Study Of Girder Bridge's Damage Identification Based On Modal Strain Energy Method

Because of various, factors, damage, which will lead to serious consequence, will inevitably appear in civil engineering structures. Therefore,it is necessary for engineering structures be monitored for health. Theory and method of damage identification of structures is the research core of Structural Health Monitoring. The purpose of structural damage detection is to identify the location and the extent of structural damages based on the characteristics represented by structures. Based on the basic theory of element modal strain energy change and structure shape change before and after damage,we may obtain the changed rate of modal strain energy,and it is used as the damage index to identify structure damage.In this thesis the damage index is used to identify the damage in a simple supported girder bridge and a continuous girder bridge in the numerical simulation and calculation analysis. A decrease of stiffness is simulated by modulus of elasticity reduction, the mode shape is computed by MIDAS. The changed rate of modal strain energy computed by program which is designed by myself is used to identify the damage .The numerical results show that the damage location can be identified exactly for the first mode to the first five mode through identifying one or several damages, damages of different extent or damages of different places. The damage index of the damage location is bigger than that of the normal location, and the damage index would be increased using the same account mode to calculate as the damage extent gets larger. So this damage index can be used to qualitatively estimate the damage extent. The numerical results show that the effect of damage identification of a simple supported girder bridge is better than that of a continuous girder bridge.The thesis is emphasized on the resistibility of the noise with the element modal strain energy method, Moute-Carlo method is introduced in the thesis, the noise is added to the model to simulated the measured error of the model, enough numerical tests are carried out and identification algorithm procedure based on MATLAB is given to obtain all identified times and the changed rate of element modal strain energy in every test. Then we can obtain steady identified probability and the average diversification rate of element modal strain energy in different noise level and different damaged extent, according to which we may obtain their change law and come to know the change law about the rate of various element modal strain energy in damage of different extent and the same noise level.The numerical results effected by noise show that in the same condition, the more the modal combined, the better the effect of identification; the higher damage extent, the better identified probability; the higher damaged extent or noise level, the larger changed rate of damaged element modal strain energy. In different damaged extent, the relative changed rate of various element modal strain energy is basically the same to the condition without noise when damage identified probability is relatively larger.