Return to index: [Subject] [Thread] [Date] [Author]

RE: ASD vs. LRFD

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Charles:

Since I'm generally tired of the ASD/LRFD debate, I'll be reasonably
quiet.  However, I believe that I can elaborate on a point Rick made
so that it becomes apparent that the reasoning was not circular.

Rick and you wrote:
> >Another reason for structural engineers to support LRFD
> >design is that all U.S. seismic codes are strength design
> >based, including 1997 UBC, 1997 NEHRP Provisions, 1999 BOCA,
> >ASCE 7-98, and 2000 IBC.
>
> This is no more than a circular argument. Here, the
> egg is chasing the chicken. The code basis is as arbitrary as
> is the steel design choice. Either could just as well be the
> other way around.

A significant portion of seismic design is based on calculating some
limiting forces.  In such cases the primary design parameter is the
expected "real" capacity.  This "real" capacity is slightly increased
(for conservatism) and used as a demand on other parts of the
structure.  (Using ASD terms) if we want to have a uniform "factor of
safety" against being wrong in our calculation of such "real"
capacities we need to use an LRFD-style approach.  ASD hides the
factors of safety (which also differ for various limit states).  Once
we decide that we want to use "an LRFD-style approach" (as is done in
a lot of ASD seismic design and detailing for the reason described
above), I believe it is a reasonable step to simply use LRFD.

Respectfully,
Mike Valley

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E.                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.              Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699      Fax:        -1201