Yongle Li

Weighting ensemble least-square method for flutter derivatives of bridge decks

Abstract In this paper, a reliable but simple identification method, here called the weighting ensemble least-square method (WELS), has been developed to extract all eight flutter derivatives of bridge deck from free vibration records. For every wind speed, free vibration test of section model is generally repeated several times in order to obtain more reliable parameter estimates. In the WELS method, many free vibration records at the same wind speed are regarded as an ensemble. The common mode parameters are then identified simultaneously from the ensemble data. The parameter fit is obtained by a nonlinear least-square method. Weighting factors are proposed to make each experiment record with the same weight in total residual error analysis. The ensemble composed of many records can reduce the effect of the colored noise of few records on the convergence of least-square iteration process. The flutter derivatives of two section models are identified to indicate the reliability and effectiveness of the WELS method.

Aerodynamic Coefficients of Inclined Circular Cylinders with Artificial Rivulet in Smooth Flow

A series of wind tunnel tests were carried out to measure drag and lift coefficients of inclined circular cylinders with artificial rivulet for later investigation on wind-rain-induced cable vibration. The axial flow effect was naturally included in the tests, compared with previous non-yawed horizontal cylinder tests. The test Reynolds number for smooth circular cylinders in uniform flow ranged from 9.38 × 104 to 1.93 × 105. A special test apparatus was designed and manufactured for easy execution of measurements. The effects of artificial rivulet, its shape, and its size on aerodynamic coefficients are discussed in this paper based on wind tunnel test results. The influence of wind yaw angle, wind speed, and cable diameter is also estimated. Furthermore, the possible transformation of wind tunnel test results to the cylinders of different inclinations and yaw angles from those used in the tests is explored.

Dynamic Analysis of Wind-Vehicle-Bridge Coupling System during the Meeting of Two Trains

In order to investigate the characteristics of the vibration of the wind-vehicle-bridge (WVB) coupling system during two trains meeting each other, the dynamic response of the WVB system has been analyzed when the two trains meeting on a large span suspension bridge using the self-developed software BANSYS. Particularly, the aerodynamic coefficients of the bridge and the two trains before and after the meeting are determined through wind tunnel tests. It is shown that the wind loads and the responses of the leeward vehicle change abruptly in the meeting process compared with the case with a single train. And the responses of the leeward vehicle increase with the growing of wind speed and vehicle speed, and decrease with the enlarging of the lateral distance between the two trains. The processes of two trains meeting each other are the control factors for the WVB coupling system.

Spectrum Models for Nonstationary Extreme Winds

AbstractNonstationary extreme winds are responsible for many structural damages. However, empirical models are not available for these winds, especially for fluctuations, mainly due to the difficulties in mathematical treatments. In this study, nonstationary wind characteristics are studied and analytic models are also proposed based on field measurements of wind speeds. Firstly, the discrete wavelet transform and kernel regression method are used to infer the time-varying mean and variance of the nonstationary extreme wind, respectively. Then, based on the estimated evolutionary power spectral density (EPSD), transient features of nonstationary winds are examined. Results show that spectral variations in nonstationary wind fluctuations including concerned downbursts and typhoons are relatively weak. This means that these nonstationary fluctuations can be modeled as uniformly modulated processes. Also, the validity of nonstationary wind spectrum models directly extended from current stationary wind spectr...

Wind Tunnel Study of a Sudden Change of Train Wind Loads due to the Wind Shielding Effects of Bridge Towers and Passing Trains

With crosswind actions, the wind loads on trains can change dramatically as the trains pass through the wake of bridge towers or when two trains pass each other, which will influence the stability of trains and the comfort of passengers. Taking advantage of the developed testing system of a wind tunnel test rig with moving train models, a three-car train model and a single-car train model were used to simulate rail vehicles and road vehicles, respectively. Sudden changes in the aerodynamic forces were measured when the train was passing through the bridge tower or when two trains were passing each other, and the effects of wind speed, train speed, rail positions of trains, and train types on the train’s aerodynamic coefficients were investigated. The analysis showed that for these studied cases, the aerodynamic coefficients of trains all have sudden changes. However, train-induced wind can weaken the sudden change of a train’s aerodynamic coefficients and lengthen the sudden-change region of wind shielding. The effect of sudden changes of wind loads acting on the road vehicle is more obvious than that on the rail vehicle.

Numerical Simulation of the Protective Effect of Railway Wind Barriers Under Crosswinds

Computational fluid dynamics (CFD) was employed for the simulation of a train model subjected to crosswinds. The numerical model was validated by comparing the flow structures and aerodynamic coefficients. A discussion of the protective effects of wind barriers at yaw angles in the range 0 < β ≤ π/4 is provided, along with an analysis of the flow distribution, and a proposal for a transformation of aerodynamic coefficients in the presence of wind barriers. The results show that the protective effect of a wind barrier for end nose are different for various yaw angle; the separation lines of the end nose at the leeward side of the vehicle bottom shift to windward in the presence of a wind barrier. The coefficients obtained through this transformation are found to be applicable for the side drag and lift coefficients of the head nose, which were obtained through numerical simulation, and are larger than the coefficients of the end nose at a majority of yaw angles. The yaw angle has a little effect on the absolute errors of the transformation for the end nose. The wind-reduction factor and absolute errors can be used to evaluate the protective effects of railway wind barriers to avoid the appearance of the different protective effect in a wind barrier.

Control of Seismic Response of a Building Frame by using Hybrid System with Magnetorheological Dampers and Isolators

The traditional isolation approach can suppress the seismic responses of upper structure and at the same time induce substantial deformation of isolation layer. Excessive base drift may cause the degradation and even the damage of the isolation system. Therefore, supplemental control devices can be implemented in the common base isolation system to construct hybrid control system and reduce the base drifts of structures. The seismic mitigation of a building frame with hybrid control system is carried out in this study. The mechanical model of magnetorheological (MR) damper is presented by involving the effects of brace stiffness of the damper. The equation of motion of a frame structure with stiffness eccentricity incorporated with intelligent hybrid control system disturbed by seismic excitations is established by considering the effects of both isolators and MR dampers. A clipped-optimal strategy based on fuzzy control principle is proposed for MR dampers. A building frame is taken as the example to examine the feasibility and reliability of the proposed intelligent control approach. The efficacy of the hybrid control approach is compared with the base isolation approach. An extensive parametric study is carried out to find the optimal parameters of MR dampers, by which the maximum reduction of seismic responses may be achieved, and to assess the effects of earthquake intensity and brace stiffness on damper performance. The work on example buildings showed that the installation of the smart dampers with proper parameters and proper control strategy could significantly reduce seismic responses of structures, and the performance of the smart damper is better than that of the common base isolation system. The optimal parameters of the MR dampers could be identified through a parametric study.

Numerical simulation of wave conditions in nearshore island area for sea-crossing bridge using spectral wave model

The assessment of wave conditions is a primary task for the design, construction, and structural analysis of sea-crossing bridge. This article presents a numerical simulation and a field measurement to study the wave conditions in the nearshore island area for sea-crossing bridges. Pingtan Strait sea-crossing bridge site was selected as an example of nearshore island area. A 3-day high-energy wave event was measured and simulated using spectral wave model. The parameters of the numerical model were calibrated through the comparison with the field measured wave data. The spatial pattern of wave conditions along the bridge and the wave-breaking zone were analyzed based on the calibrated model. The analytical procedures suggested in the Chinese Code for nearshore wave prediction were finally testified through the comparison with numerical results. The research showed that (1) spectral wave model predicts the wave conditions in bridge site reasonably; (2) seabed terrain and islands significantly influence wave conditions, wave spatial pattern, and breaking wave zones; and (3) analytical wave simulation procedure recommended in the Code is not suitable to island area for sea-crossing bridge. This research allows a better understanding of wave conditions for sea-crossing bridge site and could provide useful reference for engineering practice.

Wind tunnel test and numerical simulation of wind characteristics at a bridge site in mountainous terrain

Wind tunnel test and computational fluid dynamics simulation were conducted to study the wind characteristics at a bridge site in mountainous terrain. The upstream terrains were classified into three types: open terrain, open terrain with a steep slope close to the bridge, and open terrain with a ridge close to the bridge. Results obtained from the two methods were compared, including mean speed profiles in the vertical direction and variations of wind speed and angle of attack along the bridge deck. In addition, turbulence intensities at the bridge site obtained from wind tunnel test were discussed. For mean speed profiles in the vertical direction, two methods are reasonably close for open terrain, while mountain shielding effects are evident for open terrain with a steep slope for both the methods, but the extents of effects appear different. Wind speed and angle of attack along the bridge deck are mainly influenced by the local terrain. Strong downslope wind is generated at the lee slope for the case of wind normal to top of the ridge. The comparative results are expected to provide useful references for the study of wind characteristics in mountainous terrain in the future.

Analysis of Rain-wind Induced Cable Vibration using Spatially Measured Aerodynamic Coefficients

In most currently-used analytical models for studying rain-wind induced cable vibration, wind velocity relative to cable motion was determined by trigonometric decomposition and the aerodynamic coefficients of a cable were measured through wind tunnel tests using a horizontal cable model with two-dimensional flow field. In this paper, the aerodynamic coefficients of a cable were measured using a spatial cable model with a three-dimensional flow and an analytical model was developed to predict rain-wind induced cable vibration. The feasibility of the proposed analytical model was examined through the comparison with wind tunnel test results. The mechanism of rain-wind induced cable vibration was also investigated from viewpoints of rivulet motion and system energy. Further, an extensive parametric study was carried out in this study to investigate the properties of rain-wind induced vibration of a stay cable. The observation indicates that rain-wind induced cable vibration is essentially similar to cable galloping with limited amplitude, which is mainly induced by negative slope of lift coefficient curve and rivulet oscillation. The results from the parametric study indicate that the aerodynamic and motion parameters of cables may affect the maximum amplitude and the wind speed range for the occurrence of rain-wind induced vibration.