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RE: LRFD, ASD, and USD

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The big difference IMHO between USD  and WSD in concrete is that in  USD the
design assures that in case of  failure of  an  RC member, a beam for
example the steel will fail first by yielding which will not be abrupt
unlike concrete failure. Max steel percentage allowed in USD is 75% of steel
required for Balanced condition as in WSD (concrete and steel fails
simultaneously).

ALEX C. NACIONALES, C.E.


-----Original Message-----
From:	Charles Greenlaw [mailto:cgreenlaw(--nospam--at)speedlink.com]
Sent:	Tuesday, July 28, 1998 1:22 PM
To:	seaint(--nospam--at)seaint.org
Subject:	Re: LRFD, ASD, and USD

At 10:57 PM 7/27/98 EDT, Carl S. wrote:
>Christopher Wright wrote (concerning the distinction between LRFD and ASD):
>
><< It seems like a distinction without a difference.  >>
>
>Good quote.  I'm going to save this one.
>
>Would you say the same about Ultimate Strength Design for concrete?  We
have
>three decades of experience to go on.  So, here's Question No. 1:  Has
>Ultimate Strength Design made a difference?
________________________________
        Do you mean a difference to the designer, or to the structure as a
whole, or just to the concrete?  I doubt that the concrete itself can tell
the difference, and the structure would be influenced mostly by whether the
old factor on strength that gave allowable stress was turned into its
equivalent reciprocal or not, when made a multiplier of loads. The biggest
difference in these three decades would seem to be in the designer's
stepwise procedures.

        Five decades ago I was taught how to tie my shoes, and have been
told I have an unusual way of doing this little everyday structural
connection. (Can't rely on toenails for much anymore, especially not in
withdrawal, and heel joints have become suspect in some quarters as well.)
The knot I tie comes out just the same as the "regular" way of doing it, but
the procedure my fingers follow is different. The safety factor, or whatever
it might be called now, seems to be according to the properties of the
laces, and secondarily on how hard I preload the final tensioning. Those
funny ways my fingers move in the earlier stages seem not to matter as to my
shoes staying on securely. It's another distinction without a difference.

        Please don't ask for details of how I tie shoelaces, because I
really don't consciously know what I'm doing. (My fingers however know
perfectly well what they're doing.) Your method might well be "more
rational" than mine, and that is usually held as good grounds for mandating
a change in required procedures. Fortunately there is as yet no code writing
group vested with an interest in shoe tieing. And that suits me, because my
mindless old way works just fine, is efficient enough, and without the risk
of mistake or  misinterpretation that comes with new methods. Hopefully this
detail of life will remain beneath the interests of those experts who
otherwise are so invariably helpful in correcting the errors of our ways.
Likewise I hope nobody will present a paper at the convention on what the
shoetieing "standard of care" supposedly is, or push for a required problem
on the PE Exam to verify attainment of "minimum competency" in this
essential skill.

Charles O. Greenlaw, SE        Sacramento, CA