"correct" One-Third Rule to mature urban
trees, reality starts getting complicated and
forces us to change our perception.
Arborists trying to practically apply the One-
Third Rule quickly realize something. Compared
to the common slender forest conifer with a
centrally rotten zone in a stem with a more or
less circular cross-section, the typical mature
urban tree to be inspected in terms of safety is
much more challenging (Fig. 5). We note that:
• The cross-sections of the trunks at the stem
base are commonly not circular, and
• The defects are mostly not located in the cen-
ter (of an irregularly shaped cross-section).
As a result, the One-Third Rule simply cannot
be applied because there are hundreds of dif-
ferent t/R values in the same cross-section; one
of them alone cannot adequately represent
changes in breaking safety due to defects.
Currently, the only non-destructive way to estimate
the loss in load-carrying capacity of a given trunk
cross-section due to defects is by tomography. This
does not mean that a sonic tomograph (patent: Rinn
1999; 2014b), with technology available from sev-
eral companies, has to be applied. In many cases,
a visual assessment is sufficient and the cross-sec-
tion can then be "painted" manually on a smart-
phone for an instant determination of the loss in
load-carrying capacity for all wind-load directions.
Sometimes, a resistance drill (Rinn 1988, 2012, 2016;
patent: Rinn 1990) can help to assess the location
and extension of voids and decayed areas more
precisely. However, this only works when it is clear
what the measured profiles really mean in terms
of wood condition. This is not the case for many of
the 20 different types of resistance drills brought to
the market since 1987 by at least six different com-
panies; for only very few of these resistance drills
is a significant correlation between the obtained
profiles and wood density shown (Gao et al. 2017).
Figure 4. This free SIA Tree Stability Assessment online form determines the so-called "basic
stability" (breakage safety factor of the tree with an intact stem) as well as the "medium
required residual wall" for sufficient safety. Using this form with the oak tree shown in Fig. 2
with a total height of more than 25 meters (75 feet) and a stem diameter of 2.5 meters (100
feet), the minimum wall thickness for sufficient safety has to be 1 cm (0.4 inches) according
to SIA. This obviously incorrect result is a consequence of inappropriate reference values
and the fact that the section modulus calculation does not correctly reflect the load-carrying
capacity of thin-walled wooden tubes (Fig. 3).
34 CityTREES
