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Fw: asd vs lrfd

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> From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
> To: INTERNET:seaoc(--nospam--at)seaoc.org
> Subject: Re: asd vs lrfd
> Date: Thursday, October 16, 1997 4:31 AM
> 
> Joe McCormick wrote:
> 
> >Load factors are used
> >to predict what might happen under "ultimate" loads, not service
> >loads.
> 
> That's NEWS to me!
> 
> Load factors and their accompanying resistance factors are used to
> provide a *factor of safety*, the same reason that allowable stresses
> are somewhat lower than yield or ultimate stress.
> 
> The method of analysis to determine how a structure behaves under
> *ultimate* loads is commonly referred to as plastic analysis, in which
> plastic hinges are developed until the building [model] "collapses". 
> I have recently seen this referred to on the listservice as "push-over
> analysis."
> 
> When USD became *the* preferred method of design for reinforced
> concrete, a letter was printed in Engineering News-Record (it was a
> much better magazine in those days!) that warned about the use of load
> factors.  I scoffed at that letter!  But you know what?  As time goes
> by, I am seeing more and becoming more convinced that that letter
> writer was more correct than wrong.
> 
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona  
> 
> 

I'd be willing to bet that USD is most often used along with the results
from an elastic analysis - I would be very surprised if most USD design was
done using collapse mechanisms, since this is much more complicated (for a
good sizes structure) than just runnning a simple elastic analysis program.
 That isn't to say that collapse mechanism analsysis is overly difficult or
impossible, but rather that it would take longer for anything but the
simplest structures.

As I recall from my college days, although both ASD (allowable stress
design) and USD attempt to implement a margin of safety in a design, they
do it differently.  ASD, as Mr. Turk points out, deals strictly with an
elastic situation, and it applies the same factor of safety for a
particular failure mode (eg flexure, flexure+axial load, shear etc),
regardless of the origin of the loading (dead load, live load).  In ASD
there is some allowance made for seismic/wind versus gravity loading in the
allowable stress increase.

In USD, the assumption is made that the member is failing in a particular
mode under the factored loading (hence it is called ultimate).  The
difference in the margin of safety determination is that the factors for
overload and the factors for member understrength or severity of the
failure mechanism (eg flexure versus shear in R/C gives phi factors of .9
and .85 due to the more severe condition of shear failure when compared to
flexural failure) are separated.  This allows the designer to assign load
factors proportioned to the possibility of overload occurring (eg 1.4D+1.7L
for R/C, since there is a greater chance of live load being exceeded than
dead load).  The nominal strength reduction factors are also chosen to
reflect the severity of the failure mechanism as explained above.  Hence,
for USD in R/C versus structural steel, the phi factors used to reduce the
nominal shear strength of a beam lets say would be higher for steel than
R/C since steel beams can better handle shear failures than can R/C beams.


T. Eric Gillham PE