| Abstract | SFT is a new type of long-span structure which is considered as an economic solution for large span crossing. However, its dynamic characteristics have not been clearly understood yet. In this study, the Rayleigh-Ritz method, one of energy approaches, is applied to formulate a mathematical model for three dimensional circular-section SFT with vertical tension anchoring. The contribution from surrounding water is taken into account by applying the modified Morison's equation. The obtained governing equations show that the dynamic characteristics of SFT in each vibrated direction: vertical, horizontal, torsional, and longitudinal vibrations, can be separately considered if the vibration is assumed to be infinitely small. Then the normalized linear governing equations of motion in each direction are obtained and the governing dimensionless parameters which control the dynamic behaviors of SFT are identified. The possible ranges of these parameters are calculated based on a SFT whose dimensions and physical properties are approximately evaluated by using the information from prior studies. Ten and twenty-span SFTs are considered and two types of tension leg, cable and steel pipe, are investigated. The modal analysis is then performed to determine modal frequencies, mode shapes, curvatures, torsional gradients, and rotational gradients of all directions. The natural frequency charts are established to show the changes of natural frequencies and other dynamics properties with the variations of the governing dimensionless parameters. The dynamic properties of vertical and torsional vibrations are found to be governed by the tension leg's stiffness, particularly in the vertical vibration. The natural frequencies of the vertical vibration are spaced very closely when the governing dimensionless parameter is large. In contrast, those of horizontal and longitudinal vibrations are found to be controlled by the contributions from tunnel's bending and axial rigidity respectively, and they can be approximated by using the dynamic properties of the tunnel without any intermediate support. The possibility of mode localization in the vertical oscillation is also investigated since SFT can be regarded as a periodic structure in this vibrated direction. The irregularity due to the non-uniform sea floor is taken into consideration. The dynamic properties of the irregular SFT are determined and the effect of mode localization on the dynamic response is evaluated by conducting harmonic load test. The localization occurs in the first three modes and these cannot be neglected since they give larger dynamic responses compared with those of the regular one. |