A CALCULATION METHOD FOR DYNAMIC CHARACTERISTICS OF THE CANTILEVER-TYPE STRUCTURE IN OPEN SYSTEMS

Structures are always built on sites to form open systems of soil-structure interaction. To calculate natural frequencies, mode shapes and modal damping ratios including the influence of radiation damping for the cantilever-type structure in open systems, an impulse-load-based modal analysis method is proposed. In the method, the finite element model of soil-structure interaction is excited by an impulse load to obtain the dynamic response, and then the dynamic characteristics can be obtained by modal identification method.Then, a cantilever-type five-story frame structure is taken as an example to investigate the effect of soil region and excitation location on the dynamic characteristics. On this basis, the influence of soil material damping on the modal damping ratio is discussed, and the results are compared with those of lumped-parameter models and the direct modal analysis method. The numerical results show that, the dynamic characteristics of the structure would converge to exact values with the increase of soil region. When the size of soil region is two times wider than the wavelength corresponding to the fundamental frequency of the structure, the relative errors of structural natural frequencies are less than 1%, and the relative errors of modal damping ratios are less than 5%. Any location, as long as it is not the modal node, can be used as excitation location to get accurate dynamic characteristics. However, there are obvious errors on the modal damping ratios by the three lumped-parameters model, and the fundamental frequency would approach zero with the increase of the size of soil region using the direct modal analysis method. Compared with the radiation damping, soil material damping has slight contribution to structure modal damping ratio.