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RE: Cantilevered Wood Diaphragm

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Section 2315.1 of the 97 UBC states:

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

It appears Monty H. is within the limits of the code as long as rotation is
provided for. Not sure if that means that a rigidity analysis needs to be
performed or not. Based on symmetry, Monty's argument makes sense, but what
about accidental torsion? Again, this is not required if the diaphragm is
flexible, but does that constitute rotation being provided for? Treating the
diaphragm as a continuous beam over supports is actually more rational that
breaking it up into simple spans. I dont' see how/why you should be required
to break it up. It is usually done for simplicity.

Probably not the greatest structural system and I agree that drift may be a
problem. But I see no code violation here or anything that would be
prohibited in S. Cal or anywhere else in CA.

-Gerard

> It was stated:
> 
> >>>In Los Angeles and other cities in So. Cal, we would not be able to
> design this with cantilevers as you have attempted to do. We would have
> to use moment frames on the exterior walls.<<<
> 
> I would appreciate it if somebody could explain how the proposed design by
> Mr hart violates the code and what problem the perimeter moment frame
> would
> correct.
> 
> If the concern is with deflection of the diaphragm then it would imply
> that
> the spans of simply supported plywood diaphragms could not exceed 50 feet.
> 
> 
> 
> Mark Gilligan
>