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RE: CBC/UBC Section 1633.2.9.3 - R for flexible diaphragms supporting concrete or masonry

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Glen -

Your point is well taken, but you have slightly missed the issue I am

I'm not debating whether to limit the R for the purposes of diaphragm
design. This code requirement was also in effect in the 1994 UBC where the
diaphragm design was limited to Rw=6. My question is directed at the fact
that, in the 1994 UBC, the limitation for the diaphragms was the same as the
structural system Rw for "all other light-framed walls" (item 1.1.b in Table
16-N), but not so in the 1997 UBC. I'm wondering if this was intentional or
due to poor editing. I don't have a Blue Book older than the 1999 edition,
so I can't say if the wording in the Blue Book changed from the previous
edition. In the 1999 edition, no reference to any event or research
information subsequent to the previous edition is made.

To summarize, for Structural Systems described as item 1.1.b of Table 16-N,
the ratios of structural system R (or Rw) value divided by the diaphragm R
(or Rw) value are as follows:

1994 UBC: Rw/Rw = 6/6 = 1.0
1997 UBC: R/R = 4.5/4 = 1.125


T. William (Bill) Allen, S.E. (CA #2607)
San Juan Capistrano, CA

:-----Original Message-----
:From: Glen Underwood [mailto:gunderwood(--nospam--at)]
:Sent: Friday, April 02, 2004 8:46 AM
:To: seaint(--nospam--at)
:Subject: Re: CBC/UBC Section 1633.2.9.3 - R for flexible diaphragms
:supporting concrete or masonry
:You can find some good discussion of this topic in the 1999 SEAOC Blue Book
:Commentary (C108.2.9 pg. 138).  They are explicitly limiting the R factor
:for the diaphragm regardless of the R factor for the lateral system.
:My interpretation - (I'm not a committee member) - It makes sense when you
:think of a flexible diaphragm supporting say - masonry moment frame walls
:(R = 6.5) versus a diaphragm supporting masonry shear walls (R = 4.5).  The
:diaphragm should be designed for the same force for these two systems (in
:the transverse/loaded direction it doesn't matter what kind of wall is
:pushing/pulling on the diaphragm).  The extra energy dissipation associated
:with the R = 6.5 doesn't really kick in until the load gets from the
:diaphragm to the vertical elements.
:It also makes sense that an element that is further up the load path be
:designed for a slightly higher force than elements further along the
:line.  At an R of 4, the diaphragm will be designed for a higher force
:level than most lateral systems.
:Glen Underwood, S.E.
:Sacramento, C.A.

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