Output-Only Dynamic Experimental Investigation of a Stand-Alone Trees

The load-bearing capacity and stability of a tree depend on several factors, among which: size and shape of the trunk, material properties of the wood (modulus of elastic and strength), root-soil mechanical interaction, etc. As the tree grows, the trunk grows and branches and roots increase in diameter and architecture thus expecting that its load-bearing capacity together with global stability increase. At the same time, the growth of trees may be accompanied by a decay of stiffness and strength of the wood and anchoring capacity of roots with a consequent reduction of its load-bearing capacity and stability. By using an engineering approach, since the tree can be modelled as an evolutionary mechanical system, useful information about its load-bearing capacity can be obtained by monitoring its dynamic properties over time (e.g. trunk and branches structural frequencies). Moving in this direction, however, there is still a lack of experimental data that allow to correlate the physical-mechanical decay of wood stiffness and strength of the tree with the changes of its dynamic properties. This paper aims to contribute to cover this issue by reporting the results of three-days continuous monitoring performed on a tree over two periods (March 2-4 and June 2-4). In the first period the tree had no leaves, while in the second period it had leaves. To perform the experimental tests, 8 seismic high-sensitivity accelerometers were employed and acceleration time histories of selected sections of the tree under ambient vibration were recorded. In particular the collar, the diameter at breast height, and the tree fork of the trunk were monitored in order to obtain lateral and torsional vibration modes. The results confirm that it is possible to identify main frequency of the tree under ambient vibration and report variation of main frequency over the measurement period.