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Re: ASD vs LRFD

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Paul:

According to Salmon and Johnson (fourth edition, page 32) the target 
LRFD reliability index, beta, has the following values for different 
loading conditions (and I've added the number of "failures" expected 
in brackets):

D+L (members)   3.0   [1.3 per thousand]
D+L (conns)        4.5   [0.0034 per thousand; yes 3.4 per million]
D+L+W               2.5   [6.2 per thousand]
D+L+E                1.75 [40 per thousand]

For comparison, the reliability index for ASD beam flexure (D+L) 
varies from 2.4 to 3.1 [8.2 to 1.0 failures per thousand] depending 
on the relative magnitude of the less predictable live load.

According to McCormac (1989), "[failure] means that those structures 
at some time will be loaded into the plastic range and perhaps the 
strain-hardening range.  As a result deformation may be quite large 
during the overloading and some slight damage may occur.  It is not 
anticipated that any of these structures will completely collapse."  
He goes on to discuss the fact that loads and capacities are 
variable, so the probability of failure is never zero.  According to 
him, "the goal of the preparers of the LRFD Specification was to keep 
this chance to a very small and consistent percentage."  He 
summarizes the shift of emphasis from ASD to LRFD by saying, "the 
AISC has introduced LRFD not for the specific purpose of obtaining 
immediate economic advantages but because it helps provide a more 
uniform reliability for all steel structures whatever the loads, and 
it is written in a form that facilitates the introduction of the 
advances in knowledge that will occur through the years in structural 
steel design."

-Mike

*****************
In response to Roger Turk's:
> > "Reliability" is the compliment of "failure" or "error" and is a euphemism
> > coined to make it sound better than to report that the analysis "... results
> > in an x percent failure or error."  People would much rather hear how correct
> > or safe something is than hear how wrong or unsafe it is and 97 percent
> > reliability sounds much better than saying 3 out of 100 will fail.  (Even a
> > reliability of 60 percent sounds safe, when it really isn't.)

...Paul Crocker wrote:
> What I actually meant by that was I have heard figures quoted like 1 in 100,000
> rather than 3 in 100.  Does anyone have any literature to offer on what the
> statistics are (or what the academics intend them to be) for gravity loading?

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Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201