There are two partial solutions to allow relatively elastic cordages such as polyester to be used as guy ropes :
Pre-tension
Using multiple legs of rope
By applying pre-tension up to the working load limit of the cordage , the available extension is reduced . Additionally , if the tree becomes dislodged , moment is already applied in the opposite direction to the lean .
This method requires a good understanding of forces and a good ability to estimate the load that has been applied .
One ‘ quick and dirty ’ way is to measure the guy line before applying tension . If the rope ’ s elastic elongation at the working load limit is known , the amount of slack that must be taken up by pre-tension can be easily approximated .
For example , consider a 30m guy line that has an elastic elongation of 10 % at the WLL . The rope must be tensioned until 3m of slack has been generated .
Note that semi-static ropes such as these will still allow significant elastic elongation as forces pass the working load limit . This method still allows significant movement of the tree and must be used with care .
By using multiple legs of the same rope , the effective elasticity will be divided by the number of legs in use .
To return to the previous example of a 30m guy line with an elastic elongation at WLL of 10 %, if the rope was bent through double pulleys at either attachment and secured so that four parts of rope were under equal load :
• The effective strength of the system has been multiplied by four .
• An equivalent load will be shared between multiple parts of rope , so that each part experiences ¼ of the elastic elongation that would occur in a single line .
Whilst these numbers are not strictly true , as elastic elongation in many cordages is not directly proportional to load , the principle helps to explain the reason why this technique should be considered if ultra-static cordage is not available .
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