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Re: Clamping & Friction of Wood

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Thor A. Tandy has a good point about dimensional changes of the wood causing
the frictional force to vary.  I would worry at least as much about the
connection getting wet and decaying, as parapet braces are incredibly hard
to waterproof reliably for the long term.  Four suggestions:

1.  For an upper bound on the clamping force,  use the perpendicular to
grain capacity of the wood over the area being clamped. Or the area crushed
by the "U"  looped around the other side of the truss member.
2.  Choose a low-end coefficient of friction value.
3.  Go with your instincts and use a direct connection to the top chord.
Those nice self-drilling screws  "SDS" from Simpson  would meet edge
distance requirements.  Require that each truss be located by driving nails
up through the roof sheathing on each side of the top chord so the screws
can be accurately located in the center of the chord.
4.  Emphasize the importance of waterproofing--work with the Architect to
get the most "waterproofable"  detail.  Also emphasize continued maintenance
and inspection by the Owner.

Another way we used to brace parapets was with a continuous element
extending up from the attic area, placed directly against the wall
itself--this did not create an "island" of waterproofing  where a diagonal
brace penetrated the roofing.  A 2x2 angle against every other truss, that
sort of thing.

Thor Matteson

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