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

• To: seaint(--nospam--at)seaint.org
• Subject: Re: CBC/UBC Section 1633.2.9.3 - R for flexible diaphragms supporting concrete or masonry
• From: Glen Underwood <gunderwood(--nospam--at)clarkpac.com>
• Date: Fri, 02 Apr 2004 08:46:19 -0800

```Bill,

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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.
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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.
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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.

I don't recall seeing any discussion on this topic, so I've got to ask.

According to Section 1633.2.9.3, one must use an R=4 for the design of
flexible diaphragms supporting concrete or masonry. Of course, in the
"typical" building (that I work on anyway), the building R is 4.5 so an
additional calculation must be made with R=4 (or using ratios) to =
determine the diaphragm stresses.

In the 1994 UBC, the values are the same (see 1994 UBC Section =
1631.2.9.3), Rw = 6 for the diaphragm and Rw = 6 for the building system, so =
additional calculations to determine the diaphragm stresses were not necessary.

My question is: Why the change? Is there anyone out there who was on the
committee(s) for the 1997 rewrite and translation of the Rw values to R
values where this issue was specifically discussed and conscientiously
determined that an increase was warranted? If so, I am interested in hearing
the rationale.

Regards,

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

V/F (949) 248-8588

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