From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
Date: Fri, 11 Feb 2000 11:13:53 -0800
At 07:19 AM 02/11/2000 -0700, Jake Watson wrote:
>Let me throw another problem in here. If you use the LRFD loading for
>seismic, you end up with larger overturning. In many cases an ASD
>approach would work out with no net overturning (stable) and with the
>LRFD the system is unstable. (....)
> So my point is, how do you design footing soil pressure with ASD and
>the footing with LRFD when the footing is no longer stable? Note this
>only applies to seismic conditions, wind is a whole other topic.
Ahhhh, you have rediscovered the often forgotten reality that the
static-force seismic code approach is a mere contrivance, a convenient
simplification that when originated, catered to engineers' familiarity with
wind design, whose loads are more credibly static.
As a contrived simplification, there are anomalies and sinkholes in the
array of concepts and sub-provisions the static seismic code bears. There is
no integrated, harmonized, seamless conceptual whole that is free of
Both the ASD and LRFD methods are aimed at design of elements of a
structure. They do not have an interest in resolving the conceptual gaps in
static-force seismic design loading provisions.
Dennis Wish used an apt metaphor in a posting earlier today: "accounting
system that will balance." Many engineers expect that every design approach
will comprehensively "balance" in the same sense. They are blown away when
disparate portions don't reconcile. But these engineers simply cannot be
satisfied at every turn. Engineering is not bookkeeping. Creative
engineering is the essence of engineering, but creative bookkeeping is
rightly deemed disreputable.
The map is not the territory, and code static force seismic loads are not
the structural reality. Yet each of these shortcut representations has its
usefulness in spite of being a "book" that will not balance. The other half
of the deal is recognizing and accommodating the limitations, the provisions
that inherently won't balance.
"Instability" as inferred from seismic static forces accounting is not
necessarily a real instability that relates to structural performance that
matters. Understandings in this matter are still fuzzy, as Frank McClure's
many revelations and inquiries have revealed. How fuzzy it really is has not
been admitted to very forthrightly by the "learned paper" people.
Yes, you have thrown another problem in, and a good one. It was a young
person who spoke up about the emperor's transparent clothes. I don't have a
solution except to counsel tolerance for the limitations in simple design
abstractions, and for these limitations to be taught openly. Trying like
manic accountants to close all the gaps and attain intellectual purity will
only continue to destroy the intended simplicity and ruin the entire method
for nearly everyone.
Charles O. Greenlaw SE Sacramento CA