Arboriculture & Urban Forestry 45(6): November 2019 Arboriculture & Urban Forestry 2019. 45(6):259–269 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & Boulevard Tree Failures During Wind Loading Events By Gary Johnson, Chad Giblin, Ryan Murphy, Eric North, and Aaron Rendahl Abstract. Wind loading events vary in their intensity and degree of damage inflicted on urban infrastructure, both green and gray. Damage to urban trees can begin with wind speeds as low as 25 miles per hour, especially when those trees harbor defects that predispose them to struc- tural failures. The tree damage triangle integrates the three main factors that influence tree failures during wind loading events, namely the site characteristics, the (wind) loading event and any defects of the trees in question. The degree of damage that trees experience is generally a func- tion of these factors overlapping each other. For instance, when the potential damage from wind loading events is exacerbated by poor tree architecture and compromised site conditions, the likelihood of significant damage is realized. Two studies on the damage to urban trees and the predictability of damage are reviewed; one study is a long-term gathering of wind loading events and accompanying damage to trees while the other is a case study of one storm in one city on one day. Both studies revealed critical pre-existing conditions that left trees vulnerable to whole tree losses: large trees in limited boulevard widths and severed roots as a result of sidewalk repair. Keywords. Construction Damage; Tree Architecture; Wind Loading Events. 259 INTRODUCTION When winds and trees collide, sometimes some trees fail; rarely do all trees suffer failures. Failures may be catastrophic, where entire trees are uprooted or broken and cause signif- icant damage or injury while other failures may border on insignificant, resulting in little to no damage to the tree or nearby targets. The range of damage can be explained to a great degree by the range of critical factors affecting fail- ures and their degrees of impacts. An equilateral storm damage triangle (Figure 1) provides a model to visualize the integration of the three major factors or sides of the tri- angle, to wit: the loading event, site characteristics for where the tree is located, and any tree defects that predis- pose the tree to failures. These three impact factors have been identified together or separately before by other authors (Mitchell 1995; Lopes et al. 2008; Schindler et al. 2012 ), and in fact mirror the variables for assigning levels of risk to urban trees, namely “Site Factors,” “Load Factors,” and “Tree Defects and Conditions Affecting the Likelihood of Failure” (Smiley et al. 2011). Each variable on its own can account for the potential of tree or tree part failure, but when the storm tri- angle is used as a holistic approach to predicting wind loading damage to trees and the extent of said damage, it becomes a more valuable tool for managing trees in urban spaces. Loading events, in this case wind loading events, are relative factors. The most commonly referenced wind loading scale relative to potential damage to trees is the Beaufort Wind Scale, conceptualized by Sir Francis Beau- fort, U.K. Royal Navy in 1805 (Table 1). Based on wind speed, the scale describes impacts on trees assuming the trees are not structurally defective. When tree defects such as codominance, branch attachments with included bark, decay, stem-girdling roots or compromised root plates are factored in, the tree would then be considered structurally defective to some degree (Pokorny et al. 2003). The direction of the wind loading event has also been Figure 1. Components of the Storm Triangle. shown to influence the extent of damage to urban trees (Lopes et al. 2008; Lopes and Fragoso 2009). Most wind loading events that cause tree damage occur during the ©2019 International Society of Arboriculture
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