City Trees

Over the years, the criticism of the "mono-causal" VTA criteria developed into a mono-culture of SIA dominating nearly all tree-care conferences, jour- nals, and educational institutions in the area of tree safety. This trend is not only seen in Germany where the debate started but worldwide: even at an international tree-biomechanics conference in the U.S., for example, nine out of ten presentations criticized VTA (and drilling) and instead promoted one company's SIA and related diagnostic products. Mattheck and Bethge responded clearly to the criticism of Gruber, Wessolly, and others (2007; 2008) and claimed that VTA and the One-Third Rule are valid results of reliable scientific studies and that SIA is based solely on unproven asser- tions. In 2009, Fink (a widely respected forest pathologist) confirmed Mattheck's statements and clearly contradicted Gruber's positions as well. This debate left many arborists and experts in con- fusion; even in 2018, by far the most frequently asked question in my workshops around the world is, "Who is right: VTA or SIA?" This is fre- quently in conjunction with, "Can we still use the One-Third Rule for safety evaluations although most well-known experts now prefer and pro- mote SIA?" Thus, there is a need for clarification. Literature from the very few tree-biome- chanics scientists without economic inter- ests in the tree-safety-business (Spatz & Niklas 2013) brings two clear answers: 1. t/R=1/3 is an important turning point of the curve describing the load-carrying capac- ity of hollow cross-sections (Fig 3). 2. The SIA-calculation of breaking safety strongly overestimates the load-carrying capac- ity of hollow trees, partially because the wrong formulas are being used (Fig. 4). Thus, the scientific aspects are very clear (and explained in more detail in: Rinn 2018b). But when applying this knowledge about the >> Figure 3. The blue line in this graph shows how the relative load-carrying capacity (% LCC) of a cross-section as represented by the "section-modulus" depends on the ratio of shell-wall thickness divided by the radius. Starting at top left with an intact cross-section, the load-carrying capacity is 100%. Going right, the centrally located void gets bigger and the curve shows how this impacts LCC. Surprisingly not only to many arborists, with t/R=1/3, the central void covers approximately 45% of the cross sectional area but leads to a loss in load-carrying capacity of only about ~20%. From this point on, however, with further decreasing shell-wall thickness and increasing void size, the loss in load-carrying capacity increases more significantly than before. Because of that, t/R=1/3 is seen by many as a turning point at which to start worrying about stability. But this is, as explained in the article, only valid for trees still growing in height. Spatz and Niklas showed (2013) that the real load-carrying capacity (red dotted line) is significantly smaller as the section modulus (used by the SIA concept) suggests as soon as the shell wall gets significantly thinner than 1/3 (depending on void length and other specific properties), mainly due to shear and torsional stresses. www.urban-forestry.com 33

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