Mechanical behavior of coniferous trees subjected to wind loading Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rf55zb48x

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  • The behavior of coniferous trees subjected to wind loading was investigated through a series of experiments and also through simulation modeling. Previous studies that measured the natural oscillation frequencies and damping ratios were reviewed and equations were developed for predicting the natural frequency of a tree from the ratio of diameter at breast height to total tree height squared (i.e., DBHJH2). Experiments were performed on nine plantation-grown Douglas-fir (Pseudotsuga menziesii Mirb. Franco) trees to quantify the effects of crown removal on natural frequency and damping ratio. Results showed that pruning increased natural frequency; however at least 80% of the crown mass needed to be removed before this increase was noticeable. This effect was consistent across all trees and a single equation was developed that enabled the natural frequency of a tree of given size and pruning intensity to be predicted. The dynamic behavior of three Douglas-fir trees was modeled with the finite element method (FEM). Bole and branch geometry were measured in detail for each tree, enabling the first order branches from each tree to be represented as individual cantilever beams attached to the bole. Predicted natural frequencies of the three trees were in close agreement with observed values, and both natural frequencies and damping ratios were very sensitive to branch modulus of elasticity. Damped free vibration tests were conducted on two instrumented Douglas-fir trees by displacing the bole and measuring the oscillations of the bole and selected branches. Results showed that branch oscillation frequency was similar to the natural frequency of the tree, but different from the natural frequency of the individual branches themselves. In this situation the branches behaved as forced damped harmonic oscillators with the forcing frequency equal to the natural frequency of the whole tree. Measurement of the oscillations of selected branches during strong winds indicated that branches primarily oscillated at frequencies below their natural frequency. Some higher frequency oscillations were observed, however these may be noise in the data. Results did not appear to support the theory that branch natural frequencies are similar to the first harmonic of whole tree natural frequency.
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