Modal Identification of Boiler Plant Structures Using Seismic and Microtremor Records

This paper deals with a modal identification method for large-scale structures such as boiler plants in thermal power stations. Practical and accurate modal identification has been carried out using seismic and microtremor records observed in boiler plants in operation. Based on the multi-input/output identification method, 3-D mode shapes were identified from the microtremor records. They are in good agreement with the results of the FEM simulation. Identification using seismic records was then executed by the following two stages in order to evaluate the amplitude dependence of modal damping ratios : ( 1 ) processing frequency transfer functions by the autoregressive (AR) spectral analysis, and ( 2 ) an extraction of modal parameters by a curve-fitting technique. Introduction of two techniques, namely, decimation of time data and FPE criterion to optimize the order of AR models, have realized effective and accurate identification. As a result of this study, our modal identitication method has proven to be effective for seismic modeling of large scale structures.