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RE: Rigid vs. Flexible Wood Diaphragms

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John,
Wasn't the ABK method intended for determination of the performance of wood 
diaphragms in Unreinforced Masonry Buildings?  I believe the outcome was that 
these diaphragms which were constructed typically of straight sheathing at 
the roof and 1x diagonal sheathing at the floors were to be considered 
flexible. 
I think the code bears this out as the performance of plywood sheathing is 
more critical when used as a diaphragm supporting or supported by concrete or 
masonry. I would certainly understand the distinction in this case.

For those unfamiliar with the ABK method was a methodology for treating shear 
distribution through a typical wood diaphragm of a multi-story structure in a 
manner different from traditional proportional distribution methods. It has 
been a very long time since I have read the document which I obtained from 
John Kariotis' office who happens to be the 'K' in the ABK. 
If I recall, ABK was intended as a methodology to be used in the retrofit of 
Unreinforced Masonry building which differed from the traditional 
proportional distribution of shear by tributary area. The method considered 
the diaphragm flexible in a URM building and distributed shear by the lessor 
of the tributary shear from analysis or the diaphragms maximum capacity. 
I may be wrong in this one as I thought the ABK was essentially the precursor 
for what became the 1991 UCBC Appendix Chapter 1 and formerly the City of Los 
Angeles RGA 1-91. 
The difference between these and traditional distribution methods introduced 
crosswalls which dampened the diaphrams deflection rather than shearwalls 
which direct a tributary area of shear to the panels. Crosswalls were also 
allowed to deflect in excess of the typical story drift, but limited by the 
diaphrams maximum allowable deflection of I believe 1" per story.

It is not an easy method to understand, but I think the root of the issue 
might be that it was intended for consideration with Masonry rather than wood 
framed buildings. I think the use of wood diaphragms in wood framed 
structures is an intirely different animal.

I look forward to your comments,

Dennis