Wind speed increases exponentially with increasing height above ground level. Therefore, tree height plays a significant role in estimating wind load. This is exacerbated as the force is amplified through the trunk, which acts as a lever creating a moment which is at a maximum at the base of the tree. Thus, as the height of the tree increases, wind load increases due to the increased wind speed in combination with the increased bending and overturning moment.
Since the force introduced into the tree from the wind occurs mainly via the crown area, its distribution over the height of the tree must be evaluated. This is usually achieved through taking a photo of the tree from the direction of the greatest wind load. However, trees do not face the wind with rigid, impermeable canopy surfaces. Tree crowns are permeable, yielding structures whose leaves and branches deflect and deform when exposed to the wind. This is accounted for by applying a typical drag coefficient (Cw value), reflecting the typical characteristics of a tree species (Wessolly & Erb 2014).
The shape of a tree and the structure of the crown also affect a tree’s vibration characteristics, which can be incorporated ito an evaluation using common models for describing natural frequency and damping. Whether, and how much, the oscillatory response of a tree contributes to the increase of peak loads is still under critical investigation (Schindler & Mohr 2019). In many cases, swinging up in the wind is used as a justification for applying a load-increasing gust response factor in dynamic wind load analysis. It is possible, however, that the dynamic response of a tree to gusty wind may actually result in reduction of the wind load transferred into a tree (Spatz et al. 2007, James 2013).
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James, Ken. 2013. „Die Dynamik der Bäume im Wind“. S. 79–86 in Jahrbuch der Baumpflege. Braunschweig: Haymarket Media GmbH & Co. KG.
Schindler, Dirk, und Manuel Mohr. 2019. „No resonant response of Scots pine trees to wind excitation“. Agricultural and Forest Meteorology 265:227–44. doi: 10.1016/j.agrformet.2018.11.021.
Spatz, Hanns-Christof, Franka Brüchert, und Jochen A. Pfisterer. 2007. „Multiple Resonance Damping or How Do Trees Escape Dangerously Large Oscillations?“ American Journal of Botany 94(10):1603–11.
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