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Re: 94 UBC and Rigid Plywood Diaphragms??????

[Subject Prev][Subject Next][Thread Prev][Thread Next] Dennis:

I was surprised to find that the current definition of flexible versus rigid diaphragm first appeared in the 1988 UBC (2312(e)6 on page 148).  Notwithstanding this, there was a common practice to assume all wood diaphragms, at least in total wood frame buildings, as flexible.  I think this was supported, at least in part, by the language in 1997 (and earlier) Section 2315.1 second paragraph which permits "unlimited" wall deflection subject only "structural integrity" .

Even ICBO admits in there notes for Seminar 109 (1997 UBC Earthquake Regulations) that "Design practice was to consider wood floor and roof systems flexible  ...".

The emphasis on changing design practice seems to have come from recent seminars not any code change.


Seaintonln(--nospam--at) wrote:

I'm embarrassed to admit that I could not answer Ben Yousefi's question:

The 1994 and 1997 UBC both contain the same sections that requires the
engineer to test the diaphragm for stiffness and to determine whether or not
it is to be designed as a rigid element. However, building officials (and the
professional community) did not endorse this method for residential
construction in the life of the '94 code.
If they did, then I must have slept like Rip Van Winkle during the '94 Code
cycle since no one that I know considered a plywood diaphgram as rigid.

Therefore, what changed? When and why did it become mandatory in the 1997
code to follow the methodology that was presented in a review of wood framing
seminars for the '97 UBC.

I thought it might have had something to do with the lack of an adequate
method for determining the true deflection of an unblocked diaphragm.

Can someone educate me (and Ben) as to why it was not supported by building
officials during the reign of the 94 code and what occured that (other than
Northridge) that made it mandatory in the '97 code.

I am somewhat embarrased because I have been so adamently against the rigid
diaphragm provisions in the '97 code that I have failed to do my homework and
discover that it was already existing in the '94 code.

Can someone enlighten us?