# Re: ASD vs. LRFD

• To: seaint(--nospam--at)seaint.org
• Subject: Re: ASD vs. LRFD
• From: Paul Crocker <PaulC(--nospam--at)ckcps.com>
• Date: Wed, 16 Jun 1999 16:29:51 -0700
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Roger Turk wrote:

> ASD is not based on "empirical" or "arbitrary" stress limits.  ASD is based
> on a factor of safety with respect to first yield.  LRFD is based on a factor
> of safety with respect to a fully plastic (yielded) section.

I'm not sure that this shows that ASD stress limits are not arbitrary, because
while ASD attaches a specific factor of safety to first yield, the choice of the
factor of safety was probably arbitrary or empirical.  How were the factors of
safety chosen?  Why use 1.67 on the gross section and 2.00 on the effective
section as in your example.  What rationale does the ASD system have for these
choices?  If it is based on testing, then it is empirical.  If it was chosen
based on the judgment of practicing engineers or academic, then it is arbitrary.
Really, empirical and arbitrary are NOT bad words.  Read the ACI Chapter 11 shear
calculations (particularly for corbels) and those two words may come to mind, but
I still believe that the ACI is a fine and informative code.  LRFD attempts to
create systems where you can say that there is an X% probability of exceedance,
and designs the system on that basis.  You quite rightly pointed out that you can
go back through the LRFD calcs and determine what the corresponding ASD factor of
safety would have to be to get the same results.  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.  This creates a certain uniformity of logic in design, since
probability of exceedance is a central principal in calculating code wind
pressure, seismic forces, rainfall amount, and probably many other things that
I've never worked with.

Paul Crocker

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