Page 105 - Dynamic Loading and Design of Structures

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forces consequent upon motion or deformation of the structure. It is instructive first to
consider these two mechanisms separately, but unfortunately interaction between them is
commonly crucial to the severity of the effects on the structure.
Anyone encountering such problems is well advised to read the seminal survey by Scruton
and Flint (1964). Further introduction, particularly relevant to bridges, is available in Wyatt
and Scruton (1981). The basic introduction to the stochastic model of vortex shedding given
by Vickery and Basu (1984) is also desirable preliminary reading. Blevins’ ‘Flow induced
vibrations‘ (1994) is widely respected for reference. There is a very wide range of design
specifications, which will be introduced later.

3.2.2 Vortex shedding: deterministic representation

The starting point for the dynamic effects of vortex shedding is the von Karman vortex street,
illustrated by Figure 3.4. This represents a cross-section through the flow field round a long
prismatic structure; the circular section has been selected for illustration partly on the grounds
of familiarity, but also because of the freedom from galloping-type aeroelastic behaviour. The
quasi-steady behaviour of this section has already been addressed in the context of gust action,
and has been shown to give unconditionally positive damping of structural oscillation,
increasing in proportion to windspeed. The vortex street as shown indicates that vortex
growth occurs alternately on opposite sides of the structure. When there is a large attached
vortex on one side, the wake is displaced laterally and there is a lateral component of force on
the structure. When this vortex is shed and (in this case) replaced by growth on the other side,
and the process repeated, there is clearly a cyclic lateral excitation. Although the actual
variation is unlikely to be sinusoidal, it is usual to extract the first Fourier component to give a
coefficient of fluctuating lift Č L; that is, describing force p(t) per unit length of prism as

(3.27)

in which ns is the frequency of the shedding.

Figure 3.4 The vortex street.