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RE: We're Not Getting Older, We're Getting DUMBER

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"Are fewer collapsing?"

Chris, since the number of buildings designed to the "limit states"
standard in the U.S. is vanishingly small, I don't know that this
question is valid right now.

The limit state theory and its application as embodied in the AISC LRFD
design specification(s) (along with other such, e.g. the AASHTO LRFD
Bridge Design Code) come from the same "source" as have all our building
code advancements: Academia + Industry + Practicing Engineers.

One of the mistakes I think AISC and co. made in "marketing" the
original edition of the LRFD spec was to try to portray it as "saving
money." The TRUTH is that sometimes it did, and sometimes it didn't. At
best, it was a wash.

However, when you get down to the brass-tacks details, structures
designed using the "limit states" approach are supposed to have "uniform
reliability." IOW, given a specific limit-state loading (or other
effect), buildings designed to this standard should "behave the same"
(all stand up, all fall down, etc.)

This is because probabilistic effects have been taken into
account--which was never true for the old "non-limit state" design
philosophy.

Now, I haven't had time to really compare, but it seems to me that with
the newer AISC specification, the prior equating of "LRFD == Limit
State" and "ASD == Non-Limit State" is no longer valid, because the
limit state approach has now been couched in terms of ASD as well as
LRFD.

That's probably too simplistic a statement--because assigning different
factors to load cases does account in part for the probability
effect--but I think it's close to the reality.

So I would say that "the newer is better" if you're on board with the
notion that uniform reliability of building design is a good thing.

-----Original Message-----
From: Christopher Wright [mailto:chrisw(--nospam--at)skypoint.com] 
Sent: Wednesday, January 25, 2006 1:37 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: We're Not Getting Older, We're Getting DUMBER 

On Jan 25, 2006, at 11:17 AM, Scott Maxwell wrote:

ould someone please tell me what's the 
improvement with limit design? Are fewer buildings collapsing than 
before? 

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