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Re: ASD vs. LRFD

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>But, the fact remains that the LRFD calcs ideally lead you to a system 
>with a known (or estimated) probability of exceedance rather than a factor 
>of safety with an unknown probability of being exceeded.
Not to sound like an old you-know-what myself, but I think this is just 
arm-waving. First the ASD design margins evolved over years of noting 
what falls down and what doesn't. Eventually some very clever people 
formulated mathematics to help classify these observations and the math 
eventually found its way into the Codes. It  was a marriage of 
theoretical mechanics, observation, some controlled testing and some 
inferences drawn from viewing wreckage. It was neither arbitrary, nor 
exclusively empirical or a matter of judgement. system 
>with a known (or estimated) probability of exceedance

The ASD design margins provide satisfactory service, which is reason 
enough for considering them valid figures of merit. I doubt that 
additional quantification is of any practical value. We're kidding 
ourselves if we think the statistical arguments are any more firmly based 
than the ASD 'safety factor,' if only because we're not dealing with a 
true statistical base. Natural events are unique, so are buildings--seems 
presumptuous to postulate a 50 year storm for an area where 
meteorological records haven't been kept longer than 100 years. Putting a 
figure on a design margin (whatever the name) by extrapolating limited 
data doesn't create a 'system with a known probability of exceedance,' no 
matter how many significant figures we carry it out to.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw