| The steel-concrete composite beam (short for "composite bridge") bridge, which is composed of steel girder and concrete slab integrated by shear connectors, is a new-type bridge arose after steel and concrete bridges. By combining the advantages of steel (tensile) and concrete (compressive) members, composite bridges are widely applied in highway, urban road, rail transit and railway bridges. However, there are inevitably some obvious problems owing to the material and structure limitation, such as cracking in negative moment regions, bigger deflection and cracks due to concrete shrinkage and creep, connection performance degradation under long duration loads, uncertainty of slip and mechanical behaviors under dynamic loads, etc. In this paper, the dynamic behaviors of composite bridges, including natural vibration characteristics, dynamic responses of external force load excitation, vehicle-bridge interaction dynamic analysis and damage identification are investigated, by theoretical derivation, numerical analysis and model test. The research is sponsored by the State Fundamental Research Funds "973" Program, the National Nature Science Foundation of China and the Scientific Research and Development Projects of Ministry of Railways of China. The main work and conclusions are as follows:(1) By employing the direct equilibrium method and treating the steel girder and concrete slab as Euler-Bernoulli beams, the fundamental motion equations of simply-supported straight composite bridge are established, considering relative slip at the interface of steel girder and concrete slab, as well the damping, and the equations are solved for certain boundary conditions. The effect of relative slip on the natural vibration characteristics of the composite bridge is analyzed. On this basis, the orthogonality conditions for the mass, stiffness and damping matrices are obtained, which provides a theoretical basis for further vehicle-bridge interaction analysis.(2) By the established fundamental theory, and considering the relative slip and harmonic load, the general series expressions of composite beam displacement under concentrated forces (or other types) are derived. Then a certain transformation is conducted to meet the summation conditions of the series. By substituting the corresponding Bernoulli functions and Bernoulli numbers to the above expressions, the general displacement expression of simply-supported composite beam is obtained, which is consistent with that derived by static methods, and can reflect the influences of slip and harmonic load, providing an analytical method for solving the displacement of composite beams.(3) The concepts "dynamic stiffness reduction factor (DSRF)" and "frequency reduction factor (FRF)" are proposed based on the mechanical behaviors of composite beams. The DSRF expression is obtained by theoretical derivation, and compared with the expression in the Code for steel structures (GB50017-2003). The valuing range of static stiffness reduction factor, the connection and difference, and the application area are analyzed. It shows when conducting dynamic calculations, the static expression cannot be directly applied to the equivalent stiffness of composite beam, for it will lead to obvious error.(4) The dynamic analysis model of the composite bridge concerning vehicle-bridge interaction is established. In views of the characteristics of train running, dynamic interaction between vehicle and composite beam structure, the equations of motion of composite beams under moving loads, harmonic load, sprung mass and vehicles are derived and solved. Especially, the relative slip effect on the dynamic responses of composite beams under moving loads is analyzed. The theoretical results agree well with the test results. Under certain conditions, the general deflection expression of composite beams under concentrated force can be obtained using the proposed dynamic theory.(5) The dynamic test was conducted on six composite beam models with full and partial connection stiffnesses, in which the natural characteristics, dynamic responses under model vehicle, slip law under impact load, etc., are studied. In the test, the dynamic responses of the composite beams are measured for six pre-set damage conditions with different damage degrees of shear connectors. The results from the test, theoretical analysis and numerical calculation verify well each other, which further reveals the mechanical behaviors of railway composite beams under static and dynamic loads. It shows the connection stiffness has a significant impact on the dynamic behaviors of composite beams, while localized damage has little impact.(6) The dynamic assessment and damage identification of composite beams are studied. Aimed at the structural features, the applicability of existing damage identification methods in composite beams is assessed and studied, especially for the identification method for local damage of studs. Through analysis on the stiffness characteristics of composite beams, a suitable identification method is proposed. Taking a model composite beam as an illustrative case study, the curvature model method is employed to analyze the identification method for connector damage. The results show that due to shear connectors, the integral stiffness of composite beam is discontinuous and distributed in a staged form; the identification methods based on global indices such as natural frequencies and modes are unsatisfactory and difficult to position and qualify the damage; by comprehensive analysis on the first3curvature modes, the stud distribution and damage position can be well identified.Altogether there are109figures,40tables,259equations and146references in this paper. |
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