Application of SIA
Trees can be assessed to have sufficient safety margins if, taking all defects into account, the ratio of load-bearing capacity to wind load is at least 1.5 (Wessoly 2014). Especially for old trees, whose height development stops whilst trunk diameter continues to increase, the basic safety factor is often many times higher than 1.0. Therefore, even with pronounced cavities and trunk damage, a residual safety factor of well over 1.5 often remains.
An SIA can be performed by using online input templates, e.g. the SIA calculator freely provided by the SIM group or other providers. Based on the tree species, crown shape, measured tree height and the trunk diameter, the minimum required trunk diameter and the basic safety factor assuming a defect-free cross-section are determined.
Alternatively, diagrams/printed graphs can be used (Wessoly & Erb 2014). The basic safety factor assuming a defect-free cross-section are first determined. Following this further graphs can be referenced to derive the necessary residual wall thickness, following decay, that the trunk cross-section must have (in the plane under consideration) to ensure fracture resistance during the threshold storm event by maintaining a minimum safety factor of 1.5.