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

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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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