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Re: Cross-grain Tension

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Roger.
 
Yes.  You are correct.  Our timber code CSA O86 allows for certain sized notches that probably keep stresses low enough (?) to avoid the onset of tension across the grain.
 
I recently came across a "notch" made up of holes drilled for electrical cords etc at the end of a joist.  The engineer had strapped and shimmed it (mechanical).
 
It got me thinking about the forces/deflections that would cause a split if the load (and/or deflection) was large enough to initiate a split at the notch root.  I have repaired several minor things at home with just that problem and thought to put some arithmetic on it.
 
Ergo ....
 
Thor A Tandy  P.Eng, MIPENZ
Victoria BC
Canada
e-mail: vicpeng(--nospam--at)telus.net
 
----- Original Message -----
From: Roger Turk
Sent: Friday, October 25, 2002 8:04 AM
Subject: Re: Cross-grain Tension

Thor,

Your response to Daryl indicates a different situation other than just
cross-grain tension.  What you are describing is a notch, and the theoretical
tension stress at a reentrant corner is *infinity*, (see any theory of
elasticity text) which is why glass breaks so cleanly when it is scored.

While the codes do permit such notches and kerfs, I will not accept them
without mechanical reinforcement.  I have seen splits start at these notches
in lightly loaded residential rafters, and if the slope of the grain is
upward, extend to the top of the rafter.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona