| Civil engineering structures exposed to wind flow frequently experience vortex-induced vibrations. The detrimental effects of these vibrations are a direct result of fluid passing across an aerodynamically bluff body, separating along its contours, and forming a wake. At a critical value of Reynold's number, an instability in the separated layers develops, and vortices form in a complicated process that involves a nonlinear interaction and a feed-back mechanism. The shedding of vortices exerts a small fluctuating thrust adverse to the direction of the incident flow. As the wind velocity increases, the frequency of shedding synchronizes with one of the structure's natural frequencies causing large amplitude oscillatory motion. This condition of synchronicity persists over a wide range of wind speeds causing fatigue or even failure in part or entirety of the structure.; In this study, a systematic approach to vortex-shedding analysis of cable-stayed bridges has been proposed. First, the deformed configuration of the structure is determined using a nonlinear static analysis model. Then the lowest natural frequencies and modal shapes are extracted utilizing the subspace iteration method and the Lanczos starting iteration vectors. Finally, the maximum vortex-induced response is approximated using the nonlinear dynamic approach developed during the course of this study.; The objective of the proposed approach was specifically aimed at extending the versatility of Scanlan's model to more general and diverse structural systems. This improvement has been achieved by implementing the finite element concept in conjunction with Scanlan's nonlinear model to formulate the equation of motion for a three-dimensional beam element. This element may be used as a building block in representing framed structural systems enduring vortex-induced vibration. The anticipated oscillatory response may then be determined by assembling and solving the constituent nonlinear equations of motion using a direct integration scheme and an iterative process. Investigating the response of a sample cable-stayed bridge, the proposed method demonstrated good conformity and close agreement with the results obtained from Scanlan's model. In addition to the improved analysis and representation properties, the proposed empirical-numerical approach may also be conceived as one that reaffirms the results acquired from Scanlan's model, particularly when treating complex structural systems exhibiting variable cross-sectional and aerodynamic properties. |
