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

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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, which would be 12.5% higher.


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.


My guess all along has been that different people were assigned to translate the Rw values to R values for the building system and for diaphragms. One rounded up and one rounded down. But, then again, I tend to (over) simplify.


If this issue has already been discussed and beat to death, my apologies in advance.




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

V/F (949) 248-8588

San Juan Capistrano, CA