Modal analysis of cable-tower system of twin-span suspension bridge

A three-dimensional finite element model is developed in order to analyze the free vibration characteristics of the tower-cable system of a triple-tower twin-span suspension bridge during the construction phase and right after the erection of the main cable. The dynamic characteristics of each component in the tower-cable system, the isolated side span, main span cables and free-standing towers, are first analyzed separately. The natural frequencies and the vibration modes of the isolated side span and the main span cables obtained from the finite element analysis closely matched the analytical solution from the linear free vibration analysis and verified the validity of the finite element model. The local natural frequency and global natural frequency were defined to categorize the characteristics of the free vibration. The calculation results show that not only does the tower-cable system maintains the information on the modal characteristics of each component in the system, but it also contains its own unique modal characteristics and other important information about the dynamics characteristics of the system. At lower natural frequency range, the swinging motion and in-plane motion are uncoupled. The coupled modal information of the towers and the cables are separated into two groups based on the natural frequencies of the vibration of the main component of the tower-cable system. Some additional natural frequencies and vibration modes are obtained from the finite element analysis depicting the dynamic interaction between the towers and the cables. Furthermore, it is observed that the lower order modes of side span cables couple with the higher order modes of the main span cables. Either in phase or out of phase, local or global modes, the tower-cable system exhibits many new coupled mode combinations that reveal useful information.