| The placement of sensors and the selection of index system can greatly affect the identifying results in the health monitoring and diagnosis of complex structures such as suspension bridges .In the resent years the scholars have done a great deal of reserch and got a series of achievement in the two fields, but because of the complication of health diagnosis and damage detection,the acheivement is just primary and there are also many theoretical and practical problems to be studied.Compared with the dofs of the structures, the number of sensors in monitoring and diagnosis system is finite, thus the measurement of structures' response is also incomplete, therefore it's impossible to find a decided and doubtless answer for the detection; considering the complicacy of damage cases and the uncertainty of working environment, it's important to adopt reasonable sensors' configuration and select fit healthy state indices in suspensions' health diagnosis.The configuration of sensors and the selection of indices is quite correlated in the damages detection. At the present the indices are generally grouped into two types by the data source: the indices based on the static data and the indices based on the dynamic data. The suspension bridge is a complex system constituting multi-materials and different components, the damages in different positions have different effect on the indices. It's difficult to identify all types of damages using single index. This paper presents a comparative study of the applicability of frequency changes ratio index, modal curvature difference index and static strain difference index for the three common damage types in the suspension bridges(girder damage hanger damage and cable damage) in the absence of noise based on finite element model modeled by ANSYS software. From the simulation results a practical damage detection methodology of suspension bridges is proposed in this paper by combination usage of those three indices.
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