Rigging of trees
a new technique
The main objective for rigging or bracing trees is to protect people,
vehicles or buildings from possible injury or damage. Therefore, the reliability
of the installation is of the utmost importance.
Any type of mechanical installation may break under stress. When a support
(rigging) breaks, it is not necessarily due to a technical error made at
the time of installation. In most cases, it is evident that the installation
was competent and carried out according to normal procedures and with suitable
hardware.
The item most often damaged when a brace gives way is the turnbuckle.
They are the most vulnerable to wear down and to let go.
The cable or guy wire is also susceptible to wear at the attaching points
and is particularly affected by the corrosion that forms into the cracks
of the protective coating. These cracks are often caused by the acute bending
of the cable. This can be avoided by using a thimble or other method to
limit the bend radius.
According to some experts, the ideal type of cable should be made of
stainless steel. A brace made of this superior material would be much more
reliable and last much longer.
Under tension, hook screws have a tendency to open up, but an eyescrew
or eyebolt provides a greater safety marging. Often eyescrews are torn
out from the tree, interior decay is generally responsible for such an
event. In such cases, an eyebolt should be used.
To reduce the failure rate, the number of mobile parts must be reduced,
be more reliable and rust resistant. A new approach is that all metal parts
such as hooks and connecting items be coalesced with the tree 
in
subsequent years. 
I
have experimented with a new and simple technique. It
consists of only four items: two eyebolts, a cable and a cable clamp. The
procédure consists of drilling two holes of appropriate size and
to install the eyebolt to within two or three centimetres of the bark.
The cable is then fed through the eye of the bolt, wrapping it around the
shaft and out through the eye. This should look like a reef knot. The procedure
is repeated at the other tie point. The ends of the cable are then brought
together near the mid point where they are tied with the cable clamp. The
eyebolts are then turned until the cable and the bark bring nearer thus
tightening the installation.
This type of rigging must be tight at all times. It is not an effective
method to support long flexible branches. Strong winds would cause its
tension to become null, and small "e"s would form along the rope. Even
one made of stainless steel would degrade very rapidly.
This technique offers many advantages and i have not noticed any drawbacks with it.
The main points are:
a) the hardware is reduced,
b) the eyebolts and the cable are quickly surrounded by the new growth.
The grasping force increases as the tree grows. Rust would not affect the
cable inside the tree,
c) twisting and doubling the cable provides a greater elasticity,
d) rubbing (faying) of the cable strands reduces the stress on the clamp,
e) the alignment of the eyebolts is not as critical as with other types
of braces,
f) the higher quality of hardware is offset by reduced cost of installation.
This article may be of special interest to other nurserymen who are willing to try this new technique. I would welcome questions and comments from anyone regarding this new procedure.
N B Translated by Jacques Pelletier, published by Arboriculture-Québec,
march 1993
For informations: arbres@canada.com
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