Tensile test

Trees are exposed to wind loads which they carry off from their top via the trunk into the roots.
In case of very high wind loads they may no longer be able to brace such forces and the reaction is fracture of the top, branches or the trunk or they lose their anchorage in the ground and tilt.

In comparison with other measuring procedures, the tensile test allows for measuring of both safety against fracture and anchorage force and consequently enables clear, profound and understandable evaluation of stability. The soundness and reliability of data collected from the tensile test in comparison with other methods has been confirmed in many situations. The latest verification was a practical test with twelve trees carried out by the research group on examination equipment of the Fachverband Geprüfter Baumpfleger e.V. under Bodo Siegert’s lead.

Tree statics examinations by means of tensile tests are carried out on the basis of the tensile methods developed by Dr. Ing. Lothar WESSOLLY and Günter SINN. During the test a tree is exposed to a simulated wind load. Then, the practitioner examines in what way this wind load has an impact on the load-bearing capacity of the trunk and the anchorage force of the tree in the ground. The results provide an engineering-level, technically well-founded evaluation of the static condition of a tree.

In more detail, at first, the wind load is estimated to which a tree is exposed in case of a hurricane (wind force 12 Beaufort) in consideration of the relevant topographic factors. Then, a simulated wind load is introduced in the trunk by attaching pulling equipment and a rope to the tree and pulling the tree in a controlled manner in individual load steps until a specific limit is reached. The reaction of the tree to this impact of force is strain (i.e. compression and extension) of the trunk’s edge fibres that are digitally recorded by means of strain sensors. At the same time, the base of the trunk tilts. This phenomenon is measured by means of inclination angle sensors.

The measuring data obtained and the basis data of the tree are input in TSE and evaluated by calculation. By projection of the recorded data and comparison with empirical measuring values (dynamic behaviour of green wood, natural tilting behaviour of trees) the stability and safety against fracture of a tree can be established for situations of large wind loads. The result of this entire process is a safety value that is used for prognosis of the stability and safety against fracture of a tree under hurricane-like wind conditions.