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RE: LRFD, ASD, and USD

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I'll jump in here. USD in concrete certainly reduces the amount of
reinforcing required and overall sections. However, IMO it is too easy to
overlook service performance (deflections and cracking) for certain elements
in certain conditions. Also, some engineers I have had experience with
(admittedly, the younger ones) don't have a clue about the strain
transformation as the load increases, first starting elastically, cracking
and still elastic then into plastic deformation. I have reviewed some
calculations of foundations (sized for soil pressure) for example and, even
with factored loads, the strain on the section is way below 0.002. However,
the engineer still used the USD formulae.

Without having a good knowledge of this transformation or taking the time to
check crack control, invariably a design could end up with too large of bars
too far apart.

Further, I feel that it is a "numbers game" unless one is designing a frame
for example and using limit state design or plastic hinging analysis. Using
factored loads and distributing them elastically is still too conservative
when checking for an "ultimate" condition, whatever that is.

It doesn't seem like too many engineers (especially us old geezers) are too
"warm and fuzzy" about LRFD in steel, although I just read a study in Modern
Steel Construction whereby one could save about 15-20% in tonnage when using
Gr. 50 steel and LRFD vs. Gr. 36 steel and ASD.

Regards,
Bill Allen

-----Original Message-----
From: CarlS95(--nospam--at)aol.com [mailto:CarlS95(--nospam--at)aol.com]
Sent: Monday, July 27, 1998 7:58 PM
To: seaint(--nospam--at)seaint.org
Subject: LRFD, ASD, and USD


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?

Thanks.

Carl S.