A method for predicting the possible site of failure in trees during mechanical loading

Abstract

In order to determine if it is possible to predict the site and mode of failure under mechanical loading, different tree species were subjected to bending tests. Four species were examined: European beech, English oak, Douglas fir and Norway spruce. All trees were of a similar age and size. Strain gauges were placed on the surface of the trunk and along the lateral roots. Strain gauges measure deformations on the surface of a material during loading. Trees were winched horizontally using increments of force of 0.25 kN, and strain was measured in each of the strain gauges for each increment of force applied. Resistance of species to mechanical failure was in the order: beech, oak, Douglas fir and spruce. Beech required over three times as much force to cause stem breakage, compared to spruce which generally broke in the root system. Oak also broke in the trunk whilst Douglas fir usually broke at the root/stem joint. In those species which broke in the trunk, strain was always found to be higher in the stem than in the roots during winching. However, in Douglas fir and Norway spruce, strain was always highest at the root/stem joint and in the root system, respectively. Root system architecture may influence the mode of failure as beech, oak and Douglas fir all have deeper growing roots than spruce, and they all broke above ground level. However, a more detailed study of tree architecture and wood properties would be needed to verify this hypothesis. It appears that through the use of winching tests and strain measurement, it may be possible to predict how and where a tree might fail under mechanical loading. More species and trees of different ages and in different states of health would need to be examined using this method.