Tower Design and Manufacture - Small Wind Turbines: Analysis, Design, and Application - page 200

The next section describes the design of monopole towers. They are the only
type whose generic form can be analysed simply without finite element analysis
(FEA). Subsequent sections will cover the remaining tower types, but in less depth.
For simplicity, it will be assumed that tower design starts after the main loads
from the turbine-top—the platform, tail fin, blades etc.—are known. These can be
calculated using the SLM as was done for other turbine components in Chap. 9 .
IEC 61400-2 states that ''support structures shall also meet local codes and reg-
ulations''. Many standards set the extreme wind speed as well as the methodology
of determining the wind loads due to the drag on the tower and turbine. A sum-
mary of extreme wind values around the world is given in Appendix D of Holmes
[ 3 ]. The value of 50 m/s used for the examples in this chapter is a typical one from
the IEC safety standards for both large and small turbines. For example, a Class III
wind turbine has a reference wind speed of 37.5 m/s. This value must be multi-
plied by 1.4 to get the 3-s gust ''50 year return'' wind speed which is 52.5 m/s
from Table 9.2 . In general, and obviously, wind turbines should be designed for
the windiest possible location in the region covered by a particular local code. In
some standards, the tower natural frequency determines the drag coefficient, and so
influences the wind loads in a manner that has no counterpart in IEC 61400-2. The
requirement to ''meet local codes'' makes it difficult to provide design guidelines
that are universally valid. Fortunately, however, the methodology of many codes is
similar and, in many cases, so is their outcome, Carril et al. [ 4 ] and Chap. 15 of
Holmes [ 3 ]. A brief discussion of Eurocode 3 [ 5 ] for wind loading is given by
Geurts and van Bentum [ 6 ] and its application to lattice towers is covered by
Baniotopoulos [ 2 ].
The main tower-top loads are the maximum thrust, T max , and weight, m tt .In
addition, it is assumed that the tower height, h, has been decided. The tantalizing
question of what is the best height for a particular installation is considered in
Chap. 12 as are the additional loads during raising and lowering. These can be
larger than the loads set by the extreme wind speed, even when raising and
lowering occur only in calm weather. It is further assumed that the turbine con-
nection to its tower fixes the minimum tower diameter, d 0 , for any type.
For simplicity, the analysis will be restricted to static-linear behaviour. Thus the
tower material is assumed to respond linearly to the imposed load and that the
deflections are small enough to make the original (unloaded) shape sufficiently
accurate for stress analysis. ''In the analysis of towers [in general] the largest
uncertainty is an accurate knowledge of the wind loads. Highly sophisticated
methods of analysis cannot improve this. A static-linear-three dimensional struc-
tural analysis is sufficient for almost all lattice tower structures'' [ 7 ]. ''Three
dimensional'' means that the analysis must capture the actual tower shape. This is
relatively easy to do with sufficiently powerful FEA software. In this chapter it will
be assumed that all structural elements of a tower have the same material properties.
Designing for extreme or ''ultimate'' loads for which the drag coefficients are
well known usually means that a lower safety factor can be used than those
encountered for the IEC SLM in Chap. 9 . In fact, all the wind loading standards
known to the author stipulate safety factors lower than the SLM. Finally, it is noted