Theoretical And Experimental Studies On Vibration Control Of Stay Cables Using Magnetorheological Damper

Due to high flexibility, relatively small mass and very low inherent damping, stay cables of the long-span bridges are susceptible to vibration under wind loading, or wind incorporated rain excitation, or deck motion. Large-amplitude vibration of cables may reduce the life of cables and threaten safe traffic, and has been considered today as one of the most critical problems for this type of bridge. Therefore, investigation of mechanism and countermeasures of cable vibration is very important for construction and maintenance of the cable-stayed bridges. As semi-active control devices, magnetorheological (MR) dampers have an excellent variable damping property and their damping forces can be changed by adjusting the input of voltage to the dampers. With the aid of an appropriate control strategy, the semi-active MR system control for cables vibration can achieve much better damping effectiveness than the passive control system. This thesis is concerned with the modeling of MR dampers, and the semi-active control and the passive control for cables vibration by using MR dampers. Theoretical investigations, laboratory validation and field experiments are carried out in this study.1. A series of cycle vibration tests with varying response amplitudes and different frequencies are first carried out for both the Adjustable-Permanent-Magnet Magneto-Rheological (APM-MR) dampers and the RD-1005 MR dampers to experimentally obtain hysteretic loops associated with the two kinds of dampers. Based on the experimental observations, a nonlinear parametric model (NLP model) is developed to describe the hysteretic loops of MR dampers. A recursive algorithm which is convergent under the time step less than order of 10-3 second is applied to discretize the governing equation of motion for MR dampers. The hierarchical compact genetic algorithm (HCGA) is proposed to identify the parameters of the NLP model. The HCGA combines the attractive merits of the global searching ability of genetic algorithm (GA) and the stable convergence of dimidiate searching root method to find the solution. It is proved that the HCGA quickly converge to the global optimum with a high probability in correctly identifying nonlinear parameter system, and the proposed NLP model matches the experimental results well over a wide range of operation conditions.2. A geometrical nonlinear finite element formulation making use of the proposed NLP model of MR dampers is presented for analyzing the dynamic responses of the cable-MR dampers system in time domain, and applied to a field cable of Hongshan cable-stayed bridge incorporated with APM-MR dampers. The effects of response amplitude, installation height and the magnetic field intensity of dampers on the equivalently linear modal damping ratio, displacement reduction effecting (DRE), modal frequency are investigated in detail. The theoretical study concludes that large installation height is in favor of suppressing cable vibration, and magnetic field intensity whose optimum value depends on cable vibration amplitudes affects significantly the performance of dampers.3. Two semi-active control algorithms for closed-loop cable vibration control with MR dampers are developed based on measurement displacement. Numerical simulation shows that the first three equivalent modal damping ratios obtained with semi-active control increases 67%, 49% and 44%, respectively, than those obtained with the optimum passive control with MR dampers tuned to a particular mode, and the corresponding displacement decrease 37%, 39% and 18%, respectively. The switch time of input voltage is vital in the semi-active control algorithm whenΔτ(time with zero voltage) is 0.1s, 0.08s and 0.06s, respectively. For the first three modes, the maximum equivalent modal damping ratios are obtained. The advantage of the semi-active control attenuates with increase of the installation height of MR dampers and modal frequencies of stay cables when compared to that of passive control.4. An experimental platform based on dSPACE, Matlab/simulink and MR dampers is established in laboratory to study the performance of semi-active control algorithms. The above-proposed semi-active algorithm with MR dampers is applied experimentally to the laboratory cable, and is compared to the passive control algorithm by using MR dampers tuned to a specific mode. The experimental study validate the semi-active algorithm is superior to the passive control tuned to a particular mode. The equivalently linear modal damping ratios experimentally obtained from semi-active control is slightly smaller than those theoretically obtained from semi-active control for all the three modes (78%, 89%, and 74%).5. Field tests are implemented on a stay cable incorporated with AMP-MR dampers of Hongshan Bridge in Changsha, China to evaluate practical mitigation vibration performance of MR damper. A series of sinusoidal exciting tests is used to measure modal damping characteristic of cable-MR dampers system. The displacement, acceleration and force time history data is obtained, then the filter and the least squares algorithm are employed to obtain the first three equivalent modal damping ratios. Vibration mitigation performance of the cable using the APM-MR dampers are compared to that using oil damper, the result shows that the APM-MR damper can more significantly suppress cable vibration than oil dampers, and have an obvious increase in equivalently linear modal damping ratios. The displacement response reduces at the location of damper of the cable when the dampers are installed. The displacement reduction effecting is studied by displacement time history data of the cable. Relationships between the DRE and the amplitude and the mode of the cable vibration and the magnetic field intensity of the APM-MR dampers are established for future applications. The field experiment results are generally in accordance with those of theoretical study.