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RE Plywood Roof Diaphragms/ Swedge Anchors

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David (from Lake City):

Nice to hear from another SE in FL, I am south of you in Longwood, near
Otown. Our company actually did the new FDOT office up there. Anyways, great
observation and question about ridge vents! I actually stopped work to look
into it, since we do our fair share of plywood roofs. This is something I
have not really considered in the past and I would imagine I am not alone. I
may be wrong, but this is what I came up with, and I think Ron Martin was
right on. (But Ron, you worried me when you said, "have yet to see one with
any visible problems from this practice." As SE's, we are not likely to see
problems during normal loading events, especially when we are talking about
lateral force resisting systems. Now if you did not see any failures or
problems after a sizeable wind event, a TS or hurricane, then that is
different."

What Ron said is right, and assuming about 98% of residential homes are
simple box shapes with a shear wall on each side of the building. Unless I
am missing something, the roof diaphragm acts as a horizontal deep beam,
which resists the lateral loading from the walls. The roof diaphragm is
considered simply supported and so the shear formula is the simple span
formula where shear is essentially zero in the center and R/2 and the edges,
or walls. At the ridge vent, when the wind direction is parallel with the
ridge vent, will cause a shear force of nearly zero at the ridge vent. Am I
missing something? Then the problem I guess would be perpendicular to the
ridge vent. But there I believe the top chord of the truss would act as a
drag strut to carry the shear over the opening. I guess tensile failure of
the plate at the top chord COULD be an issue, but I doubt that is the worse
case shear load on that plate. I can see several ways of detailing around
this problem, but are the contractors going to do this? Are inspectors going
to catch it? Is it a problem in single story structures with relatively
small shear values? Feedback would be appreciated as I am working on a
project with a very large roof plan, non-residential, in a high speed wind
area near the FL coast.


ANd pardon my ignorance Keith Fix,  but what is a "swedge-anchor"? Also,
what kind of structure are you designing with a 300kip uplift??!!! That is a
large number to say the least. You must have quite a foundation if by SBC or
other code you have to provide 1.5X uplift and overturning resistance.

Appreciate all the info!

Andrew Kester, EI
Longwood, FL



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