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Clamping & Friction of Wood

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Title: Clamping & Friction of Wood

I have an architect client who has retained me to do some parapet bracing engineering. He has asked me to confirm the adequacy of his method of fastening diagonal bracing to an existing roof. He uses a 5/8 dia. All-thread rod bent into a U shape that surrounds the top chord of existing wood truss and uses a nut to clamp this hook device tight.

When the braces are perpendicular to the truss, he uses a bolt at each end of a sleeper turned and hooks onto a truss at each end. This locks in the sleeper via bearing of the shank of the hook against the side of the truss top chord. He uses a very similar connection, one clamp at each end of the sleeper, when the sleeper/brace is parallel to the truss. I have questioned the validity of this because it relies on friction. The truss top chord is 1.5 wide and I would be concerned about the following items:

1)      How does he (and more importantly the guy in the field) achieve the required tension through nut tightening to get enough friction.

2)      Tightening of the nut may dig into the top chord and fail it in compression perp to the grain.

Can anyone refer me to something that will give me the following information: Coefficient of friction between steel and wood and how to calculate clamping based on number of turns of the nut or torque (beyond finger tight) on a threaded bolt.

I just cant seem to think this will work without a direct screw in into the top chord. Lags would do the trick but the width of the top chord is too narrow for a lag and I could easily see the top chord being split.

Thanks if anyone can help.

-gerard

Santa Clara, CA