Other structures - Wind Loading of Structures - page 335

is blowing obliquely on to the rear of the paraboloid. As shown in Figure 14.14, the effect
is to increase the moment about the altitude axis and decrease it about the azimuth axis
(Wyatt, 1964).
In Figure 14.14, the moment coefficients are defined as follows:
(14.8)
14.5.2 Microwave dish antennas
The drag forces acting on small dish antennas used for microwave frequency
transmission are of interest for the structural design of the towers supporting them. In the
past, total drag forces for tower design have been obtained by simply adding the drag
measured on the antennas in isolation to that determined for the tower without antennas.
This will overestimate the total drag in many cases, as usually the antennas will shield
part of the tower, or vice versa; also the drag on an antenna itself in the presence of the
tower will be different to that on the antenna in isolation.
Figure 14.15 shows the drag coefficient for an impermeable unshrouded dish obtained
as a function of the wind incidence angle measured from the normal to the plane of the
dish, in both smooth (approximately 1% turbulence intensity) and turbulent flows (10%
turbulence intensity; Holmes et al., 1993). The reference area is the projected area of the
dish, π ( b 2 /4).
The drag coefficient for the isolated dish is maximum with a wind direction normal to
the plane of the dish, but does not reduce much in an angular window within 30° to the
normal. The maximum drag coefficient based on the disc area is about 1.4. A large
reduction occurs for wind directions from 40-80° to the normal. The effect of turbulence
intensity is small.
The concept of interference factor is illustrated in Figure 14.16. The drag of an
isolated antenna should be multiplied by this factor to give the measured incremental
contribution to the total tower drag. The sum of the drag on the tower segment, D t , and
the incremental contribution from the antenna, K i ,D a , gives a total effective drag, D e .