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

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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.

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.

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.


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

V/F (949) 248-8588

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