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

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> From: Christopher Wright <chrisw(--nospam--at)skypoint.com>

> 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

Ditto.

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

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

It is necessary to make a distinction between design philosophies and
things that are happening in the AISC realm.

On a case by case basis and for "typical" design, you may not find much
difference in design results between ASD 9th and ASD 13th, if you do a
full due diligence design (e.g compare oranges to oranges). Charlie
Carter recently alluded to this.

For some light commentary on, "Why Limit Sates?," see these references:
Limit States
http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd221_e.html
Wood Design
http://irc.nrc-cnrc.gc.ca/pubs/ci/v7no4/v7no4_5_e.html
Bridges
http://lrfd.aashtoware.org/sites/aashtoware/docs/Why%20LRFD.pdf
Mostly, these reflect the simple points that I made previously.

If you do a Google search for limit states, you will see how this is
coming together across many material design standards, in many
countries, to improve compatibility of design and consistency of
performance.

If you want to reserve the discussion to AISC standards, only: I am
aware of circumstances where using ASD 9th as the -> sole <- basis for
design, results in more than 50% difference between response ratios when
compared to latest LRFD basis (specified/allowable vs
factored/strength). The problem is not in the standard, the problem is
in the blinkered application (names withheld). 

> > left to rot (but remained popular due to commercial/regulatory inertia

> 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

You are describing a "simplified approach" vs "rigourous approach": more
material vs design time/quality control. As you know, limit states is a
philosophy shift not an adjustment of design limits. Since the LRFD was
calibrated to ASD results for "typical" conditions, the apparent
economic benefits are not visible.

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 are working with designs that are controlled at service limits
(e.g. deflection, fatigue), there really is no change in design
regardless of what label is assigned. If you are working on the edge
where either service or strength limits control, you will now be doing
limit states strength analysis regardless of the value of the final
number (I foresee some Mars crashes).

There may be a desire among the engineering community to have a
"simplified" standard. I believe this is the real preference of many ASD
"holdouts," to avoid doing the rigourous analysis that is, mistakenly,
attributed to limit states design. I am not aware of a "simplified"
structural steel design standard, anywhere.

> 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.

ASD 9th is on crutches and needed to be replaced.

-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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