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[SEAOC] ASD/LRFD One More Time

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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)skypoint.com        | 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

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