Wind-induced torsional vibration in a ponderosa pine tree

Abstract

The overall response of above-ground tree parts to wind loading consists of torsional and bending vibrations. While bending vibrations have been frequently measured in previous studies, measurements of torsional vibrations are scarce. Therefore, three-dimensional tree response measurements were conducted along the stem and on several branches of a ponderosa pine (Pinus ponderosa) tree. The measurements reveal that tree parts respond to external forces with torsional and bending vibrations even at low wind loads. As wind loading intensifies, the contribution of torsional vibration to the overall tree response rapidly diminishes, stabilizing at a lower level Concurrently, the share of bending vibrations becomes more prominent, accounting for nearly the entire tree response. Torsional vibrations are most pronounced in the crown space and more localized than bending vibrations. Under the wind loading conditions during the study period, they were hardly measurable in the lower stem parts. If one aims to quantify torsion in above-ground tree parts, multiple measurements are necessary depending on the research objectives. The results also indicate that depending on the wind load, one- and two-dimensional measurements of the wind-induced tree response made in lower stem parts are only sometimes suitable to capture the overall tree response to wind loads.