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cantilevered flexible diaphragms / section 2315.1

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i need clarification regarding the last paragraph of labc section 2315.1 
regarding limits on cantilevered wood framed diaphragms in wood ramed buildings.  

i have a project where the architect wants to cantilever the upstairs portion 
of a structure approximately 6' beyond the first floor wall below.  the 
backspan distance between lateral load resisting elements is about 20'.  the aspect 
ratio of the backspan diaphragm is approximately 3:1 and the cantilever 
diaphragm is less than 1:1.  i have received conflicting and vague answers from the 
city plan checkers i have spoken to and am curious what other have done in 
this situation.  

the wording of this section of the code limits cantilever diaphragms 
supporting roofs and floors above to 15% of the backspan distance between lateral load 
resisting elements.  does this refer to a diaphragm that supports a shear 
wall offset from above, and if i have floor and roof diaphragms that is designed 
to cantilever I can go further as long as the aspect ration is less than 4:1?

the provision seems overly restrictive and stiffness and allowances for 
performing deflection calcs does not seem to come into play.  it would seem to me 
that a cantilever diaphragm should be governed by aspect ratio stiffness and 
not blanket 15% allowance that does not consider the geometry's of the 
diaphragm.  a blocked diaphragm is stiffer and stronger than unblocked but this is not 
mentioned.  also, how is this different from a plywood shear wall which is 
essentially a cantilever diaphragm?

if plywood diaphragms can in fact only cantilever 15% what do others do?  
Horizontal braced frames or weak axis bending of frame beams?

thanks in advance for your help.

paul franceschi, s.e.  

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