Fracture Mechanism Of Trees By Wind And Snow Disaster And Risk Assessment And Cabling,Bracing And Other Support Systems For Trees

Wind and snow disasters can not only have a significant impact on the stability of forest ecosystem,but also bring huge economic losses to the production of forestry.On the other hand,some tall shade trees of the city and ancient and famous trees in the scenic area have varying degrees of potential safety problems after a long-term wind and rain baptism.They will fail under bad weather and can damage to people and property and causes economic loss.Global climate change models show that all kinds of extreme weather will increase in frequency.Therefore,it is important to study the tree failure under wind and snow for risk assessment and safety protection of shade trees and ancient and famous trees.Also,it can promote the basis research of forest ecology and forest resources protection.In this paper,the Pinus hwangshanensis and Cinnamomum camphora were selected as testing materials to study the mechanics characteristic and fracture behavior of trees under wind and snow.The theoretical derivation and the integration of experiment and practice were adopted.The main works were as follow:1.First introduced the characteristics of the wind and snow,including the composition of the wind,wind speed and wind direction,the types of snow: dry snow,wet snow and freezing rain.The trees response under different characteristics of wind and snow disasters was also studied to help understand the tree fracture,risk assessment and protective measures.2.The material mechanical properties of green wood were tested.The critical strength of trees strength criterion was determined by testing the material strength of green wood.In this paper,the digital speckle method was adopted to test the elastic coefficient of the green timber,because there were some problems of the electrometric method to measure the elastic coefficient for green lumber.First,the strain gage was hard to glue on the surface of wood specimen due to the high moisture content;second,the elasticity modulus of the resin glue which bigger than the elasticity modulus of wood leaded the distortion of deformation of strain gage.The finite element method was adopted to analyze the maximum bending strain and torsional shear strain of the trunk and determine the trunk damage behavior and manner.The Γ type hollow trunk mechanical model was established by ABAQUS.The F force was applied at the free end.The model base was set as fixed constraint.The results showed that the critical hollowness of tree breakage was 0.7.That was,when the trunk hollowness wasbigger than 0.7,the bending and torsion failure coefficient increased significantly.For a tree with a certain degree of asymmetric crown,the trunk would split firstly along longitudinal direction,then the stiffness was dramatically reduced and overall breakage would happen,when the trunk hollowness increased.3.A new analysis theory and method of tree stems strength was presented and the method of testing tree trunk deformations was first proposed in this paper.It could measure the maximum bending strain and torsional shearing strain of a stem under combined deformation state at the same time.And the mathematical expressions of the measured strains were not affected by the wind direction.To validate the measurement approach,tree-pulling tests were performed to model wind forces.After the bending strain and torsional strain were measured,the strains were converted to stresses by Hooke’s Law and compared with the theoretical results.The results demonstrated that the experimentally measured stress values were in good agreement with the theoretical values.Then,the maximum bending strain/stress and torsional shearing strain/stress of a camphor stem in real wind was measured based on the above method.Meanwhile,an anemorumbometer was set in front of the sample tree to measure wind speed and direction simultaneously.The results showed that for a gust with multiple peaks,the sway motion of tree was very complex duo to the dynamic interaction of branches.It was difficult to find a consistent relationship between trunk stresses and wind speed square.For a gust that the wind speed continued to increase to a maximum value and then decrease,moreover,there was a period of no wind or soft wind time before the single wave gust occurred.The trunk stresses and wind speed square had a consistency linear correlation.Furthermore,the linear relationship established in other different time based on the single wave gust had a consistency slope K.When put the material strength into the above linear relationships,we could calculate the critical wind speed necessary to break trees.This would provide important help to tree protection under strong wind.4.The strength defect of trees was summed up.And,it could be classified into 5 groups:hollow trees,decay,deadwood,crack and structure defects.The stress wave tomography and resistograph were used to check the hidden decay hollow trees.5.The elastic cabling and bracing protection method was put forward.It combined with tree mechanics,tree structure and external environment.Furthermore,some cabling and bracing examples of ancient and famous trees were listed.The self-developed elastic bracing rods which achieved good effects in practical application were also introduced.