Test And Analysis Of Wind-resistance Ability Of Green Tree Under Wind Load

Wind disasters often bring damage to forests,and stem breakage or tree uprooting may occur under wind loads.However,the existing methods which are used to assess and predict wind damage of trees are often due to the limitation of model parameters mostly applied in standard forests in Europe,but less reported in the Asia-Pacific region.Moreover,wind is an irregular air movement.The theoretical and experimental methods to analyze the tree strength under oriented load are not applicable to the situation that the wind direction rapidly changes.In fact,the breakage/ uprooting of trees under wind load is a mechanical behavior,which occurs when the load on trees exceeds the limit state of tree stem or when the bending moment at the stem base is greater than the resistance moment.If the maximum stress on the stem or the critical tilt angle of the stem base can be determined under different wind speeds,the stability of an individual tree can be monitored in real time and the critical wind speed when it is damaged can be predicted.In this paper,the dynamic relationship between tree stability and wind load based on mechanical method was studied,and the dynamic relationship model between the stress on the stem/ the tilt angle of the stem base and the squared wind speed is established.In order to verify the validity of the model,in the two typhoons,the stress/strain of Chinese fir(Cunninghamia lanceolata)and the camphor tree(Cinnamomum camphora)were respectively tested by strain meters in summer,2018.And the critical wind speed induced stem breakage was predicted.In the typhoon season of 2019,the strain/ stress on stem of street trees in Shanghai was measured by tilt sensors,and the critical wind speed induced stem breakage of Platanus(Platanus hispanica)was predicted.According to the output value of the tilt sensors of the stem base,the critical wind speed induced tree uprooting was preliminarily predicted.In terms of the three experimental results,in the range of measured wind speeds,the prediction range of critical wind speed induced stem breakage of Chinese fir is:36.4-45.1m/s;the prediction range of critical wind speed induced stem breakage of camphor tree is: 34.0-39.3m/s;the prediction range of critical wind speed induced bough breakage of Platanus is: 28.2-29.9m/s;the prediction value of critical wind speed induced uprooting of Platanus is: 48.7m/s.Due to the fact that there is a good linear relationship between the squared wind speed and strain/stress or tilt angle of the stem base in the range of measured wind speeds.This relationship can be used to predict the wind-resistance ability of an individual tree.With respect to the model value,because the stress on the stem and the tilt angle of the stem base and the wind speed are calculated by the sensors,the model was of high accuracy and could be used in different forests.Meanwhile,the data obtained by this method are all measured in real time,then any tree can be monitored in real time through computer technology(for example,the stability monitoring of Greeting Pine in Mount Huang),and an alarm can be given when damage may occur.If the model will be applied in a forest,it is necessary to divide a forest into several blocks,then setting test points in each block,and obtaining vulnerable areas in the forest by predicting critical wind induced trees damage at different positions,so as to provide scientific and reasonable forest management guidance for forest operators,and provide decision support for enhancing forest resistance to wind disasters.