Modal properties of a structural system are evaluated conventionally by using the measurement records of input forces and output responses of the structure. However, it is very difficult to measure the time-varying input force or of existing structures under the natural operational loading conditions. Therefore, it is desirable to develop a method for the estimation of modal properties based on the response records only. One of the possible methods is the one using the random decrement technique.
In this paper, the random decrement technique is studied for the purpose mentioned above. The study focuses on the estimation of the natural frequencies, modal damping ratios and mode shapes of structures with multi-degrees of freedom. The effectiveness of the method is investigated for the cases with different response components measured, such as displacement, velocity and acceleration records of the structures. Studies are carried out for various input forces with different frequency contents. From numerical example studies based on the measurement data artificially generated using the Monte Carlo simulation technique, it has been found that the random decrement technique is a powerful tool to estimate modal properties of structures subjected to random loadings.
