Tree rescue manual | Page 116

2.3 Evaluating tree anchors

2.3 Evaluating tree anchors

A key challenge for emergency responders performing tree rescue will be evaluating the strength of potential anchor points in tree structures . Trees are complex biomechanical structures , capable of sustaining large static and dynamic loads , but are vulnerable to defects and decay with some species having the potential to fail unexpectedly under low loads . Anchor point assessment for climbing and rigging is a crucial skill for working arborists , and can take years to master .
A basic understanding of physics and some simple structural engineering is required to discuss and evaluate the impact of force on tree structure .
This section of the manual introduces some simplified concepts for evaluating forces and assessing tree strength . It is important to note that there is no need to perform any calculations to assess the potential strength of an anchor point during a rescue . At the end of the section , some basic ‘ rules of thumb ’ will be provided to allow quick assessment of a potential anchor without the use of equations . This section is provided because a strong underpinning knowledge of the forces generated by different rescue techniques , and the strength of tree structure under different loads , will allow accurate on-the-fly assessments of the suitability of different rescue plans .
Key terms
Force
Newtons ( N )
Any interaction which if unopposed will change the motion of an object . One newton is the amount of net force required to accelerate a 1 kg mass by 1 m / s 2 .
A mass in kilograms can be converted into the downward force applied by that object by multiplying the mass by the acceleration due to gravity ( 9.81m / s 2 ). In this document , gravity has been approximated as 10 m / s 2 , therefore a 1 kg mass exerts a downward force of 10 newtons ( N ).
Lever arm
Metres ( m )
Bending moment
Newton metres ( Nm ); or
Kilonewton metres ( kNm )
Stress
Newtons per square metre ( N / m 2 ); or
Pascals ( Pa )
The distance between the point of application of a force and an axis of rotational movement .
The rotational effect caused on a point by a force that is applied at a given distance away from that point along a lever arm . Also described as torque , or turning moment .
Force multiplied by length .
The force applied to a material , divided by the material ' s crosssectional area .
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