Natural sway frequencies and damping ratios of trees: influence of crown structure.

Moore, John R and Maguire, Douglas A (2005) Natural sway frequencies and damping ratios of trees: influence of crown structure. Trees - Structure and Function, 19 (4). pp. 363-373. ISSN 0931-1890

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Natural frequency and damping ratio were measured for nine plantation-grown Douglas-fir (Pseudotsuga menziesii Mirb. Franco) trees from the Oregon Coast Range under different levels of crown removal. Natural frequency of trees, in both their unpruned and completely de-branched states, was linearly related to the ratio of diameter at breast height to total tree height squared (i.e., DBH/H2), as expected from the theory governing the oscillation of a cantilever beam. Pruning resulted in an increase in natural frequency; however, at least 80% of the crown mass needed to be removed before this increase was noticeable. A single equation was developed that enabled the natural frequency of a tree of given size and pruning intensity to be predicted. Damping ratios of unpruned trees varied considerably from 8% to almost critical, while those for completely de-branched trees ranged from 1% to 8%. Two different trends in damping ratio were observed during pruning. Some trees exhibited an increase in damping ratio with initial crown removal, followed by a sharp decrease when the uppermost portion of the crown was removed. Others showed little or no change in damping ratio followed by a sharp reduction upon removal of the uppermost portion of the crown. Damping was mainly due to aerodynamic drag and preventing interference with neighbouring trees had little effect. Theoretical analysis using the finite element method indicated that changes in natural frequency as a result of pruning are not due to changes in damping ratio, but rather changes in mass distribution. This analysis also suggested that treating branches as lumped masses rather than individual cantilevers attached to the main stem may not be appropriate.

Item Type: Article
Print ISSN: 0931-1890
Electronic ISSN: 1432 2285
Uncontrolled Keywords: Pseudotsuga menziesii Mirb. Franco; Douglas fir; damping; crown removal;
University Divisions/Research Centres: Faculty of Engineering, Computing and Creative Industries > School of Engineering and the Built Environment
Dewey Decimal Subjects: 500 Science > 580 Plants (Botany) > 582 Plants noted for characteristics & flowers > 582.16 Trees
600 Technology > 620 Engineering > 624 Civil engineering
Library of Congress Subjects: S Agriculture > SD Forestry
T Technology > TA Engineering (General). Civil engineering (General)
Item ID: 1931
Depositing User: RAE Import
Date Deposited: 19 Jun 2008 10:30
Last Modified: 16 Apr 2013 15:51

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