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I realize a structure doesn't "know" if it's a building or a non-building,
but how does the structure "know" if it has a pair of inverted pendulums,
sitting on pad footings with a grade beam connecting them as opposed to an
upside down OMF (Rw=6)? The deflection would be about the same.

Bill Allen

-----Original Message-----
From: Bill Cain, S.E. <bcain(--nospam--at)>
To: seaoc(--nospam--at) <seaoc(--nospam--at)>
Date: Friday, April 10, 1998 3:59 PM

>At 14:24 4/10/98 -0700, Bill Allen wrote:
>>I guess I'm the only one who believes that a structure designed using
>>"flagpoles" needs to use an Rw=3. Or maybe it's just that my head's
>>into 1997 UBC. I recall that I have seen this provision somewhere referred
>>to as "inverted pendulums" (maybe SEAOSC blue book?).
>>Bill Allen
>[Bill Cain]  The '90 Bluebook (and hence the '94 UBC) has a category of
>"inverted pendulum type structures" with an Rw=3.
>It has been accepted practice to increase the design loads for "inverted
>pendulum" structures for a very long time in school design.  The common
>type of structure at a school site that would come under these provisions
>is an exterior corridor supported on pipe columns cantilevering from a
>foundation.  I know OSA was enforcing the increased loads when I worked
>there checking school plans from 1977-1979.  In fact, it was one of the
>more frequent errors made on plans submitted.  I don't remember the exact K
>value required by Title 21 but believe it was either 2 or 2.5 which would
>translate to an R value under the '97 UBC of 3 (K=2) or 2 (K=2.5).  The '97
>UBC value (Table 16-N, page 2-32) is for R=2.2 (which would be equivalent
>to Rw=3) for "cantilevered column elements."
>Although common practice and codified for schools, the UBC doesn't clearly
>talk about such structures until '94 for non-building structures and '97
>for "cantilevered column elements" as structural systems.  The 1990
>Bluebook, under Table 1-I (p.36) for non-building structures,  called for
>an Rw=3  (or R=Rw/1.4=2) for Non-Building Structures.  Two things to keep
>in mind are: 1) The structure doesn't know if it is a building or not; and
>2) once the column hinges you have a mechanism and almost sure collapse
>unless there are other redundancies.
>Bill Allen has once again brought some sound common sense advice (even if
>gummit engineers were the first to codify it :<)   ) that should be taught
>to young engineers and usually is not.  Basic common sense needs to
>constantly be a reality check on all our designs.