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Re: horiz. diaph. blk'g.

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Mark Baker wrote:

>>I guess the next question to follow would be, am I the only naive fellow out
there... how many of you have ever adjusted horiz. diaph. capacities given in
the UBC based on blocking nail penetration?<<

No, Mark, you aren't the only "naive" person out there!  I have always 
assumed that the blocking was to serve two functions:  

1. Prevent the plywood edges from displacing out of plane, and,

2. Inhibit parallelogram movement of the joists by preventing relative 
movement of sheets of plywood.

Considering the wood industry's poor history of liberalizing code 
requirements, I would be very hesitant to accept any new liberalization that 
is proposed.  While there have been several liberalizations in the past 30 
years that have been rescinded, the one that comes immediately to mind was 
the increase in allowable bolt capacities when metal side plates are used.  
That increase was changed from 1.25 to 1.75 between two editions of the NDS 
and startled me so much that I wrote to NFPA (now AF&PA) and asked them if it 
was a typographical error.  They responded by saying that, no, it wasn't a 
typographical error; that tests by West Virginia University [where else does 
all liberalization come from?] showed that this was the case.  They also sent 
me copies of the research, however, I could not see anything in the research 
that would justify liberalization.  The next edition of the NDS rescinded the 
increase and changed it to a sliding scale, which I am still reluctant to use.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona