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

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On Jan 29, 2006, at 4:37 PM, Paul Ransom wrote:

ASD 9th is on crutches and needed to be replaced.
Again--does that mean that buildings constructed to ASD 9th are structurally inadequate? I don't think so and apparently you don't either

As much as the new 2005 standard is presented as dual LRFD & ASD, it is
really LRFD and LRFD/omega or Allowable STRENGTH Design. The limit
states philosophy is being put into terminology that may be more
palatable to some Allowable STRESS Design converts. It is also providing
a standard to wean the building codes off an obsolete ASD 9th.
If you'll look at the document 'Basic Design Values' you'll see that the ASD allowable loads differ form the LRFD allowables by 1.5 in nearly all cases including stability criteria. 1.5 is the factor used to morph estimated service loads into estimated ultimate loads. You can take the ASD approach multiply by 1.5 and get the LRFD numbers. In effect you can do an elastic analysis and claim to call it a limit analysis if you multiply the results by 1.5. I think what they've been doing is to take classical ASD and add some provisions that allow the designer to estimate ultimate loading. That in itself is a very useful thing, particularly for seismic work where some damage is permitted.

BTW by limit analysis, I'm not referring to a 'limit state' but an estimate of upper bound limit load for a component. The estimate is OK because it's conservative and builds on existing service experience with ASD methodology. A real limit analysis would be more like what's called a pushover analysis which is concerned with the kinematic effects of plastic hinges.

You are describing a "simplified approach" vs "rigourous approach": more
material vs design time/quality control.
I don't understand your point. You had said that Div2 was adopted and Div 1 allowed to 'rot.' That statement is flat wrong. Div 2 and Div 1 are equally alive and well.

As you know, limit states is a
philosophy shift not an adjustment of design limits.

Idon't think so. Seems to me the term 'limit state' is jargon whomped up to sound neat-o, not anything particularly new. It's a small thing really. Paraphrasing Queen Elizabeth I comments on the old learning vs the new learning in reformation England: 'Steel is still steel; all else is a quarrel over trifles'

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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