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The additional damping ratios provided by viscous dampers (VDs) and viscous inertial damper (VID), were tested in the laboratory to estimate the effectiveness of these dampers on mitigating the multimodal vibrations of stay cable. First, two plate eddy current dampers (PECDs), in which the damping coefficients can be conveniently adjusted, were specially designed and manufactured. The mechanical property of these PECDs was obtained by tests and numerical simulations. Second, the natural frequencies of a test stay cable were carefully determined by tests and numerical analysis, and the effect of cable sag was carefully analyzed. Third, a series of tests including 12 test cases were carried out to validate the effectiveness of a single VD, a single VID, and two VDs, in which PECD is a typical VD, an additional mass is attached on the test stay cable near the PECD and the output is similar to that of a VID. The amplitude and frequency of the vibration have little effect on the mechanical properties of PECD. The inherent damping ratios of the first 25 modes are in the range from 0.068% to 0.22%. It appears that low efficiency to simultaneously enhance the damping ratios of the first 25 modes can be found by adding a single VD and a single VID with the installation position of 4.75% l. However, two VDs installing on 2.75% l and 4.75% l has a significant improvement on the damping ratios of the 15th∼25th modes, and should be considered to a reasonable countermeasure to simultaneously reduce the low-order rain-wind induced vibrations and high-order vortex-induced vibration by raising the modal damping ratios. [ABSTRACT FROM AUTHOR] more...

Based on VIV time histories growing from still to limit cycle oscillations, the motion-amplitude dependence of the VIV loading model is determined. Energy-trapping properties corresponding to the entire process of motion evolution are described by the model parameters. The influence of nonlinear mechanical damping on the VIV loading model parameter is analyzed. Prediction of VIVs under higher mechanical damping ratios is performed by the identified VIV model. The results show that, while the model parameter 휀 varies drastically with the motion amplitude, the aerodynamic damping involves gently and monotonically towards a final value corresponding to an LCO state. Neglecting the nonlinearities in mechanical damping would result in significant deviations in model parameters and predicted VIV responses. By comparing the traditional fixed-휀 model with the proposed nonlinear-휀 model, it is found that the VIVs predicted by the former are about 1.5 ∼3.0 times those predicted by the latter or tested, which underlies the necessity of consideration of motion-amplitude-dependence of an aerodynamic model. [ABSTRACT FROM AUTHOR] more...

Stay cables of several cable-stayed bridges with a span of nearly 1000 m are confronted with the simultaneous actions of low-mode rain-wind induced vibration and high-mode vortex-induced vibration. It is difficult to effectively reduce these two kinds of cable vibrations by installing one damper at the end of the stay cable. In this paper, the effectiveness of two dampers attached at the lower end of the stay cable, including viscous dampers and viscous inertial mass dampers, is investigated by the finite difference method. First, the stay cable was simplified as an inclined cable with sag, and equation of motion governing the stay cable attached with two dampers was derived. This equation was then numerically solved by the finite difference method. Second, taking Cable A30 of the Suzhou-Nantong Yangtze River Bridge as a reference, a series of numerical simulations were carefully conducted to obtain the damping ratios of the first fifty modes. The results show that it is impossible to enhance the damping ratios of all of the concerned modes up to the minimum target value only by installing a single viscous damper or a single viscous inertial mass damper at the end of the stay cable. However, two viscous dampers installed at the lower end of the cable (l 1/ l = 2%, l 2/ l = 5%) can overcome the shortcoming of a single viscous damper or viscous inertial mass damper to enhance the damping ratios of the first 34 modes up to the minimum target value (0.5%). In general, the optimized relative location of the two dampers (l 2/ l 1) should be near 2. The optimized ratio of the damping coefficients (η 1/ η 2) should be lower than l 2/ l 1, and the lower limit of η 1/ η 2 is determined by the minimum target values for modal damping ratios. Moreover, it appears that a lower installation position of two viscous dampers can realize the mitigation of higher modes even up to the 40th ∼ 50th modes. [ABSTRACT FROM AUTHOR] more...