Calculating the required rating of a guying system ; worked example
The rescue team have been called out to an extremely challenging tree rescue situation .
An arborist was attempting to dismantle a tall Monterey pine tree ( Pinus radiata ) in a river easement behind a house when it lifted out of the ground and became hung-up in another tree , which has also partially lifted . The arborist is trapped between the two trees and suffering severe crush injuries . The other arborists on site are not able to perform a rescue as they are nervous of dislodging the trees and causing complete collapse .
There is no access for a rescue appliance and the rescue team decide to guy the tree to prevent further movement before climbing to extract the injured arborist .
The crew begin setting lines and establishing anchors whilst the team leader conducts a quick whiteboard analysis to confirm safety margins :
The tree :
• is approximately 40m tall ,
• has a diameter at the base of 1.6m
• is leaning by approximately 30 °
• has a density of 760 kg / m 3 but the team leader elects to use the ‘ standard ’ density of 1000 kg / m 3 in order to provide a safety factor .
• has a centre of gravity at approximately the mid-point ( 20m ) due to a fairly consistent branching structure .
• has an attachment point where the guys can be set from the ground at 30m .
Unfortunately , the only available anchor point on the up-lean side is another pine tree approximately 30m from the base of the tree . Using either trigonometry or eyeball estimation , the team leader determines that the angle between the guy line and the tree will be around 30 ° and that the team will need just over 50m of rope to form the guy .
In a real-life situation , the strongest available ropes on site may be the arborist rigging ropes . Ropes should be doubled if necessary to achieve the highest possible rating . This example is provided solely to illustrate the process and provide some guidance on rope selection for this task .
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