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RE: Rigid vs Flexible residential diaphragm discussion

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> This would be section 2303 of the 1991 UBC (I could not
> locate my 88 code)
> and is similar to section 1605.2.1 in the 1997 UBC.
> Dennis S. Wish PE

See also section 1630.6 (1997 UBC). This is where, IMO, it would be prudent
to include an exception for residential, wood framed structures three
stories or less, so that diaphragms are considered flexible. Rationale? I
hope that, at the August 13th Seismology Committee meeting, someone acts as
Frank Lew's proxy by standing up and shouting: "Where are the bodies?"

I would be very interested if someone could cite specific (real world) cases
where there was a structural failure attributed solely to the fact that the
original analysis was performed based on a horizontal distribution of forces
assuming the diaphragms were flexible AND, based on an analysis that
accurately modeled the stiffness of the diaphragms and shear walls, the
shear walls failed based on this analysis. I am not interested in cases
where the original designer used (or misused) the "rotation" provisions of
past codes since this provision can be addressed specifically without
affecting the entire methodology of lateral analysis of wood framed

Also, shouldn't the seismic provisions for wood framed structures be
compared to the Conventional Framing Provisions? For example, if a structure
was *slightly* deviant from the Conventional Framing Provisions and had to
be designed in accordance with section 1630, shouldn't the structural design
produce only *slightly* more hardware, nailing, etc? The only
"anti-argument" I could see here would be that a "prescriptive method would
be *most* severe (not less) as they are in other model codes such as the
Title 24 Energy Compliance regulations. In these regulations, there are (or
at least used to be) three methods of compliance: Prescriptive, Points
Method and Computer Analysis; each progressively more rigorous and each
progressively less restrictive. This is opposite in concept from the seismic
design of wood framed structures.

Those who have an influence in the code editing procedure and who know the
truth about the flaws and ridiculous impracticalities of the 1997 UBC with
regards to wood framed construction should stand up now and fix this mess
with an addendum.


Bill Allen, S.E.
Laguna Niguel, CA