Date of Award
Civil and Environmental Engineering
Cable-Stayed bridge, Galloping, Stay Cable, Turbulent, Unsteady Wind, Wind-Induced Vibration
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Due to their long flexible nature and low inherent damping, stay cables on cable-stayed bridges are prone to dynamic excitations such as wind. Of specific interest to this research study is the dry inclined cable galloping phenomenon which has been addressed as a potential risk to stay cables. However, due to the complex nature of flow on a bridge site, differentiation between different wind-induced phenomena and their mechanism is very challenging. Several wind tunnel studies have confirmed the occurrence of dry inclined cable galloping under circumstances that could be easily satisfied on site. On the other hand, the transferability of wind tunnel test results on rigid circular cylinders to real cables on cable-stayed bridges might be questionable due to the unsteady nature of the natural wind and flexibility of stay cables which are hardly addressed in wind tunnel studies. The current research is therefore an effort to develop analytical models that could qualitatively investigate the dry inclined cable galloping phenomenon under more realistic conditions. A single-degree-of-freedom (SDOF) linearized model is developed to estimate the aerodynamic damping of stay cables in the context of wind flow past an elastically supported cylinder with arbitrary cross-sectional shape under unsteady wind conditions. The stability of wind-induced cable response could thus be judged based on the aerodynamic damping parameter. Further, in order to account for aerodynamic nonlinearities, a nonlinear two-degree-of-freedom (2DOF) aeroelastic model is proposed in which the nonlinear equations of motion of an elastically supported cylinder having an arbitrary cross-section subjected to the general case of unsteady wind are numerically solved. To assess the impact of aerodynamic nonlinearity, wind-induced response of an elliptical cylinder are computed using the proposed SDOF model and 2DOF model. Since these two models utilize the aerodynamic force coefficient data as input, a series of wind tunnel tests were conducted on a static elliptical cylinder to measure its aerodynamic drag and lift force coefficients. Results suggested that the nonlinear model was superior to the linearized models in prediction of aerodynamic instabilities. At last, a three-dimensional (3D) aeroelastic model for galloping of a long stay cable considering its flexural rigidity and sag is proposed, which, similar to the 2DOF model, numerically solves the equations of motion of a stay cable subjected to general unsteady wind. Results yielded from the current study suggest that the galloping response amplitude of real stay cables may be significantly reduced by several practical factors on site that could not be accounted in the wind tunnel studies such as unsteady/turbulent nature of wind, atmospheric boundary layer type wind velocity profile, and pre-existing small amplitude multi-mode oscillations. Therefore the possibility of the occurrence of dry inclined cable galloping in field may be in fact significantly less than that predicted by a wind tunnel study.
Raeesi, Arash, "Wind-induced response of bridge stay cables in unsteady wind" (2015). Electronic Theses and Dissertations. 5452.