Flutter is a potentially destructive vibration and it is self-feeding in nature. The aerodynamic forces on a bridge, which is in nearly same natural mode of vibration of the bridge, cause periodic motion.
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When wind flows around a bridge, it would be slowed down when in contact with its surface and forms boundary layer. At some location, this boundary layer tends to separate from the bridge body owing to excessive curvature. This results in the formation of vortex which revises the pressure distribution over the bridge surface. The vortex formed may not be symmetric about the bridge body and different lifting forces are formed around the body.
Two types of bridge vibration that are of special concern are:
(i) Flutter, which is self-induced vibration characterized by occurrence of vertical and torsional motion at high wind speeds.
(ii) Vortex shedding, which is the vibration induced by turbulence alternating above and below the bridge deck at low wind speeds.
Gravity anchorages consist of three main parts, namely the base block, anchorage block and weight block. The weight block sits on top of anchor block and its weight is not used for resisting the pull of cables. Instead, its vertical action presses the cables vertically downward so as to turn the pull of cables against the foundation.
For one cable plane to be adopted, the requirement of high torsional stiffness of bridge deck is necessary to enhance proper transverse load distribution. Moreover, owing to the higher stiffness of bridge deck to cater for torsional moment, it possesses higher capacity for load spreading. As a result, this avoids significant stress variations in the stay and contributes to low fatigue loading of cables. On the other hand, the use of one cable plane enhances no obstruction of view from either sides of the bridges.