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RE: ASD vs. LRFD

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Bill

I am totally agree with your statements. With the endless re-
emergence of this topic, I was scarcasticly pointing towards what 
you said. i.e. use of potential energy...Second Piola Kichoff stress 
matrix...first principles have not changed.

I like to think that understanding the concepts of elasticity that was 
presented in my Structural Analysis courses dealing with the 
Cauchy stress tensor etc is a lot harder to grasp than learning to 
apply LRFD.

Peter McCormack

From:           	"Polhemus, Bill" <wlpolhemus(--nospam--at)sbinfra.com>
To:             	"'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject:        	RE: ASD vs. LRFD
Date sent:      	Thu, 27 Apr 2000 15:08:28 -0500
Send reply to:  	seaint(--nospam--at)seaint.org
Organization:   	http://www.seaint.org

> On Thu, 27 Apr 2000, Christopher Wright wrote:
> 
> > Imagine someone walking into a fluid mechanics 
> > class and announcing that viscosity is an obsolete term and thereafter 
> > shearing stress is actually the velocity gradient divided by a better, 
> > more up-to-date physical property called 'slipidity.' And never mind that 
> > the physics hasn't changed--in five years viscous drag will be completely 
> > replaced by non-slipitude, making the world a better place in which to 
> > live. ;-> 
> 
> Except that you are here confusing "first principles" with "derived method".
> 
> The "first principles" of structural design--physical principles embodied in
> statics and dynamics, for example--aren't being "changed" or even "looked at
> in a new way". What is changing is the synthetic derivations we use to apply
> them.
> 
> Engineering design in general, and certainly structural design in
> particular, must use simplifying assumptions to model system behavior, so
> that those systems become susceptible to design using real world materials,
> real world fabrication, real world construction, etc. The simplifying
> assumptions are often arbitrary. In fact, the "coarseness" of such
> assumptions due to the lesser level of understanding in former times can
> seem rather quaint to us today, in light of current knowledge.
> 
> For example, once reinforced concrete "two way slabs" were designed assuming
> that, for some reason, statics didn't apply. There were actually "patented
> systems" sold to building owners, designed on the basis of some sort of
> suspension of the laws of statics. The assumptions WORKED; the slabs were
> designed and built and those buildings lasted decades. But the "simplifying
> assumptions" were wrong, nevertheless. Subsequent formulations of the
> theories of behavior of such slabs eventually made such design methods
> laughable.
> 
> With LRFD, no one is suggesting that "first principles" have changed. What
> is suggested is a closer approximation to real structural behavior, and that
> of individual elements.
> 
> Mc/I is a sacred "simplifying assumption" used as the basis for beam and
> beam-column design under ASD. But any grad student who has fooled around
> with fine analysis of structural sections knows that it is "invalid" when
> you look at it on a finer level. The sections really DON'T behave that way,
> but assuming they do makes them susceptible to design, with appropriate
> factors of safety. "Good enough" is "good enough," I suppose.
> 
> But when someone says, "there's really no major difference between the two
> methods as far as results are concerned," that's not something to be
> dismissed. Getting from here to there, using LRFD, is done with asssumptions
> that fit more closely to REAL behavior. And in some cases, such insights can
> save time, money, and add to the overall reliability (read "safety") of a
> given structure.
> 
> So, we're not inventing a "new statics" when we do LRFD design. The "old
> statics" still pertains, but the behaviors of the materials and elements are
> modeled more closely to reality.
> 
>