Modal analysis and extraction method for the frame structure considering soil-structure interaction effect

Abstract

For a soil-structure interaction (SSI) system, the direct method can analyze the dynamic characteristics of the arbitrary complicated soil deposits. However, to determine the radiation damping effect on the structural dynamic characteristics, the complex modal analysis using the direct method needs to calculate many complex modes for obtaining the few exact structural modes due to a large number of soil degrees of freedom. The needed structural modes among a large amount of soil modes are difficult to be extracted. In this paper, a new complex modal analysis method using sequential eigenvalue-shift is developed for the calculation of the modal parameters of the frame structure under SSI effect. Accordingly, a new index of the local modal mass ratio is proposed to quickly and accurately extract the structural modes from many combinations of modes from structures and soil. Several comprehensive examples are presented to validate the proposed method, and the influence of artificial boundary on the natural frequency, mode and modal damping ratio of the structure is investigated.