218 Spatz and Pfisterer: Mechanical Properties of Green Wood and Tree Risk Assessment Arboriculture & Urban Forestry 2013. 39(5): 218–225 Mechanical Properties of Green Wood and Their Relevance for Tree Risk Assessment Hanns Christof Spatz and Jochen Pfisterer Abstract. In a biological context, the mechanical properties as elasticity and strength of green wood, particularly as measured in the axial direction, influence the stability of trees against static loads (e.g., snow, ice, rain) and dynamic loads (i.e., wind). Extensive collections of data on mechanical properties are listed in three different catalogs edited in Canada, Great Britain, and the United States. A statistical anal- ysis shows that the density of the wood is a major predictor for the mechanical properties as measured in axial direction. In this respect, coni- fers from temperate zones and deciduous trees both from temperate and tropical zones do not differ significantly from each other. A com- mon, nearly linear relation between the modulus of elasticity and the density at 50% moisture content is found. Relationships between strengths in bending, compression, and shear and green wood density have ordinary least squares scaling exponents around 1.2, but can almost equally well be approximated by linear functions of wood density. Therefore, if the density of stem wood of a given tree is known from direct measure- ment and differs from the tabulated value, the values tabulated for mechanical properties can be corrected for by a simple rule of proportion. Pulling tests as tools for tree control are discussed with emphasis on how the method is based on the knowledge of the mechanical properties of green wood, and how wood density is measured. Key Words. Conifers; Deciduous trees; Elasticity; Green wood; Pulling Tests; Strength; Wood density. For many centuries dried wood has been used for construction and engineering purposes. For this, mechanical properties had to be known, particularly the elasticity and strength in the axial direction of the wood used. Knowledge of many quite dif- ferent mechanical properties of wood, like the speed of sound, the sound radiation coefficient, and the loss coefficient, was required for making good musical instruments (Wegst 2006). In contrast, less is known about the mechanical properties of green wood. Summarizing reliable data from the work in different laboratories over a period of up to 50 years, catalogs of mechanical properties of green wood from more than 300 species have been prepared by Jessome (1977, 55 species), Lavers (1983, 161 species), and in the USDA Wood Handbook (Kretschmann 2010, 195 species). Entries in the tables are the density of green wood and key mechanical properties, like the modulus of elasticity and the strength in bending, compression, and shear, mostly but not exclusively, in the axial direction. A first statistical analysis of the data from the Lavers catalog revealed that wood density appears as a primary determinant of mechanical properties of green wood. Both elasticity and strength—important for assessing tree stability—scale with exponents near or slightly higher than 1.0 (proportionality) with density (Niklas and Spatz 2010). This was affirmed by Pfisterer and Spatz (2012) who extended the analysis of the data for deciduous trees from the three catalogs. Includ- ed in the same paper is a critical review of the widely used “Stuttgart table of wood strength” for interpreting pulling test (Wessolly and Erb 1998; Brudi and van Wassenaer 2001), where the dependency from wood density is not apparent. In this communication the previous analyses were extended ©2013 International Society of Arboriculture by including the data for conifers in the statistical analy- sis of the entries from the three most important catalogs (Jessome 1977; Lavers 1983; Kretschmann 2010). This way, the mechanical properties for conifers from temperate zones and for deciduous trees from both temperate and tropical zones can be directly compared. A comparison between the three catalogs is also presented, showing that the catalogs mutually support each other. As a result of the statistical analysis, the authors present simple equations, which relate the mechanical properties of green wood, as measured in axial direction, to reference values of its density. The relations between the maximal bending strength, the yield strength, and the maximal compression strength, are also given. For a diagnosis of living trees, suspected of being unstable, pulling tests can be executed (Milne 1991; Wessolly und Erb 1998; Bruechert et al. 2000; Brudi and Van Wassenaer 2001; Peltola 2006). The following details how mechanical properties, like the modulus of elastic- ity and the strength properties, enter into the calculations of tree stability and why a reliable source of data on the mechanical properties is essential for the interpretation of the data of such tests. Special attention is also paid to the problem of estimating wind forces acting on trees. One of the goals of this communication is to extract and sum- marize some of the information hidden in the comprehensive tables presented by Jessome (1977), Lavers (1983), and Kretschmann (2010). Another is to draw attention to the wealth of informa- tion in these catalogs and to encourage arborists to use these well-documented and reliable data, in particular those from the easily accessible USDA Wood Handbook, in tree risk assessment.
September 2013
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