Y.L. Xu

Dynamic response of damper-connected adjacent buildings under earthquake excitation

A formulation of the multi-degree of freedom equations of motion for fluid damper-connected adjacent multi-story buildings under earthquake excitation is presented. The ground acceleration due to earthquake is regarded as a stochastic process, and a pseudo-excitation algorithm in the frequency domain is implemented in a computer program to handle non-classical damping properties of the system. The 1940 El Centro earthquake time history is also used for dynamic analysis of the system in the time domain. The effectiveness of fluid joint dampers is then investigated in terms of the reduction of displacement, acceleration and shear force responses of adjacent buildings. Finally, an extensive parametric study is carried out to find optimum damper properties for adjacent buildings of different stiffness ratios and different heights. Results show that using fluid dampers to connect the adjacent buildings of different fundamental frequencies can effectively reduce earthquake-induced responses of either building if damper properties are appropriately selected.

Tuned liquid column damper for suppressing pitching motion of structures

Abstract Tuned liquid column damper (TLCD) was developed mainly for the purpose of suppressing horizontal motion of structures. No relevant research has been found on the suppression of structural pitching vibration by using TLCD. This paper thus aims to investigate the possibility and effectiveness of applying TLCD to suppress pitching motion of structures. Both theoretical and experimental investigations are carried out. A mathematical model of tuned liquid column damper for suppressing structural pitching vibration is developed. The TLCD-structure interactive equations are derived and solved in both time domain and frequency domain. A series of free and forced vibration experiments with different TLCD configuration and parameters are performed. The influences of variable TLCD parameters on control effectiveness are determined. Numerical simulations corresponding to the experimental cases are carried out and compared with the experiments. A close agreement is obtained between the experimental results and theoretical simulation. This also verifies the developed theoretical model. Both theoretical and experimental studies show that TLCD can efficiently reduce structural pitching motion.

Dynamic Stress Analysis of Long Suspension Bridges under Wind, Railway, and Highway Loadings

Computation of the dynamic stress of long suspension bridges under multiloadings is essential for either the strength or fatigue assessment of the bridge. This paper presents a framework for dynamic stress analysis of long suspension bridges under wind, railway, and highway loadings. The bridge, trains, and road vehicles are respectively modeled using the finite-element method (FEM). The connections between the bridge and trains and between the bridge and road vehicles are respectively considered in terms of wheel-rail and tire-road surface contact conditions. The spatial distributions of both buffeting forces and self-excited forces over the bridge deck surface are considered. The Tsing Ma suspension bridge and the field measurement data recorded by awind and structural health monitoring system (WASHMS) installed in the bridge are utilized as a case study to examine the proposed framework. The information on the concerned loadings measured by the WASHMS is taken as inputs for the computation simulation, and the computed stress responses are compared with the measured ones. The results show that running trains play a predominant role in bridge stress responses compared with running road vehicles and fluctuating wind loading. DOI: 10.1061/(ASCE)BE.1943-5592.0000216.

Computer-aided Nonlinear Vehicle-bridge Interaction Analysis:

The interaction between railway vehicle and bridge is dynamic and nonlinear in nature. This paper aims to develop a computer-aided numerical method for analyzing coupled railway vehicle-bridge systems of nonlinear features. The finite element method is used to establish not only a bridge model (bridge subsystem) but also flexible vehicle models (vehicle subsystem). The connections between the two subsystems are considered through wheel-rail contact models with and without wheel jumps. All the nonlinear forces and concentrated damping forces in the two subsystems and the nonlinear contact forces at their interface are treated as pseudo forces to facilitate nonlinear analysis. The mode superposition method is then applied to the two subsystems, and both non-iterative and iterative computation schemes are utilized to find the best solution. The convergence of iterative computation schemes is investigated with and without wheel jumps. The explicit integration scheme is found to possess higher convergence than other schemes. The applicability and accuracy of the proposed numerical method are finally illustrated through numerical examples and comparisons with previous work.

Modal analysis and seismic response of steel frames with connection dampers

To reduce dynamic response of a steel frame of bolted connections, energy dissipation materials may be placed at a connection between the end plate and column flange or between the angle and member flange. By idealizing bolted connections and energy dissipation materials as rotational spring and damper respectively, this paper derives the mass matrix, stiffness matrix and damping matrix for the frame using a combination of the finite element method and the direct stiffness method. The complex modal analysis is then carried out to determine dynamic characteristics of the frame and to investigate the effects of connection stiffness and rotational damper on natural frequency and modal-damping ratio. By further introducing a generalized pseudo-excitation method, the vibration analysis of the frame subject to earthquake excitation is performed in the frequency domain to see how the connection stiffness and rotational damper affect the seismic performance of the frame. The parametric studies on the example frame with and without the connection dampers show that there is an optimal damper damping coefficient by which the modal-damping ratio of the frame can be considerably increased and the seismic responses, including both lateral displacements and internal forces, can be significantly reduced.

Modal analysis of tower-cable system of Tsing Ma long suspension bridge

A three-dimensional dynamic finite element model is established for the tower-cable system of the Tsing Ma long suspension bridge which is currently under construction. The two bridge towers, made up of reinforced concrete columns and deep prestressed concrete beams, are modelled by three-dimensional Timoshenko beam elements with rigid arms at the connections between columns and beams. The main span and side span cables are modelled by three-node cable elements accounting for geometric nonlinearity and large elastic deflection. The modal analysis is then performed to determine the dynamic characteristics and dynamic interaction between the towers and cables. The results show that at lower natural frequencies, the modes of vibration of the system can be reasonably separated into in-plane modes and out-of-plane modes. Dynamic interactions between the towers and cables are significant at global natural frequencies in either in-plane or out-of-plane vibration. There are many local natural frequencies at which the cables vibrate but the towers remain stationary or have relatively small modal motion only. The dynamic interactions between the main span and side span cables are also observed at some local natural frequencies. The finite element model and the analytical results presented in this paper have been verified by measuring the dynamic properties of the system.

Microvibration control platform for high technology facilities subject to traffic-induced ground motion

Abstract This paper investigates the possibility of using a microvibration control platform to isolate a batch of high tech equipment from the floor of a building subject to nearby traffic-induced ground motion. The governing equation of motion of the coupled platform–building system is derived in the absolute coordinate to facilitate the feedback control and performance evaluation of the platform based on the BBN vibration criteria with the absolute velocity being targeted. A hybrid control system composed of passive mounts and active hydraulic actuators with a sub-optimal control algorithm is designed to actively control the platform. Hydraulic actuator dynamics are also considered in the modelling of the control system to avoid possible instability of the platform. The performance of actively controlled platform is assessed through comparisons with the cases of the building without control, the building with passively controlled platform, and the building with passive base isolator. Simulation results indicate that passively controlled platform and passive base isolator can be effective in reducing microvibration of high tech equipment if their parameters are properly selected. The actively controlled platform is superior to the passively controlled platform and passive base isolator because of its high performance and robustness.

Parametric study of active mass dampers for wind-excited tall buildings

A method for selecting design parameters of active mass dampers and estimating motion reduction of wind-excited tall buildings is proposed, based on aeroelastic model tests of plain (uncontrolled) tall buildings. The alongwind, crosswind and torsional responses of two typical plain tall buildings, measured in a wind tunnel, are used to perform an extensive parametric study of the active mass damper-building system under different types of wind excitation. The results show that the wind-induced response of the plain buildings can be substantially reduced if acceleration sensors are used and parameters of the active mass damper are selected appropriately. A number of graphs are presented to show the beneficial and practical control parameters. The sensitivity study indicates that the performance of the active control system considered here is not sensitive to the small variations of structural characteristics of both building and damper, but it depends on the wind excitation type to some extent.

Triple-girder model for modal analysis of cable-stayed bridges with warping effect

Abstract In the modal analysis of cable-stayed bridges using the finite element approach, the single-girder model or the double-girder model is often adopted for modeling the bridge deck. These two models are simple and easy to embody in commercial software packages, but the warping stiffness (constant) of bridge deck cannot be properly taken into consideration. This paper thus presents a triple-girder model consisting of one central girder and two side girders symmetrically connected to each other by transverse links. The proposed triple-girder model can easily consider the warping stiffness and other section properties of the bridge deck, and at the same time keep the user-friendly features in the single-girder model. The Nanpu cable-stayed bridge, built recently in China, is then taken as a case study to verify the rationality of the proposed triple-girder model through a comparison with measured data. The study shows that the modal properties of the cable-stayed bridge obtained from the modal analysis using the triple-girder model are more reasonable and close to the measured data. Further studies of four more cable-stayed bridges in China highlight that the extent of the warping effect depends greatly on the type of bridge deck and on the type of bridge tower–cable system. A simple approach for estimating some section properties of typical open-section decks is also suggested to further facilitate the use of the triple-girder model.

Seismic reliability analysis of hysteretic structure with viscoelastic dampers

Abstract This paper presents a framework for performing seismic reliability analysis of hysteretic structure–viscoelastic damper systems with and without parameter uncertainties. The dynamic response of a hysteretic shear beam type structure with viscoelastic dampers under random seismic excitation is first evaluated in the state space utilizing stochastic response analysis and equivalent linearization technique. By taking the maximum story drift as a measure of structure limit state and the maximum deformation of viscoelastic material as a measure of damper limit state, the failure probabilities of the structure and viscoelastic dampers either for a given earthquake event or during the entire service time are then estimated using the first-order reliability method and the response surface approach for the system with and without uncertainties, respectively. Finally, the framework is applied to a ten-story building with and without viscoelastic dampers and parameter uncertainties. It is found that the existence of uncertainties reduces the reliability of the building but the installation of viscoelastic dampers of proper parameters significantly enhances the reliability of the building.