Study On Wind-induced Response And Fatigue Of Flexible Structures

Wind induced vibration of flexible structures is increasingly outstanding with the extending size of the civil structures. It is integrated and fully developed for the analysis and design of wind induced vibration. However, the traditional research methods are not applicable to wind induced vibration of flexible structures because of the characteristic of structures which results in the greater wind induced responses and the complicated interaction between wind and flexible structures. The analysis of wind induced vibration should be improved or updated according to new theories. This thesis mainly focuses on the vortex-induced vibration and buffeting which are induced by wind loadings of two typical flexible structures, supertall buildings and long-span bridges. The specific studies include the structural comfort at acrosswind and the prediction of vortex-induced vibration of supertall buildings, as well as the analysis of the local stress response induced by buffeting as well as the wind induced fatigue damage of long-span bridges.First, the wind tunnel test with the aeroelastic model is applied into the vortex-induced vibration of supertall buildings. The aeroelastic model is updated in the similitude, the parameters and the manufacturing. Wind tunnel test results show that the acrosswind response of supertall buildings is seriously influenced by the vortex-induced vibration, and it tends to be locked in within a certain wind speed range. The single-degree-of-freedom empirical model of the vortex-induced force is then introduced into the computation of supertall buildings after considering both of the calculated amount and exactitude in the engineering applications. The acceleration response at the top of the structure is computed with aerodynamic parameters obtained from the wind tunnel test with aeroelastic model. According to the comparison with the measurements, the vortex induced stable-state response in the lock-in wind speed range can be predicted accurately with this empirical model. This study is valuable in the acrosswind design of supertall buildings.The other part of the thesis is on the stress responses induced by buffeting of the long-span bridges. The research work mainly focuses on the refined three dimensional finite element model of the long-span bridge, the computation of buffeting forces in the refined finite element model and the possible application of CFD numerical simulation in the computation of buffeting aerodynamic parameters. The stiffening girder of the main span in the refined finite element model is simulated according to the authenic structure, which can represent more details than simple models. The nodal buffeting forces and self-excited forces are computed according to the refined finite element model. The computation of the nodal buffeting forces is based on the pressure distribution on the surface of the span, while the self-excited forces are calculated according to the displacement relationship between the center of elastics and the element nodes on the section of the span during the rigid motion. The application of CFD numerical simulation in the buffeting analysis of long-span bridges is explored. Reynolds-averaged Navier-Stokes equation is used to obtain the pressure distribution induced by buffeting, and the block iterative Gauss-Seidel method is extended to simulate the fluid-structure interaction. The field measurement data from structure health monitoring system of Tsing Ma Bridge in Hong Kong during typhoon York in the year1999are first analyzed and used as input data to the computation. The buffeting induced stress responses are computed in the time domain with Newmark method and the frequency domain with pseudo excitation method, respectively. Comparing with measured data of the same components of the bridge, the numerical results are satisfactory in general.Wind induced fatigue of bridges is due to the local stresses induced by buffeting. A nonlinear fatigue damage analysis method that not only includes the accumulative fatigue damage during the stage of fatigue crack initiation and growth but also accounts for the random distribution of wind speeds and direction is proposed according to the theory of continuum damage mechanics. The buffeting induced fatigue accumulated damage of monsoon during the bridge design life is evaluated by using the proposed method. The results show that the proposed procedure can well assess variation of the material properties under the long term cycling loading, and be applicable to the evaluation of buffeting-induced fatigue damage to a long-span bridge.