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LRFD vs. ASD

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     Based on the recent thread re: LRFD vs. ASD, listserve 
     participants might be interested in a perspective from 
     the wood industry on this discussion. A couple of 
     points:
     
     1.) Factoring of loads does NOT equal Ultimate strength 
     design
     
     LRFD is ONLY an ultimate strength design if you choose 
     to make it one.  If you view the factored loads as 
     representing some extreme event and if you compute and 
     define your material properties on the basis of a 
     near-ultimate condition, and if either of these 
     variables includes inherent nonlinearity, then (and 
     only then) your LRFD design equals ultimate strength 
     design.
     
     Conversely, if your LRFD material properties are 
     linearly related to your ASD properties (i.e., 
     factored), then LRFD is nothing more than a shuffling 
     of safety factors.  It's nothing more, and nothing less 
     than a different way to look at the checking equation.
     
     2.) LRFD is MORE than that -- but ASD is perfectly 
     valid as well
     
     The wood industry chose soft-conversion because we 
     believe that the safety levels embodied in a hundred 
     years of ASD history and experience are better 
     indicators of appropriate safety levels than any 
     first-order, second-moment calculation will ever be.
     
     Also, LRFD (on the load factor side) has significantly 
     smoothed safety levels over a broad range of load 
     conditions.  Even with soft-conversion, LRFD can give 
     significantly different answers from ASD -- and a big 
     reason is that load factoring provides a better way to 
     analyze the structure.
     
     
     3.) ASTM D5457 permits both soft-conversion and hard 
     conversion for developing design values for wood 
     products.  That will allow products developed/tested in 
     the future, particularly engineered wood products, to 
     move toward more theoretically precise estimates of 
     in-service reliability.
     
     SUMMARY
     The wood industry did its best to make the transition 
     as painless as possible by providing maximum 
     compatibility with allowable stress design (NDS). We 
     also plan to provide both methodologies on parallel 
     tracks for the foreseeable future. Since engineering 
     schools are primarily teaching LRFD for most materials 
     now, the primary target audience for wood LRFD 
     initially will be the academic community.