The Research On The Free Vibration Characteristics Of Tree

Road trees located in the city not only have effect s of purifying the air and reducing PM2.5,but also add luster to a city.However,coastal cities suffer higher frequency of strong winds / typhoons every year,which not only has an impact on the safety of building structures,but also causes the damage to urban roadway trees and the consequent loss of personnel and property.According to a large number of investigations,it is found that the damage patterns of trees in strong winds / typhoons are mainly divided into two forms,trunk breaking and lodging.S tudying the dynamic characteristics of trees is of great pract ical significance for reducing the damage of trees in coastal areas during strong winds / typhoons.Therefore,in this paper,the model experimental test and the finite element comput ation model analysis are carried out by taking camphor trees as the resea rch object.The main research contents and conclusions are as follows:The field model experimental test for a camphor tree at a campus in Changsha City was carried out by the method of force-hammer excitation.The model of the trunk in X and Y directions(X is along the west and Y is along the south)were acquired on field,simultaneously,the first three order vibration of measure d model on field are identified firstly.The actual measurement vi bration frequencies in X direction are all higher than those in Y direction.The first order frequencies are about 5.5 HZ,the second order frequencies are about 10 HZ and the third order frequencies are about 30 HZ.By adopting ANSYS finite element software and applying the bottom-up modeling method,the 3D finite element analysis model for measuring the camphor tree structure was established.In order to reflect the influence of branches and leaves on the structure of the whole tree,two different f inite element analysis models were established.In this paper,the first 34 order vibration modes of two finite element analysis models in the X and Y directions were extracted.In order to identify the first three-order calculated modal shapes of the whol e tree and timber structure which corresponds to the measured results,this paper defines the equivalent processing method of the calculated modal results.By applying this method,we obtain the centroid modal shapes of the first 34 orders of the two finit e element analysis models normalized in the X and Y directions.F inally,we identify the first three-order calculated modal shapes of the whole tree and timber structure corresponding to the measured results by combining the modal participation coefficients.This paper verifies the correctness of the finite element model through the similarity(MAC method)between the vibration modes calculated by the two finite element analysis models and the vibration modes measured by camphor tree.By comparing frequencies between the actual measurement model results and the computed model results {(finite element-actual measurement)/actual measurement)},the actual measurement results are more close to the Model Two analysis results(except for the second order in the Y direction).The minimum relative difference between the actual measurement natural vibration frequency and the computed natural vibration frequency is-8.43%,of which the maximum is-20.64%.The relative difference of the other orders is around 15%.By comparing the relative difference between the actual measurement natural vibration frequencies and the natural vibration frequencies of two finite element analysis models,it can be concluded that branches and leaves have great influence on the natural vib ration frequencies of tree structures.The reasons for the discrepancy between the finite element model computed results and the actual measurement results are summarized,which can be useful for acquiring better results.