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Re: WOOD MOMENT FRAMES

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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 already
>into 1997 UBC. I recall that I have seen this provision somewhere referred
>to as "inverted pendulums" (maybe SEAOSC blue book?).
>
>Regards,
>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.  

_______________

BILL CAIN, SE
OAKLAND, CA
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