An effective fixed-base model with classical normal modes for soil–structure interaction systems

To simplify the analysis of soil–structure interaction systems, various fixed-base models have recently been proposed by the author to efficiently represent the SSI system and have been shown to have good accuracy. However, the modified mass and damping matrices of these models do not hold the properties of symmetry and orthogonality. Difficulties may consequently be induced for these models in applying conventional computer codes to carry out dynamic analysis. In the present paper, this problem is further explored to establish a fixed-base model possessing classical normal modes. Formulated in the modal space, this fixed-base model is constructed through applying an iteration algorithm to incorporate the Gram–Schmidt orthogonalization process. The convergent real orthogonal mode vectors, natural frequencies, and modal damping ratios are directly determined for this model. It is demonstrated with a numerical example that this new fixed-base model retains excellent accuracy. Accordingly, the complicated SSI systems can be directly analyzed using conventional computer codes for structural dynamics with the fixed-base model developed in this study.