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Re: asd vs lrfd

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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