Ave, #3200, Seattle WA 98101-2699 Fax: -1201