Location of attachment point in the failed tree
attachment along the lever arm , but of course this is not possible in practice .
Guying cables should be installed at an minimum angle of 30 ° to the lever arm of the trunk wherever possible . Where guy lines are installed at a more acute angle than this , the support they provide will be significantly reduced and the required WLL of the guy line climbs exponentially .
As has previously been discussed ( see leverage and bending moment , pg . 119 ) the height of the guy cable and the angle between the guy and the lever arm of the tree will have a profound effect on the support provided to the tree .
Guying systems will usually provide better support the higher they are placed in a tree , but there may be limitations imposed by availability of suitable anchors , and the integrity of the tree in the upper crown . If the angle of attachment could be maintained at 90 °, the moment applied by the guy would be directly proportional to the height of the
This can be illustrated by comparing the bending moment applied by two guys installed at different heights in the same partiallyoverturned tree , and tensioned to the same anchor . Increasing the height of the attachment increases the lever arm but simultaneously decreases the angle at which force is applied to the lever arm .
The images below show a change in anchor height from 5m to 10m in a tree that leans back at 30 ° approximately 10m from a suitable guying anchor . In both scenarios a 10kN force ( approx . 1 tonne ) is applied to the guy line to support the tree : we can measure the effectiveness of the change in anchor point by calculating the bending moment applied at the fulcrum ( base of tree ).
M = r F sinø M = 5 ( 10,000 ) sin37 °
M = 30,090Nm
M = r F sinø M = 10 ( 10,000 ) sin20 °
M = 34,202Nm
As can be seen , the advantages of lever arm provided by moving the attachment point higher are largely offset by a reduction in angle between the guy cable and the tree .
© Arboriculture Australia 2022 - 207 -