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Re: ASD vs LRFD: Say It Ain't So

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And to be fair, Charlie has warned people about this when he has talked
about the "joint" ASD/LRFD spec.  He has stated MANY times that we should
be careful what we wish for as we needed to keep in mind the ASD
spec/method that we were used to using was last updated in 1989.  Thus,
an updating of the spec was bound to end up looking a lot like LRFD as
there were LOTS of things that were updated in LRFD that ASD has not
caught up to.  In otherwords, Charlie repeated stated that an updated ASD
would NOT look like what we were used to...and realistically, it should
not...not when you add in more than 15 years of advancements to it.

Regards,

Scott
Adrian, MI


On Mon, 18 Oct 2004, Cliff Schwinger wrote:

> Bill,
>
> It ain't so. The ASD methodology that we've used in
> the past is gone. It's been replaced with a new type
> of ASD design that is really just strength design
> divided by a safety factor.
>
> The next edition of the Specification provides for
> both ASD design as well as LRFD design – but the ASD
> design methodology is not the same one everyone is
> used to.
>
> Regardless of whether you choose ASD or LRFD design,
> the first step you do is to compute the ULTIMATE
> STRENGTH (Rn) of the member or connection that you’re
> looking at.
>
> If you’re using LRFD design then you do the following:
>
> -	Compute the factored loads and then compute the
> factored moment, reaction, axial load or whatever it
> is that you’re looking to get the required strength
> (Ru).
> -	Compute the useable strength (Ø x Rn)
> -	Verify that the usable strength is greater than the
> required strength (i.e., verify that Ø x Rn > Ru).
>
> If you’re using ASD design then you do the following:
>
> -	Compute the service level moment, reaction, axial
> load or whatever it is that you’re looking at to get
> the required service level strength (Rs).
> -	Compute the useable strength which is the ultimate
> strength divided by a safety factor (i.e., Rn/Ω)
> -	Verify that the usable strength is greater than the
> required strength (i.e., verify that Rn/Ω > Rs)
>
> The bottom line is that the ultimate strength of the
> member or connection is computed regardless of whether
> you are using ASD or LRFD.
>
> I may be off a little with some of the terminology and
> abbreviations (I’m sitting in front of my TV at home
> watching Boston play the Evil Empire in game 5 of the
> ALCS – it’s the twelfth inning as I write this), but I
> think you get the idea. The “new version” of ASD
> design is simply strength design divided by a safety
> factor.
>
> HTH,
>
> Cliff Schwinger
>
>
> --- Bill Polhemus <bill(--nospam--at)polhemus.cc> wrote:
>
> > A young friend of mine who is a grad student at a
> > local University mentioned
> > that one of his professors has stated that "AISC is
> > going back to ASD and
> > away from LRFD."
> >
> >
> >
> > I assume that he referred to the upcoming AISC
> > Manual of Steel Construction
> > which will, reportedly, have ASD side-by-side with
> > LRFD in much the same way
> > that the AISI CFS manual does.
> >
> >
> >
> > Or is there something in the works that I don't know
> > about? Enquiring minds
> > want to know.
> >
> >
>
>
>
>
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