Page 113 - Dynamic Loading and Design of Structures

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likely to suffer stronger periodic excitation than the originating body. This commonly arises
where a power station, for example, is served by two or more chimneys. Clearly this only
operates when the wind is within a small range of direction, but extends to separations as
large as 15 times the diameter. The most unfavourable effects occur when the chimneys are
identical, as resonant motion of the upwind element leading to enhanced regularity of the
vortex street will coincide with resonance of the affected downwind element. For this case it
has been suggested that response of the downwind element may be twice that predicted for an
isolated stack if the separation is 5D, or 1.5 times the isolated value if the separation is 10D
(Vickery, 1981; see also informative annex C.3.2.3 of the Eurocode ENV 1991–2–4).
Serious consideration must be given to this problem when slender modern structures are
located in proximity to existing structures causing greater disturbance to the flow. The effect
may be to present a significantly organized vortex street, changing with increasing separation
to greater resemblance of a normal turbulence field but with strongly enhanced strength in the
range of frequencies likely to embrace the structural resonant frequency. The term ‘buffeting’
has been expressly applied to this effect in UK usage, in distinction from its application to
turbulence effects in general in the aeronautical field and elsewhere. The seminal example
was the decision to build a stiff lattice arch bridge in place of the proposed suspension bridge
over the Mersey at Runcom, in proximity to the nineteenth century through truss railway
bridge (Scruton et al., 1955; Grillaud et al., 1992; Bietry et al., 1994).
Two other resonant potential responses to vortex shedding should be borne in mind,
although generally out of range or readily circumvented when the excitation derives from the
natural wind. There is a weak in-line excitation, with one cycle for each vortex shed, giving
resonance at one-half of the cross-flow resonance windspeed. The Scruton number limit to
effectively avoid a structural problem is perhaps K =7. This has been observed with wind
s
excitation only in exceptional circumstances such as aluminium tubular members in a frame
with very low damping, but can be a serious problem in water (e.g. for pile columns
supporting a jetty). The second phenomenon possible at this reduced velocity is excitation of
the ovalling mode of the structural section, which should be circumvented by ensuring a
sufficiently high frequency, by stiffening if necessary, so that resonance is out of the practical
windspeed range.

3.2.7 Vortex shedding: design rules for circular sections

The windspeed corresponding to resonance at the Strouhal frequency for a stationary structure
can be robustly estimated, and if this exceeds the practical windspeed for the site, no further
action is required. If not (as is commonly the case), the stresses caused by resonant response
must be checked. Lock on may lead to a slightly greater response at a windspeed perhaps 20
per cent greater than the basic Strouhal resonance; for transcritical Reynolds number (nD 2
significantly