A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
friction between the embedded concrete anchorage and the surrounding
foundation.
4. How does the shape of bridge deck affect the aerodynamic
behaviour?
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.
5. How do vortex-induced vibrations affect the stability of long
bridges? (AB2)
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.
6. How does flatter affect the stability of long bridges? (AB1)
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
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