Flexible bridge is very sensitive with dynamics loading especially in dynamics wind load. Dynamics wind load can make the bridge vibrate with higher amplitude. Some phenomenon in structure failure because of wind dynamics loading is Tacoma narrow bridge which collapsed in 1940 at 18 m/s wind speed and Ferrybridge Power Station which had 114 m in height and collapsed at 41 m/s wind speed.
Vortex induced vibration (VIV) are motions induce on bodies by an external flow because of vortex shedding in the surrounding bodies. If the frequency of vortex shedding coincides with one of the flexible structure natural frequency, a resonance phenomenon occurs. Vortex induced vibration is structural vibration induced phenomenon caused by vortex shedding around the building in which its frequency coincides with natural frequency of the structure. Another phenomenon in cross wind direction is lock-in, lock-in occurs, not only at one critical wind velocity but also at several wind velocities.
VIV suppression in a low mass damping system is more challenging in practice due to higher amplitude response. VIV can be seen frequently in a multitude of engineering applications such as heat exchanger tubes, cooling towers, bridges, buildings, offshore structures, and nuclear reactors . When the VIV occurs, the bridge structure can vibrate with high amplitude. If the structure cannot withstand the phenomenon then the bridge will be a failure because of high displacement vibration on this bridge. Under the dynamic effects from wind induced vibrations, fatigue damage would accumulate and may lead to an eventual collapse of bridges.
Environmental influences, changes in load characteristics and random actions accelerate the structural deterioration and can cause damage leading to expensive retrofitting or bridge failure. Numerical solution, experimental, and analytical technique have been used to study about vortex induced vibration. The flow behind the structure and the separation flow could be controlled to decrease the unsteady force on the bluff body structure. Moreover the vibration because of the vortex can be made less than before. With controlling the flow around the bluff body structure where the purpose is to minimize the vibration effect is called aerodynamic modification.
Passive flow control techniques are dependent on geometry modifications for the reduction of drag and affecting the vortex shedding formation. The passive devices, such as splitter plates, fairings, helical strakes and small secondary control cylinders, require no external power input and have received much attention. From the wind tunnel experiment by using flow visualizations at circular cylinder with and without strake have different phenomenon regarding the vortex induced phenomenon. The separation points can be changed by the helical strake and refusing the interaction between two shear layers and also minimizing the vortex structure length. By suppressing the vortex structure of wake, VIV control is achievable, therefrom suppressing the lift force and ultimately reducing or controlling the amplitude response .
The other study which aims to evaluate the passive control of VIV by small control
rods had been done with a Computational fluid dynamic model coupling with a fluid structure interaction method utilized to solve the fluid flow and the motion of cylinder
group with varying Reynolds numbers . The other way to reduce the vibration effect because of vortex induced vibration is structural dynamic modification which use damper or modification at structural natural frequency. Another effective approach is
to install additive energy dissipating equipment to increase the apparent damping, such as using TMD or Active-Mass-Damper (AMD). Research findings indicated that the tuned liquid column dampers could increase the tower fatigue life . Employing aerodynamic approaches to change the external force is the most common way .
It is very important to reduce the vibration in the flexible bridge in order to make the structure have a long life time and safety for the bridge. The passive TMD is found to be a simple, effective, inexpensive, and reliable means to suppress undesirable vibrations of structures caused by harmonic or wind excitations. Tuned mass damper consists of mass, spring, and damper (optional). A tuned mass damper, or dynamic vibration absorber, consists of a secondary oscillating mass, appended to a primary vibrating structure, and tuned properly on the resonance frequency of the targeted vibration mode of the primary structure .
The flexible bridge will be designed with a sectional model with wind tunnel experiment and finite element analysis, because the sectional model is becoming popular in long span bridge design processes. It is convenient to analyze the bridge deck aeroelastic behavior and the model is simple in structure. Wind tunnel testing was used to analyze the VIV phenomenon at sectional bridge wind tunnel model and Tuned mass damper was modeled with finite element by using spring and lumped mass to tune the frequency. Vibration analysis had been done by using frequency response analysis at finite element model.