Buffeting Analysis
Buffeting response (or gust response) is an irregular vibration caused by fluctuating aerodynamic forces due to wind variations. In the wind-resistant design of long-span bridges, buffeting response is treated as a fluctuating wind load problem, and the expected value of the maximum buffeting response is estimated using random theory and reflected in the structural design.
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Especially during the construction phase, the structural stiffness of a bridge is lower compared to the completed stage, making it more vulnerable to wind effects. For construction stages considered to be susceptible to wind, both the static deformation caused by mean wind speed and the buffeting response caused by fluctuating wind speeds are calculated.
▣ Buffeting response of a bridge in the erection / completed stage ▣ Buffeting response of a bridge in various wind directions should be considered ▣ Vibration control methods such as wind cable and TMD should be considered |
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Flutter Analysis
The flutter phenomenon, which caused the collapse of the old Tacoma Narrows Bridge, is a type of vibration where self-excited aerodynamic forces—generated as the flow field around the section is altered by the structural system’s response—feedbacks into the fluid force, thereby continuously feeds motion energy into the structure. This leads to increasingly amplified vibrations and can ultimately result in structural failure. Due to its catastrophic potential, the flutter onset wind speed of a long-span bridge must be designed with a sufficient safety margin over the design wind speed.
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To determine the flutter onset wind speed for long-span bridges, the multi-mode flutter analysis method, which considers the coupling effects between different modes, is commonly used. For flutter analysis, accurate flutter derivatives, which describe the abnormal aerodynamic forces, must be obtained—typically through forced vibration tests. Inputs for flutter analysis include structural modeling, aerodynamic data (particularly the accurate acquisition of flutter derivatives), and wind load data.
▣ Flutter onset speeds of a bridge in the erection and completed stage ▣ Various combination of bridge modes should be considered |