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A Cautionary Tale (Was: LRFD)

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-----Original Message-----
From: Tom.Hunt(--nospam--at)fluor.com [mailto:Tom.Hunt(--nospam--at)fluor.com]
Sent: Wednesday, April 26, 2000 9:46 AM
To: SEAINT(--nospam--at)seaint.org
Subject: LRFD

Surprisingly (at least to me) our East Coast vendors are all using ASD.Why
is that a surprise?

-----/Original Message-----

Are you assuming that structural engineers on the west coast are using ASD
strictly for compliance with UBC?

The fact is, the REAL reason most of us long-in-the-tooth structural types
stick with ASD is far simpler: We don't like to learn something new, when we
think what worked for grandpappy oughta be good enough for us.

We've had some half-hearted discussions purported to show--in a few isolated
instances--the "superiority" of the ASD way vs. LRFD. But this nit-picking
refuses to recognize the fact that the ASD code is now over a decade old,
and any/all further developments of code methodology--not just LRFD
application, but applications of demonstrated structural behavior--will be
in the LRFD manuals.

Thus, ASD clingers-to are getting farther and farther behind the learning
curve, such that while it may be true that for the time being LRFD vs. ASD
isn't really about economics, that won't be the case for long.

EXAMPLE FROM ANOTHER REALM ENTIRELY: A year or so ago I had occasion to
encounter a "problem" (perceived, anyway) with a low-rise concrete building
that had been designed my someone else (no longer employed) in our firm some
thirteen years previously, which was being considered for upgrading to
current standards as a FEMA-authorized emergency operations center for the
City of Houston.

As such, the building was being assessed to determine if it could withstand
current wind loading at a higher importance factor from that for which it
was designed originally.

The engineer doing the review was a senior citizen, probably nearing seventy
years of age. He alarmed the owner of the building by stating that it was
his determination that the building would not even withstand the wind
loading for which it was originally supposed to be designed, much less the
proposed criteria.

He made this assessment by noting that the embedment of the beam and column
reinforcing bars at the joints were all inadequate for development of frame
moments in addition to gravity loading.

Now, I'm no expert on concrete design done before the late 1970s, but I do
know that since that time, there have been advancements in understanding of
how such joints perform, that have been incorporated in the design
codes--notably ACI 318 along with others. These "new" criteria allow more
economical design of such joints.

But this fellow was using criteria from the "old days" (ca. 1956) whereby
"bond stress" (a fictitious construct in light of later developments) in the
rebars was used for this purpose. According to the antiquated method of
doing things, these bars were prone to pullout.

I was able to show the joint designs were actually satisfactory according to
design practice, both current and at the time the building was designed.

This episode opened my eyes to the value of incorporating continuing
research into structural behavior in the design codes, and how it is
necessary to keep abreast of these developments. As it was, in order to
protect my firm's reputation I was obliged to show how this venerable
engineer was "wrong", leaving him open to disapprobation on the part of his
own employer. I hated to do this but there was nothing else for it.

Personally, I don't want to be caught that way; I'd rather quit practicing
first.

A Cautionary Tale, if you'll receive it.