Continuing the discussion of the Haeyoon Miller tragedy, an article published in the American Journal of Botany (October, 2006) verifies the instability of eucalyptus trees and warns of liability issues in urban areas. The text of the abstract reads:
Tree stability in windstorms and tree failure are important issues in urban areas where there can be risks of damage to people and property and in forests where wind damage causes economic loss. Current methods of managing trees, including pruning and assessment of mechanical strength, are mainly based on visual assessment or the experience of people such as trained arborists. Only limited data are available to assess tree strength and stability in winds, and most recent methods have used a static approach to estimate loads. Recent research on the measurement of dynamic wind loads and the effect on tree stability is giving a better understanding of how different trees cope with winds. Dynamic loads have been measured on trees with different canopy shapes and branch structures including a palm (Washingtonia robusta), a slender Italian cypress (Cupressus sempervirens) and trees with many branches and broad canopies including hoop pine (Araucaria cunninghamii) and two species of eucalypt (Eucalyptus grandis, E. teretecornus). Results indicate that sway is not a harmonic, but is very complex due to the dynamic interaction of branches. A new dynamic model of a tree is described, incorporating the dynamic structural properties of the trunk and branches. The branch mass contributes a dynamic damping, termed mass damping, which acts to reduce dangerous harmonic sway motion of the trunk and so minimizes loads and increases the mechanical stability of the tree. The results from 12 months of monitoring sway motion and wind loading forces are presented and discussed.
The article then addresses the possibility of liability issues:
Trees, especially large ones, need careful management in urban areas where failure can result in loss of life or damage to property (Fig. 1). Serious liability issues can arise if there is a perception that poor or negligent tree care practices have contributed to tree failure.
It appears that a simple “static pull test” on trees located in urban areas would help identify instable trees. It is not clear if local and state governments have a policy of testing tree stability.
Read the entire article here: Mechanical stability of trees under dynamic loads. Kenneth R. James5, Nicholas Haritos and Peter K. Ades. Received for publication March 22, 2006. Accepted for publication July 27, Published October 1, 2006.