Preliminary results of a novel approach to assess the fundamental sway frequency of trees

Trees in the urban context provide several environmental services. The current climate change crisis is seriously compromising the health of trees in urban and periurban areas leading to an increment of tree collapses. As a consequence, there is high risk of damage to urban infrastructures and risk for the safety of the people. Preventing tree failure and assessing the tree stability is a current challenge to maintain the ecosystem services provided by the trees in urban context but at the same time to guarantee the well-being of the people. Several methods and instruments are used to assess the stability of trees, among them, the dynamic analyses methods monitor the behaviour of the tree considering it as an object that sways in a dynamic manner. That behaviour can be described with several dynamic properties like the fundamental sway frequencies, a key-property to understand the motion of the tree under wind loads. The purpose of this paper is to present a novel approach to measure the dynamic response of a tree. The method is based on the assessment of the sway fundamental frequency under free vibration without notable external forces, using a non-destructive method consisting of high-sensitivity accelerometers. The method used to rigidly attach the sensors without both provoking any damages to the tree and compromising the data acquisition was improved several times. Due to the thickness of the bark of one sample in the first test the method didn't correctly measure the fundamental sway frequencies and a few enhancements have been made before the other tests. In the second test each tree was tested using three accelerometers fixed on the surface of the tree through a 3D printed support, a couple of omega shape steel bars and a ratchet strap. The technique was then tested on seven different open-grown broadleaf trees. Results of the spectral analyses shows that a reliable estimation of the peak values under free vibration and without any artificial excitations can be achieved. Moreover, the experimental design of the method allows to monitor the fundamental sway frequency rapidly and without provoking damage to the bark or the sapwood of the tree.