Effect of tree shape and green wood properties on tree stability

Abstract

Two urban tree species in Singapore with distinct tree shapes, i.e. Saman Samanea and Khaya Senegalensis were investigated for stability in regards to uprooting and stem breakage failure, under a wind gust loading condition during thunderstorm. The trees were modeled based on basic shapes extracted from 3D laser scanning and modeling of the actual trees. The tree models were analyzed in Computational Fluid Dynamics (CFD) simulation to determine drag coefficients caused by the wind. The drag coefficients were 0.55 and 0.53 for Saman Samanea and Khaya Senegalensis, respectively. It was found that over-simplification of tree models may lead to underestimation of the wind force. A portable 4-point bending test machine was specially designed and fabricated to determine green wood properties of tree branches such as modulus of rupture (MOR) and modulus of elasticity (MOE). Khaya Senegalensis performed better than Saman Samanea with regards to stem breakage. Consequently, the drag coefficients and MOE were the inputs in the numerical modeling of the tree-root-soil system. The soil experienced bearing failure when critical wind speed exceeded 44m/s and 42m/s for Saman Samanea and Khaya Senegalensis, respectively. The latter was found to perform poorer with regards to uprooting. In conclusion, the computer simulation, CFD and numerical modeling, is a reliable method to investigate tree stability. Possible improvements of this method include incorporation of more detailed tree architecture in the tree models, coupling of wind-leaves-tree-root-soil systems and optimization of bending test machine for larger sample.