Dynamic Loading of Trees

Abstract

The dynamic forces on tree structures during periods of high winds is being studied in order to determine the loads on trees and the responses of the trees to those dynamic loads. Field measurements of dynamic forces on trees, branches, and cables have been conducted on urban trees in an attempt to quantify the magnitude of these forces and to provide a basis for evaluating tree stability. Equipment was constructed to measure the dynamic wind loads on tree trunks and branches in situ. This equipment is described, and results are presented which indicate that tree structure is loaded by highly variable wind gusts and responds by behaving in a complex dynamic manner, which minimizes the energy transfer from the wind to the tree structure. The dynamic response of the tree involves a complex interaction of the natural frequencies of each component of the tree, including the trunk, main branches, sub-branches, and smaller sections. A dynamic model of trees is presented and includes mass damping that minimizes the sway energies and combines with the drag forces of the canopy to help the tree cope with large wind forces. A discussion of windthrow and tree dismantling is presented, based on the information collected from these studies.