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RE: Cantilevered Wood Diaphragm

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It was stated that  Section 2315.1 of the 97 UBC applies to the building in

>    46'      46'      46'     46'
> |          |          |          |          |  25'
> |          |          |          |          |
> ------------------------------------
> ____________________    6'
> |          |          |          |          |
> |          |          |          |          |  25'

 Section 2315.1 states:

"In buildings of wood frame construction where rotation is provided for,
depth of the diaphragm normal to the open side shall not exceed 25ft, nor
2/3rds the diaphragm width, whichever is smaller."

This provision has historically been understood to apply to what we offen
refered to as three sided buildings.  These are buildings that because they
were open on one side there was large torsional loads on the  diaphragm as
a result of translational loading in one direction.  

It was never the intent that Section 2315.1 apply to a building of this
configuration since the center of resistance of this building is near the
center of mass.  There is no more reason to be concerned about torsion in
this building than there is for most simple span diaphragms.

Accidential torsion needs to be considered and should be managable.

I agree that it is more rational to treat the diaphragm as a continuous
beam over supports when looking at loads in the transverse direction. 
Breaking the building up into seperate diaphragms for each unit defies
logic and ignores how the building will actually perform.

Mark Gilligan