Tower Design and Manufacture - Small Wind Turbines: Analysis, Design, and Application - page 201

that wind loads may be only one of a number of loads that must be considered in
designing a tower. Additional loads may include those due to snow, ice, and
earthquakes. These are not covered in this topic but the basic methodology can be
found in the standards referred to.

10.2 Monopole Towers

This analysis considers only the general form of the tower and no details such as
the turbine mounting flange and baseplate. Other important details include the
selection of the foundation bolts, the inclusion of an access panel, and possible
provision of rib-stiffeners as used on large towers, e.g. Lavassas et al. [ 1 ] and Uys
et al. [ 8 ]. A full three-dimensional FEA is usually required to design these com-
ponents. Following the discussion at the end of the last section, the remaining
parameters of the basic monopole design to be determined are the cross-sectional
shape, the thickness, t, of the steel in each section, and the base diameter, d h . These
have to be chosen to withstand the loads from the turbine as well as the tower self-
weight and the wind load. In addition it is necessary to calculate the horizontal
force and base overturning moment before designing the foundation. The simple
analysis that is now developed is used in Sect. 10.3 with an optimization process to
determine the minimum mass tower which is probably the cheapest to make and
transport. The analysis closely follows that of Kocer and Arora [ 9 ], Dicleli [ 10 ]
and Clifton-Smith and Wood [ 11 ].
The horizontal force on the tower due to an extreme wind, with a typical speed
of 50 m/s, is usually much larger than T max . This means that some care is required
in selecting the tower shape to reduce drag. For example, a circular cross-section
usually has a lower drag coefficient than, say, an octagonal section, see Table 9.5 .
On the other hand, tapered octagonal towers, similar to light poles, are easy to
make. Figure 10.1 shows the bending of a half-section for a polygonal tower. The
two halves are then seam welded to make a tapered section which should be hot-
dipped galvanized. The sections can be slip-fitted together on site. This involves
using a hand or hydraulic winch to force an overlap, typically of length 1.5 times
the local diameter. For polygonal towers, the ''diameter'' will be taken to be the
distance ''across the flats'' on the outside of the tower. For lighting towers, the slip
fit sections rely on friction to remain connected but they should be bolted for wind
turbine towers, to minimise movement due to fluctuating turbine loads and to
prevent disassembly during raising and lowering. Alternatively the sections can be
flanged and bolted without slip-fitting. For simplicity, slip-fitting, and any use of
flanges and bolts are important details that are ignored in the following analysis.
Take the co-ordinate y in the negative vertical direction with origin at the tower
top. The horizontal drag per unit height on the tower section at y, D(y), is given by
an equation similar to that used in Chap. 4 for aerofoil drag:
D ðÞ¼ 1

2 qU 2

ðÞ C d d ðÞ ð 10 : 1 Þ