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[SEAOC] Re: [SEAOC] ASD/LRFD One More Time

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Please do not confuse global "efficiency" with local (member, connection) 
efficiency.  3-D structural framing system design and performance is a 
very different topic than examinging system components.

Structural framing only performs as well as the "design" thinking that 
went into its creation.  The overall framing (whether a true 3-D system or 
a collection of 2-D vertical and horiztonal planes) does not understand the 
difference between LRFD and ASD.

_Nothing_ in any codes (past, present, or proposed) ensure that there 
will be a 3-D structural framing *system*.  This comes from one's 
education, and, sadly, very sadly, this has not been a part of structural 
engineering education for decades.  

Check out "The Northridge Warning: Has 3-D Design Been Lost?", my ASCE 
Forum article published in Dec. 1994.

Julie Cohen

On Wed, 12 Jun 1996 BRBATES(--nospam--at) wrote:

> In a message dated 96-06-12 10:18:23 EDT, you write:
> >I'd be real curious about which sorts of structures show the greatest
> >efficiencies and whether there's a correlation between the degree to which
> >lateral loading plays a part in the efficiencies. Was any comparison done
> >to determine where the efficiencies come in? (Column, girder, connection
> >economies? High rise vs small structures?) Are you implying simply lower
> >allowable design margins based on more analysis or is there something
> >inherently more reliable about LRFD that provides a better estimate of the
> >margin?
> >
> >Christopher Wright P.E.    |"They couldn't hit an elephant from
> >chrisw(--nospam--at)        | this dist--"   (last words of Gen.
> >Voice phone (612)933-6182  | John Sedgwick, Spotsylvania 1864)
> >
> >
> >...
> Mr. Wright,
> The ASD/LRFD thread lives on!
> There's more to LRFD than just lowering allowable design margins. LRFD uses
> load factors based on the predictability of the various loadings applied to
> the model, for example when DL and LL are combined, you use a factor of 1.2
> for DL and 1.6 for LL. This reflects the fact that DL can be more accurately
> predicted versus live load. This also is true for the resistance factors;
> they are based on the predictability of the strengths of the members and
> columns. For example, Phi for compression is .85, while Phi for tension is
> .9, indicating that the tensile stength of a member can be predicted somewhat
> more accurately than the compressive strength.
> I think the primary benefit of LRFD is that it puts the higher margins where
> they belong, on those values you can't predict as accurately, while allowing
> lower margins on those values that can be predicted more accurately.
> Bruce Bates
> RISA Technologies
> ...

|   Dr. Julie Mark Cohen, P.E.,    |
|   Director and Principal         |
|   Cladding Research Institute    |
|   6 Commodore Drive, Suite 330   |
|   Emeryville, CA   94608-1620    |
|   Tel (510) 654-7917             |
|   FAX (510) 4500-CRI             |