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

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Chris,

I'll start with a simple example to explain how I look at the situation.
Let me preface this by saying that I was weaned on LRFD as I graduated
during the early 90's.

Let's consider a beam with dead load and live load present.  Under ASD,
the safety factor applied to the allowable stresses (whether shear or
flexure) is constant and there is no load factor (ie LF = 1.0).  Under
LRFD, the phi factors are constant for shear and flexure (capacity
side), but there is a load factor which varies with the type of loading
present.

Why, as is done in ASD, should I logically treat both dead and live
loads the same?  Dead load is usually well understood in its value and
spatial extent.  Live loads, however, are by nature transient, variable
in value, not well-defined spatially, and completely out of the control
of the designer in service.  Note that often the same framing
members/system results, in large part because LRFD has been calibrated
to ASD.

LRFD, as the academics would express, provides more uniform reliability
in the calculation of applied load versus ultimate strength (ASD is
examining the same relationship but with a different methodology).

Another aspect to consider is second order effects (P-Delta) in frame
design.  P-Delta analysis cannot be performed at 'service' loads.  It
must be done at factored loads due to the nonlinearity of the
load-deformation relationship.  To perform this under ASD, you would be
required to insert a load factor (1.5 or 1.6 I believe) in the analysis
and then remove it from the analysis results for design (the new AISC
spec covers this in more detail).  Who wants to go through that (at
least on purpose)?

I'm sure there are other reasons but I can't think of them at the
moment.

Eric

Eric R. Ober, PE, SE
Associate
Holbert Apple Associates
Olney, Maryland
 

-----Original Message-----
From: Christopher Wright [mailto:chrisw(--nospam--at)skypoint.com] 
Sent: Wednesday, January 25, 2006 2:37 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:

> Point is that I agree that there are quite a few engineers that are 
> more than happy to bury their head in the sand and conintue to use 
> what they are used to using even though there is new and typically 
> better knowledge (or codes) out there.
I swore I'd stay out of this, but I can't help myself. This thread is 
only half a discussion--a lot of people saying 'newer and better' but 
no one saying what's improved. Could someone please tell me what's the 
improvement with limit design? Are fewer buildings collapsing than 
before? Is the owner's cost noticeably smaller? Are structural design 
fees going up? Or down? What's been gained by changing our jargon to 
include 'limit state?' So far we know that academia loves it and so 
does Canada.  New graduates seem to like it but they're taught nothing 
else, which isn't much of a reason. Anyone know why they teach it 
instead of ASD? Can anyone cite any examples of hazards avoided? Are 
buildings built under ASD design rules being scrapped because they're 
dangerous or unsatisfactory? Any correlation  between seismic 
resistance and the code rules? It is suddenly easier to determine 
loading or characterize plastic collapse?

As an old fart who's gone from slide rule design of pressure vessels in 
the 60's to FEA I've seen some real Code changes and the improvements 
they led to. The most profound was a realistic consideration of 
fracture toughness in design and the second set out an analysis-based 
design approach which allowed lower design margins in return for 
greater attention to secondary stress and fatigue. Both of these 
embodied significant performance and safety enhancement that everyone 
could point to. And like LRFD, they still haven't been universally 
embraced or understood, even as Code provisions are being regularly 
upgraded. So I think I understand the process and the reasoning as well 
as anyone. Let's have it--why is LRFD an improvement? Will there be 
fewer bodies or do people like it because some academic told them to 
like it?

I've said a hundred times on this list that I don't do buildings, so 
maybe I've misunderstood something. In the years we've discussed the 
topic I haven't gotten much of an explanation of why LRFD is 
better--maybe this time it's different. It's been repeated a hundred 
times on this list: steel doesn't care what the design rules are--it 
always behaves like steel. Charlie Carter pointed that out a couple of 
days ago, and he's right, but there's still this undercurrent in this 
whole thread of shifting paradigms and 'better' designs, along with 
snickering about old farts who who stick with what works, just because 
it's what has worked, tee-hee.

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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