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Re: Why LRFD vs ASD (was: We're Not Getting Older, We're Getting DUMBER)

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Before I get started on this , I really have to say that I don't want to get into a big flame war or sound like I'm being a pain in the butt. This is pretty much an intellectual exercise for me--the Code is what it is at this point, and there's nothing I can actually do about it, even if I could. I'm still trying to get my arms around the rationale behind the change of methodology. Right now my gut feeling is that the code overall is returning to evolution rather than revolution as it's framework for change. Moreover the similarity in approach between the basics of both approaches as set forth in the PDF versions is obvious and I think it really makes perfect sense.

On Jan 28, 2006, at 12:38 AM, Paul Ransom wrote:

I think that if you are looking for benefits,
I'm looking for effects--benefits or detriments. And If you can't compare the two sets of rules, who's to say the 12th edition has benefits or detriments. Gut feeling or motherhood statements isn't enough. Moreover, differences in load application isn't enough, either, since the changes would apply just as well to ASD as LRFD. Structural loads are just as God made them: no different or more accurate or appropriate for either methodology.

Significant ...? Strong word. However, that's my take on the issue.
But not significant enough that existing construction is demonstrably inadequate, right?

Consider if the ASME standards had been split into two different
approaches when the major changes were made some years back.
That's precisely what happened, not only when Section VIII gained a second Division, based on analysis, but more recently when Div 3 was added for high pressure vessels. And not counting many years ago when Power and Heating boilers were distinguished from Unfired vessels. and in the 50's and 60's when the nuclear codes were added.

 One was
left to rot (but remained popular due to commercial/regulatory inertia -
it costs money to change and it is supported by legal acceptance so
there is no incentive) but the other was upgraded and supported by
developing research and failure response, such as the current ASME.
That is exactly what did not happen. Div 1 and Div 2 have received the same attention to upgrading as the rest of the Sections, including the nuclear codes. As I've been preaching, there are distinct economic reasons for choosing one or the other set of rules, not the least of which is that Div 2 gives an excellent basis for assessing design details that aren't explicitly covered. Div 1 allows reduced analysis effort in setting limits on fatigue and self-limiting stress if allowable stress is reduced. In that regard it's not much different than permitting a lower level of NDT if the allowable stresses are reduced. The thinking behind those changes is a good deal more obvious than the thinking behind the change in approach from ASD to LRFD. The only reason I'm beating this whole question to death, the way I have for years, is my experience with the evolution of the pressure vessel codes. I've done a lot of complaining over the state of the ASME, but the pressure vessel codes are top notch--still best in the world.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw/


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