|Title:||Stress-level buffeting analysis of long-span cable-supported bridges with twin-box-deck under aerodynamic and aeroelastic pressures|
|Advisors:||Xu, You-lin (CEE)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Cable-stayed bridges -- Aerodynamics
Cable-stayed bridges -- Vibration
Long-span bridge -- Aerodynamics
Long-span bridges -- Vibration
|Department:||Department of Civil and Environmental Engineering|
|Pages:||xxix, 231 pages : color illustrations|
|Abstract:||Many long-span cable-supported bridges with a steel box deck have been built in recent years, and some super long-span cable-supported bridges across the sea are also under design or planning. When these bridges are located in strong wind regions, buffeting-induced vibrations and responses, including displacement, acceleration and stress, could be considerable. Because buffeting-induced stress responses are imperative for the assessment of local failure and fatigue damage, an accurate and efficient stress-level buffeting analysis of such a bridge is of great necessity. To develop an accurate buffeting-induced stress analysis method, an appropriate modelling of distributed aerodynamic (buffeting) and aeroelastic (self-excited) pressures on the bridge deck is important. Therefore, this study first presents a modelling method for the distributed aerodynamic and aeroelastic pressures on the surfaces of a bridge deck based on the proper orthogonal decomposition (POD) method. The characteristic parameters of POD pressure modes, such as covariance modes, principal coordinates, pressure modal coefficients, pressure modal admittance functions and pressure modal derivatives, are introduced in the formulation. Indicial function-based parameter identification method is developed to identify pressure modal derivatives, and the aerodynamic and aeroelastic components are then separated in terms of POD pressure modes. On this basis, pressure modal admittance functions are finally identified with the isolated aerodynamic components by using a colligated least-square method. Moreover, the span-wise correlation of buffeting-induced pressures over the surface of a twin-box deck is investigated in terms of POD pressure modes. Each POD pressure mode obtained is likely to be associated with a particular excitation mechanism: aeroelastic pressure component due to motion or aerodynamic pressure component due to both incident and signature turbulence. Because the spanwise correlation of each pressure component is different from each other, the aerodynamic and aeroelastic components are first separated by the aforementioned indicial functions. The empirical mode decomposition (EMD) method is then applied to the aerodynamic component to further separate the incident turbulence-induced component from the signature turbulence-induced counterpart. The spanwise correlation coefficient and root coherence of each pressure component can be finally figured out. To examine the accuracy of the POD-based modelling method for distributed aerodynamic and aeroelastic pressures, wind tunnel pressure tests were conducted on a sectional twin-box deck model of the Stonecutters cable-stayed bridge in Hong Kong. When the sectional model was set to be motionless, only aerodynamic components could be measured. When the sectional model was spring-suspended, both aerodynamic and aeroelastic components could be obtained from the measured wind pressures. The pressure modal coefficients, covariance modes, principal coordinates, pressure modal derivatives and pressure modal admittance functions were calculated from the measured wind pressures by using the proposed modelling method. The results show that the higher-order POD pressure modes provide a very limited contribution to the distributed pressures and could be truncated without notable loss of accuracy. The distributed pressures over the bridge deck surface could be well represented by the superposition of a limited number of POD pressure modes. The use of POD pressure modes thus greatly simplifies the identification of characteristic parameters used to describe the aerodynamic and aeroelastic pressures. It is also confirmed that the aerodynamic and aeroelastic components of buffeting-induced pressures, as well as the incident and signature turbulence-induced components, are separated successfully in terms of POD pressure modes. The spanwise correlation coefficient and root coherence of each component are also investigated. The results show that the spanwise correlation coefficients of the aeroelastic pressures are close to 1. The root coherences of the signature turbulence-induced pressures show completely different characteristics from the incident turbulence-induced pressures. Therefore, a new coherence fitting scheme for signature turbulence-induced pressures is proposed based on the empirical functions for incident turbulence component. Accordingly, the root coherence of the pressures at any two points on the surface of the twin-box deck can be reconstructed with the root coherences of POD pressure modes.|
Based on the proposed POD pressure modes, a new framework for buffeting-induced stress analysis of long-span cable-supported bridges with a twin-box deck is then presented. Distributed aerodynamic and aeroelastic pressures on the surface of the twin-box deck are modelled in terms of POD pressure modes. The substructure modelling scheme is adopted to construct the finite element (FE) model of the bridge so that POD pressure modes can be applied, and both global (displacement and acceleration) and local (stress) responses of the bridge can be captured simultaneously and accurately. The pseudo-excitation method is used to solve the governing equations for buffeting-induced stress analysis in the frequency domain. The Stonecutters Bridge is also selected to examine the feasibility and accuracy of the proposed framework, in which the global and local responses of the bridge under Typhoon Hato are computed using the proposed framework and compared with the measured ones recorded by the wind and structural health monitoring system installed in the bridge. The comparative results show a good agreement between the computed and measured responses and confirm the necessity and accuracy of the proposed framework for buffeting-induced stress analysis. To compare the new proposed framework with the traditional one, two wind loading models (lumped force or distributed pressure) and two FE models (spine-beam or substructure) are discussed and formulated for buffeting-induced stress analysis of long-span cable-supported bridges with a twin-box deck. These models and formulations are applied to the Stonecutters Bridge in the form of three combination cases: spine-beam FE and lumped force model (Case 1), substructure FE and lumped force model (Case 2), and substructure FE and distributed pressure model (Case 3). The aerodynamic and aeroelastic properties of the bridge deck section obtained from wind tunnel tests are used in the analysis. The buffeting-induced global (displacement and acceleration) and local (stress) responses of the bridge are finally predicted for all the three cases. The comparison results show that the use of spine-beam FE and lumped force models (Case 1) causes slight difference in the global response but yields distinct underestimation of the local stress response. The neglect of wind pressure distribution in Case 2 may underestimate the maximum local response to some extent. Therefore, the substructure FE and distributed pressure models are recommended to obtain accurate buffeting-induced stresses of the long-span cable-supported bridges.
|Rights:||All rights reserved|
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