You-Lin Xu

Dynamic analysis of wind-excited truss tower with friction dampers

Abstract This paper presents a rational analytical method for determining dynamic response of wind-excited large truss towers installed with friction dampers and investigates the effectiveness of friction dampers. The analytical procedure involves two models of one truss tower: one is the two-dimensional lumped mass dynamic model and the other is the three-dimensional (3D) finite element static model. These two models are alternately used to fulfill control force transformation, displacement increment transformation, and the numerical integration of equations of motion of the coupled damper-tower system. A three-story truss structure under wind excitation is used to validate the proposed bi-model method through the comparison with a precise 3D dynamic analysis. After a satisfactory comparison, a large space steel television tower is used as an application to examine the feasibility of the bi-model method and the effectiveness of friction dampers. The case study shows that the bi-model method can efficiently compute wind-induced response of the large space television tower with friction dampers and the installation of the friction dampers of proper parameters can significantly reduce wind-induced vibration of the tower.

Experimental Investigation on Statistical Moment-based Structural Damage Detection Method

Although vibration-based structural damage detection methods have demonstrated various degrees of success, the damage detection of civil structures still remains as a challenging task. The main obstacles include the insensitivity to local damage and the high sensitivity to measurement noise. A new structural damage detection method based on the statistical moments of dynamic responses of a structure has been recently proposed by the authors, and the numerical study manifested that the proposed method is sensitive to local structural damage but insensitive to measurement noise. The experimental investigation on this method is presented in this article. Three shear building models with and without damage were built and subjected to ground motions generated by a shaking table. The displacement and acceleration responses of each building model at each floor were recorded. The recorded ground motion and building responses as well as identified structural damping ratios were then used to identify damage locations and severities using the statistical moment-based damage detection method. The identified damage locations and severities were compared with the theoretical values. The comparison is found satisfactory, and the method proposed is effective and feasible.

BUFFETING ANALYSIS OF LONG SPAN BRIDGES: A NEW ALGORITHM

Abstract This paper presents a new algorithm for buffeting analysis of long span bridges, featured mainly by a complete finite element approach and a pseudo-excitation method. Aeroelastic forces on a bridge deck are first converted into nodal forces to form element aeroelastic damping and stiffness matrices. Aerodynamic forces are then converted into nodal forces to obtain an element loading vector. After the system equation of motion is assembled, the pseudo-excitation method is finally applied to determine the buffeting response, which naturally includes inter-mode and multi-mode contributions. The accuracy of the new algorithm is examined by comparison with the Scanlans method through an example bridge deck. The potential advantages of the algorithm are also pointed out.

Adjustable fluid damper with SMA actuators

The passive fluid damper is one of most widely used control devices for mitigating the vibration of stay cables in a cable-stayed bridge in practice. However, each stay cable features unique dynamic characteristics and requires a specific damper to achieve the best vibration mitigation, which engenders much trouble in the manufacture, implementation and maintenance of dampers. In this study, a novel adjustable fluid damper with shape memory alloy (SMA) actuators was developed. Instead of the fixed number of orifices in the piston head of a common fluid damper, SMA actuators were installed inside the piston head to control the number of orifices so as to change the damper parameters for the best control of a group of stay cables. The principle and design issues of the adjustable fluid damper are introduced. An adjustable fluid damper, which has ten orifices in the piston head, eight of them being controllable for achieving changes in damping at nine levels, was designed and manufactured. The performance tests of the dampers were carried out within a range of frequencies and amplitudes and for a number of open orifices and different sizes of orifice. The experimental results provide a data base for the Maxwell model of the damper and show that such a damper can provide a wide range of forces and energy dissipation capacity.

Synthesis of vibration control and health monitoring of building structures under unknown excitation

The vibration control and health monitoring of building structures have been actively investigated in recent years but often treated separately according to the primary objective pursued. In this study, a time-domain integrated vibration control and health monitoring approach is proposed based on the extended Kalman filter (EKF) for identifying the physical parameters of the controlled building structures without the knowledge of the external excitation. The physical parameters and state vectors of the building structure are then estimated and used for the determination of the control force for the purpose of the vibration attenuation. The interaction between the health monitoring and vibration control is revealed and assessed. The feasibility and reliability of the proposed approach is numerically demonstrated via a five-story shear building structure equipped with magneto-rheological (MR) dampers. Two types of excitations are considered: (1) the EI-Centro ground excitation underneath of the building and (2) a swept-frequency excitation applied on the top floor of the building. Results show that the structural parameters as well as the unknown dynamic loadings could be identified accurately; and, at the same time, the structural vibration is significantly reduced in the building structure.

Structural damage identification via multi-type sensors and response reconstruction

One outstanding obstacle that hinders robust application of vibration-based damage identification to civil structures is that the number of sensors installed on a large civil structure is always limited, compared with the total degrees of freedom of the structure, so that the limited measured responses may not provide enough information for detecting local damage. Furthermore, developments in sensor technology make installation of heterogeneous sensors on a structure practical and feasible while every type of sensor has its own merits and drawbacks for damage identification. But the benefits of utilizing heterogeneous sensors in vibration-based damage identification have not been fully investigated. This study proposes a damage identification method by combining the response reconstruction technique with the response sensitivity–based finite element model updating method to address these issues. The number and location of heterogeneous sensors, such as accelerometers, displacement transducers, and strain gauges, are optimally and collectively determined in an optimization strategy to obtain the best reconstruction of multi-type responses of a structure using Kalman filter. After damage occurrence, radial basis function network is employed to predict the mode shapes using the modal properties extracted from the measurement data by experimental modal analysis method, and these modal properties are further used to reconstruct responses of the damaged structure. The reconstructed responses are finally used to identify the damage in terms of sensitivity-based finite element model updating. In every updating, the sparse regularization is employed to increase the identification accuracy. A simply supported overhanging steel beam composed of 40 elements serves as a numerical study to demonstrate the procedure and feasibility of the proposed method. The validation of this method is further conducted by laboratory test. Both simulation study and laboratory test show that the multi-sensing approach via response reconstruction does improve the identification accuracy of damage location and quantization considerably.

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.

Settlement Monitoring of a Supertall Building Using the Kalman Filtering Technique and Forward Construction Stage Analysis

Excessive and unexpected settlement of supertall buildings during the construction stage may affect structural safety. Floor settlement monitoring is imperative during the structural construction. The current practice is to measure floor settlement within a specific time frame and then compensate for the elevation error in the next construction stage. This posterior manner is inaccurate because it does not take account of measurement error and the real settlement in the next construction stage. This paper presents a Kalman filter based settlement monitoring approach with the aid of forward construction stage analysis. The settlement of floors in the next stage is predicted with the forward construction stage analysis. Consequently the elevation of the floor to be constructed can be pre-determined. The proposed method is demonstrated through construction settlement monitoring of the 632-m tall Shanghai Tower. The structure will be the tallest supertall building of China upon completed in 2014. It shows that this method can improve the accuracy of pre-determined elevation of supertall buildings.

Imperfect Correlation of Vortex-Induced Fluctuating Pressures and Vertical Forces on a Typical Flat Closed Box Deck

A wind tunnel test of synchronized pressure measurement on a spring-suspended sectional model of a flat closed box deck at a large scale of 1/20 was carried out by taking Xiangshan Harbour Bridge in China as background. The chordwise correlation of the fluctuating aerodynamic pressures (FAP) on the surface of the box deck was then investigated at both stationary state and vortex-induced resonance (VIR) states of the deck model. Afterwards, an approximate approach was developed for estimating the spanwise correlation coefficients of pure vertical vortex-shedding force (VSF) on the deck under VIR state. On this basis, the spanwise correlation coefficients of both the integrated total vertical fluctuating aerodynamic force (FAF) and the pure vertical VSF on the flat box deck were then studied and compared to each other quantitatively at the stationary state as well as the VIR states at various wind speeds. Finally, the spanwise correlation coefficients of the surface FAP at some typical positions on the deck cross section were calculated and compared with that of the total vertical FAF and the vertical VSF. The results show that the chordwise correlation of the FAP on stationary deck is not strong at most part of the deck surface, however, can be enhanced with the increase of incident wind speed. The chordwise correlation of FAP can also be enhanced quite significantly by the deck oscillation under VIR states, but is still imperfect. Furthermore, the spanwise correlations of FAP at different positions on the deck cross section are clearly different from each other, and much weaker than that of the total vertical FAF, for both the stationary state and the VIR states of the deck. However, they are generally much stronger than that of the pure VSF in cases of notable VIR, and much weaker in the cases of slight VIR, such as near the onset of VIR. The spanwise correlation characteristics of both the pure vertical VSF and the total vertical FAF are hence very different from those of the surface pressures. Moreover, the spanwise correlation of the total vertical FAF on cross sections can be greatly intensified by the VIR because of the existence of a large portion of vortex-induced self-excited force (VISEF). On the other hand, the spanwise correlation of the vertical VSF under the VIR states is much weaker than that of total vertical FAF as well as that of the pure vertical VSF under the stationary state at same wind speeds. Finally, the variations of the spanwise correlation of both the total vertical FAF and the pure vertical VSF with the response displacement of VIR is quite complicated. The most violent VIR, the most correlated total FAF and the most correlated VSF occur generally at different wind speeds.

Temperature monitoring of Tsing Ma Suspension Bridge : numerical simulation and field measurement

12th International Conference on Engineering, Science, Construction, and Operations in Challenging Environments - Earth and Space 2010, Honolulu, HI, 14-17 March 2010