Stability calculation

(Calculation of the anchorage strength of the roots by means of inclination of the trunk base)

From the stability calculations, conclusions for the behaviour of a tree in case of high wind loads can be drawn. Trees react on wind loads by slight inclination of the root ball. The root-soil compound shows elastic behaviour as a reaction on the wind load until a limit value is reached from which on the tree starts to tilt. This “root tilting” phenomenon is mainly caused by elongation of the load-carrying roots and through deformation of the soil and results in a non-linear function (generalised tilting curve according to WESSOLLY).

During the tensile test, wind loads are simulated by introducing a substitute wind load into the trunk. The minimal tilting of the root plate (in a 1/100° range) and the resulting inclination of the base of the trunk is measured and recorded by means of inclination angle sensors. The tensile force is increased gradually in individual load steps. In the event that during this simulation critical sensor limit values are reached either for trunk strain or for root tilting, the tensile force is limited accordingly.

Then, with the help of TSE the measured values recorded for the individual load steps are projected so that prognoses are obtained for behaviour of the tree for the assumed wind forces (generally hurricane force). These values are compared with empirical limit values representing the natural tilting behaviour of trees (generalised tilting curve). In this way, one can define whether or not the reaction of the analysed tree is within the “standard behaviour” on simulated wind loads. As a result of the calculations, a factor is obtained that illustrates the stability of the respective tree.