Need a book? Engineering books recommendations...
LRFD vs. ASD[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaoc(--nospam--at)seaoc.org>
- Subject: LRFD vs. ASD
- From: buddy_showalter(--nospam--at)afandpa.org
- Date: Mon, 20 Oct 97 15:42:28 -0500
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.
- Prev by Subject: LRFD
- Next by Subject: mail
- Previous by thread: Re: BOLT- Epoxy Anchors
- Next by thread: Re: open house