Subject: More questions about rigid plywood Diaphragms
Date: Fri, 25 Jun 1999 14:57:16 EDT
It's no secret that I have strong opinions as to the consideration of rigid
diaphragms in wood framed construction. However, I need some advise to see if
my specific opinions regarding roof diaphragms are valid.
1. I would argue that a Gable roof or any roof exceeding a slope of 3:12
would be considered flexible by nature of the mechanical connections at the
peak. These can not be considered rigid and therefore the diaphragm will tend
to bend at the ridge rather than force deflection normal to the walls. The
hinge at the peak of the roof becomes the weak link.
2. I would argue that any scissor truss or vaulted ceiling which exceeds a
3:12 slope could not be designed as rigid by nature of it's performance.
3. I might be more inclined to accept a minimally sloped roof (1/4:12) as
having greater ridgidity - however, I am still reluctant to treat it as a
rigid diaphragm by nature of the quality of construction unknowns.
I don't want to waste time working through the analysis for rigidity if my
professional judgment tells me that the calculations are not adequate to
determine deflection of a pitched roof.
What are the opinions of others? Also, what would be my risk of liability if
I choose to argue this point and have the building official accept my
Dennis Wish PE