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. Flutter occurs on bridges so that a positive feedback occurs between the aerodynamic forces and natural vibration of the bridge. In essence, the vibration movements of the bridge increase the aerodynamic load which in turns cause further movement of the bridge. Consequently, it results in large magnitude of movement and may cause bridge failure.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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. As a result, the motion of bridge body subject to these vortexes shall be transverse when compared with the incoming wind flow. As the frequency of vortex shedding approaches the natural frequencies of the bridges, resonant vibrations often occur, the amplitude of which depends on the damping in the system and the motion of the wind relative to the bridges. Such oscillations may “lock-on” to the system and lead to hazardous amplification and fatigue failure.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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.

One of the important features affecting the aerodynamic behaviour of a bridge is the shape of bridge deck. The shape which provides maximum stability against wind effects is that of an airplane wing, on which the wind flows smoothly without creating turbulence and there is no separation of boundary layers. To improve the aerodynamic behaviour of a bridge, addition of wind fairings and baffle plates could be considered.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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.

Tunnel anchorages resist loads from cables by mobilization of shear friction between the embedded concrete anchorage and the surrounding foundation.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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.

For very wide bridge, three cable planes are normally adopted so as to reduce the transverse bending moment.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.