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Re: Clamping & Friction of Wood

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Clamping & Friction of Wood
• From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Thu, 6 Feb 2003 22:04:25 -0500 (EST)

```Gerard,

>From my statics book (Beer & Johnston's Vector Mechanics for Engineers:
Statics), metal on wood has an approximate coefficient of static friction
of 0.20 to 0.60.  The following page has similar values for metal on wood
(clean), plus a whole lot more:

http://users.breathemail.net/roybeardmore/Useful_Tables/Tribology/co_of_frict.htm

Another table of friction coefficients at:

http://www.carbidedepot.com/formulas-frictioncoefficient.htm

All seem to have the same values...0.2 to 0.6.

As far as  the number of turns versus torque, you could get an
approximation if you know the pitch of the threads on the all-thread rod.
This would allow you to calculate the distance the nut moves through one
turn of the nut, which you could then use with some basic structural
statics/solid mechanics/deformation/energy principles to get an
approximate of the force.

HTH,

Scott
Ypsilanti, MI

On Thu, 6 Feb 2003, Gerard Madden, SE wrote:

> 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
>
> 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 can't 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
>
>

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