A novel energy dissipation system that can achieve the amplified damping ratio for a frame-core tube structures is
explored, where vertical dampers are equipped between the outrigger and perimeter columns. The modal characteristics
of the structural system with linear viscous dampers are theoretically analyzed from the simplified finite element model
by parametric analysis. The result shows that modal damping ratios of the first several modes can increase a lot with this
novel damping system. To improve the control performance of system, the semi-active control devices,
magnetorheological (MR) dampers, are adopted to develop a controllable outrigger damping system. The clipped optimal
control with the linear-quadratic Gaussian (LQG) acceleration feedback is adopted in this paper. The effectiveness of
both passive and semi-active control outrigger damping systems is evaluated through the numerical simulation of a
representative tall building subjected to two typical earthquake records.
A new self-powered and sensing semi-active control system based on magnetorheological (MR) damper is presented.
The system includes four key parts: a rack and pinion mechanism, a linear permanent magnet DC generator, a current
adjustment MR damper, and a control circuit. Numerical simulations for seismic protection of elevated bridges equipped
with this system excited by two historical earthquakes are conducted. Linear quadratic regulator (LQR) is used for the
design of ideal active control system. LQR-based clipped optimal control as well as skyhook control is used to command
a MR damper in both the semi-active control with external power and self-powered semi-active control. It is shown that
five strategies (ideal active control, two semi-active controls and two self-powered semi-active controls) have similar
control performance in pier response as well as bearing response. It is noticed that only one accelerometer is needed to
monitor the response of the deck to realize the self-powered skyhook control, which greatly simplifies the classical semiactive
vibration control system based on MR damper.
The Dongting Lake Bridge is a cable-stayed bridge crossing the Dongting Lake where it meets the Yangtze River in southern central China. After this bridge was completed in 1999, its cables were observed to be sensitive to rain-wind-induced vibration, especially under adverse weather conditions of both rain and wind. To investigate the possibility of using MR damping systems to reduce cable vibration, a joint project between the Central South University of China and the Hong Kong Polytechnic University was conducted. Based on the promising research results, the bridge authority decided to install MR damping systems on the longest 156 stay cables. The installation started in July 2001 and finished in June 2002, making it the world's first application of MR dampers on cable-stayed bridge to suppress the rain-wind-induced cable vibration. As a visible and permanent aspect of bridge, the MR damping system must be aesthetically pleasing, reliable, durable, easy to maintain, as well as effective in vibration mitigation. Substantial work was done to meet these requirements. This paper describes the implementation of MR damping systems for cable vibration reduction.
As the world's first time implementation of MR-based smart damping technique in bridge structures, a total of 312 semi-active magneto-rheological (MR) dampers have recently been installed on the cable-stayed Dongting Lake Bridge for wind-rain-induced cable vibration control. Prior to the full implementation, a comprehensive field vibration test, has been conducted on the longest cable of 150 m to identify and compare damping performance of the cable-damper system under different damper installation setups and under a wide spectrum of voltage inputs to the MR dampers. Forced vibration experiments were carried out for the cable without damper, with single-damper setup, and with twin-damper setup, respectively. One purpose of this in-situ experimental investigation is to determine the optimal input voltage which achieves maximum system damping for the aim of designing a multi-switch control strategy. Due to geometric nonlinearity of cables and hysteretic nonlinearity of MR dampers, the equivalent modal properties of the cable-damper system are deemed to be amplitude-dependent. Keeping this in mind, a Hilbert transform based method is deployed in the present study to identify the amplitude-dependent natural frequencies and modal damping from the sinusoidal-decay response data. The experimental and identification results show that the equivalent modal damping ratios of the system are noticeably dependent on vibration amplitude and the relevance of the natural frequencies to vibration amplitude is negligible. The single-damper setup is competitive with the twin-damper setup in suppressing in-plane vibration of the cables. However, when mitigation of cable out-of-plane vibration is also required, the twin-damper setup performs much better. For both setups, the value of optimal voltage is found to be mode-dependent and amplitude-dependent.
The newly built cable-stayed Dongting Lake Bridge in Hunan, China has experienced wind-rain-induced cable vibration several times during the past months. A research/implementation project on using semi-active magneto-rheological (MR) dampers for cable vibration control of the bridge is in progress. As part of this ongoing project, one typical stay cable with 115 m length was installed with two MR dampers near the lower anchorage, and accelerometers were deployed on the damped cable and its two neighboring cables for long-term monitoring. After installing the dampers and sensors, wind-rain-induced cable oscillations were observed two times. This paper aims to investigate the vibration characteristics and to identify the equivalent modal damping of the cables with and without MR dampers in one wind-rain-excited event based on in-situ monitoring. In this wind-rain-excited event, the in-plane and out-of-plane responses of the damped cable and its two neighboring free cables were monitored. Equivalent modal damping ratios of the cables in both in-plane and out-of-plane motions are identified by means of spectral analysis of the measured data in conjunction with a curve-fitting technique. Such observed and identified results are beneficial to understanding the coupled motion of cables in wind-rain-excited conditions and the damping contribution of MR dampers to both in-plane and out-of-plane motions. The frequency-domain analysis of the wind-rain-excited responses of the damped and undamped cables also reveals the response characteristics under wind-rain excitation and the damping mechanism of MR dampers in suppressing such oscillation.
Working towards full implementation of semi-active MR dampers to over 200 cables in the cable-stayed Dongting Lake Bridge, a few selected stay cables were installed with both MR dampers and vibration sensors for trial testing before full implementation. This paper reports the field vibration tests of a typical stay cable of 115 m long before and after being installed with MR dampers. The field vibration tests were conducted by using ambient vibration excitation, and sinusoidal excitation followed by free vibration decay, respectively. The resonant frequencies and equivalent modal damping of the first twenty in-plane modes of the cable with and without MR dampers are identified. The relationship between the equivalent modal damping ratio and the applied voltage strength to MR dampers is experimentally determined. The test results show that the modal damping ratios of the free cable without dampers conform to a combined Rayleigh and frequency-independent damping model, and are almost unvaried with vibration amplitude within the tested range. Installation of MR dampers results in a slight change of resonant frequencies in comparison with the free cable, but variation in applied voltage almost does not affect the resonant frequencies. The equivalent modal damping ratios of the cable-damper system are found to be dependent on the installation location of MR dampers, voltage strength applied to the dampers, and the cable vibration level. The optimal voltage input, which achieves maximum system damping, is obtained for different modes under different vibration amplitudes. With optimal voltage applied to the dampers, the resulting system damping should be high enough to suppress both wind-rain-induced vibration and wake galloping of cables.