Subject: Re: More questions about rigid plywood Diaphragms
Date: Fri, 25 Jun 1999 20:03:40 -0600
I totally agree with Dennis. I practice in an area of high snow loads.
Seldom do we see a roof with a pitch less than 4:12. In addition dormer
openings, intersecting wings valleys, hips, etc. etc. etc.
In all the discussion about rigid vs. flexible, there hasn't been a lot
discussion of the shearwalls these supposed rigid diaphragms are
loading. Even if you make the assumption that the diaphragms are rigid,
how do you distribute loads to shearwalls where there is no method, that
I have found, for calculating the rigidity of these walls. There are
the standard deflection calcs for shearwalls, but these don't take into
account door and window openings. If you're making a lot of assumptions
and guesses to the wall rigidity, then the whole exercise of designing
to a rigid diaphragm means absolutely nothing.
joseph R. Grill, P.E.
> 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