Numerical And Experimental Investigation On The Cable-damper System

The characteristics of small mass, high flexibility and low inherent damping in stay cables are more likely to make cables vulnerable to all kinds of vibration under environmental excitation and support motion. These transverse vibrations can lead to the fatigue of the cable or the breakdown of the cable corrosion protection system, and even endanger the safety of the bridge. Thus, all the cable vibrations including rain-wind induced vibration must be taken into consideration and mitigated for the long-span cable-stayed bridges in use or under construction. To dampen such vibrations, the implementation of transversely attached dampers is very popular in engineering practice. However, the precise design of mitigation measure for vibration and proper selection of damper require us to consider the effects of cable and damper parameters, suck as sag and inclination, damper coefficient, damper stiffness, support rigidity and damper mass, comprehensively. But the coupled effect of all these parameters has not been well studied, and the experimental verification is still inadequate. The primary research works in this paper are listed as follows.(1) By establishing the mathematic model of the cable-damper system, the effects of cable sag, damper coefficient, damper stiffness and damper location on this system have been analyzed. After establishing the model of a cable-damper-bushing system, the effects of damping coefficient, stiffness and location of the bushing on the system damping have been investigated. The results show that the rubber bushing near the deck will reduce the maximum damping ratio, while busing near the tower will increase the maximum damping ratio of the system, when the damper is located near the deck anchorage. Finally, based on the development of a mathematic model and the use of Galerkin method, a preliminary study on the strategy of using double dampers at opposite positions to control the cable vibration has been conducted. It’s found that the available damping ratio from using double dampers near pposite anchorages is approximately the linear superposition of that of using only one damper near the deck and near the tower.(2) Based on the introduction of’generalized universal formula’for modal damping of the cable-MR damper system, the effects of cable parameters and damper parameters on the maximum damping and the optimal damper coefficient have been investigated. Then, by adopting the mathematic model of the cable-damper system mentioned above, numerical verification for the’generalized universal formula’has also been made.(3)In laboratory, a18.75m-long stay cable, which is a1:31scale model of the longest prototype cable from Sutong Bridge, is set up for the experimental study. Modeling dampers have also been designed and fabricated for the experiment of cable vibration mitigation. Using the noncontact monitoring technique in videogrammetric method and the modal parameter identification technique based on elliptic filter and Hilbert Transform method, experimental study of cable vibration control, which involves the influence of damper coefficient, damper stiffness, support stiffness and damper mass, has been completed. Experimental results show that the cable sag mainly has effect on the first mode, and slightly affects the higher modes. The increase of damper stiffness and the decrease of support stiffenss will reduce the attainable damping ratio of the cable-damper system. Experimental results agree well with the results by the’generalized universal formular’. The experiments on the cable attached with both dampers near the tower anchorage and deck anchorage show that the enhanced damping is approximately equal to the linear superposition of that of using only one damper near the deck and near the tower.(4) Finally, with the’generalized universal formula’, which has been validated through experiments, the method and procedure are proposed for the design and optimization of MR dampers for cable vibration control. By taking an actual large-span bridge as an example, the MR damper parameters and installation heights are determined for providing enough damping to suppress the rain-wind induced cable vibration. With the function of Matlab GUI, an interactive-interface program for damper design has been developed.