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

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Bill,

The wording of the commentary is definitely changed from the earlier edition
of the Blue Book.  The 96 Blue Book simply states "No commentary provided".
Personally, I think you are probably correct, the change from light framed
wall equivalence was not intentional, but good luck confirming it.


Paul Feather PE, SE
pfeather(--nospam--at)SE-Solutions.net
www.SE-Solutions.net
----- Original Message ----- 
From: "Bill Allen" <T.W.Allen(--nospam--at)cox.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 02, 2004 9:09 AM
Subject: RE: CBC/UBC Section 1633.2.9.3 - R for flexible diaphragms
supporting concrete or masonry


> Glen -
>
> Your point is well taken, but you have slightly missed the issue I am
> raising.
>
> 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
>
> Regards,
>
> T. William (Bill) Allen, S.E. (CA #2607)
> ALLEN DESIGNS (http://www.AllenDesigns.com)
> San Juan Capistrano, CA
>
> :-----Original Message-----
> :From: Glen Underwood [mailto:gunderwood(--nospam--at)clarkpac.com]
> :Sent: Friday, April 02, 2004 8:46 AM
> :To: seaint(--nospam--at)seaint.org
> :Subject: Re: CBC/UBC Section 1633.2.9.3 - R for flexible diaphragms
> :supporting concrete or masonry
> :
> :Bill,
> :
> :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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