| Becoming a new task in civil engineering, Structural Health Monitoring (SHM) brings many challenges and opportunities to civil engineers. Enclosing damage diagnosis, this pape studies modal experiment in ambient excitation, optimal placement of sensors and structural damage alarming using wavelet packet energy spectrum.Modal experiment is very important in verifying model design, establishing the dynamic model and evaluating the function of structure. It becomes an important part in construction finish examination and SHM. The traditional modal experiment needs to excite the structure with impulse force, which is difficult to realize. Operating Deflection Shape, a new method only using responses of structures in ambient vibration, is widely used in large structures. Firstly, the relationship and differences between the traditional modal experiment and Operating Deflection Shape are introduced. Then based on the hypothesis of ambient vibration, the characters of large span bridge and the vibration measurements equipment, Operating Deflection Shape with Measurement Set Method is pointed out as the more commonly used acquisition method. It is used in Suspension Bridge in Yangtze River of Runyang.The optimal number and location of sensors are focused on for the importance of online modal monitoring. The successive sensor placement method based on QR factorization is introduced and used in truss bridge and Runyang cable-stayed bridge module.Finally, the steps of the structural damage alarming using wavelet packet energy spectrum is introduced, which includes the selection of energy spectrum, the selections of number of wavelet packet decompose and selections of wavelet function, etc. Rungyang cable-stayed bridge module is used as an example to study the effectiveness, sensitivity and noise robusticity of the index of damage alarming. Results show that the vector of ERVD has the function of damage location when there are a large amount of damage samples.
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