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

Re: ASD vs. LRFD

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Roger:

You wrote:
> Why not use the same factor of safety (load factor) as the material
> recognizes only the total load applied to it doesn't care whether it
> is composed of dead, live, wind, or seismic loads or a combination
> of them.

You are missing the point.

Yes, the material in a member or structure "recognizes only the total 
load applied."  But no matter how many calculations you perform, you 
don't know exactly what that load is!  In ASD, we take loads that are 
slightly wrong, add them to loads that are very wrong, and use a 
single factor to cover the fact that we don't know the exact load.  
Like a good ASD designer, the LRFD method tries to take into account 
how accurate our estimates are likely to be.

To illustrate, if an owner asks you to design steel roof framing to 
support mechanical equipment and they *guess* it will weigh 10000 
lbf, you will probably apply some engineering judgement (a load 
factor) when you calculate your ASD demands because your experience 
tells you that their guess may not be correct.  The purpose of the 
LRFD method is to standardize similar decisions regarding commonly 
occuring loads and to have a framework in place that can be applied 
to strange loads.

-Mike

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley                                   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