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Canada started with Limit States Design in 1976 and it was made mandatory for building design in the 1990s. I asked the very same questions you are asking because LSD (LRFD in the US) was more work, i.e., time consuming, harder to track through the load paths with separation of all the dead, live and other forces. Also, steel people (CISC, AISC) will tell you it is not applicable to plate structures, so you have to revert to Allowable Stress Design. But now we have no structural code to use Allowable Stress (because LSD is mandatory in Canada) so now we have to dig through old books.

One reason I was given for the change was that that a transmission tower in the US failed because the dead load was over-estimated in designing the foundation and connections to the base. I think this may be one of those myths that keeps getting passed around. Another, more likely, reason was the probable combinations of forces was more efficient and economical (by reducing the dead load factor). Steel's answer to concrete's Ultimate Strength design. And, of course, wood, masonry, etc., promoters jumped on the bandwagon.

I've always felt that if the same logic had been applied to the load combinations in ASD, we could have just stayed with it. I have to go back to designing my rectangular hot-metal ladle with plate, using my 1972 ASD steel handbook, and the AISE guide for Design of Ladles (also ASD).


SGE Structural wrote:
Struggling through the 2007 AASHTO BDS, I fail to understand one very basic thing. There was a good thing invented once, called stress, why the new codes use forces instead? The use of forces - as those of resistance or loading - is so less informative, and does not give the same feel for the performance of material/component. Of course, all new equations allow conversion into stress, but why the change? V. Steve Gordin, SE Irvine CA

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