Environmental Engineering Reference
In-Depth Information
Table 10.5
Results of tower optimisation
d
min
(m)
d
0
(m)
d
h
(m)
t
1
, t
2
,t
3
(mm)
m
t
(kg)
n
1
(Hz)
Turbine
deflection (m)
0.10
0.10
0.373
2.8, 3.6, 4.3
406.9
1.34
0.893
0.14
0.14
0.384
2.5, 3.4, 4.4
439.6
1.28
0.939
0.17
0.17
0.378
2.7, 3.5, 4.8
485.5
1.22
0.966
0.17
0.17
0.384
2.8, 3.6, 4.7
487.0
1.16
0.991
0.17
0.17
0.428
2.6, 3.4, 4.3
497.2
1.12
0.953
10.4 Lattice Towers
Lattice towers are usually triangular or square in planform cross-section. They can
be guyed or unguyed but only the latter are considered in this section. Guyed
lattice towers often have a constant width and so are similar to guyed pole towers.
Both are discussed briefly in the next section. Unguyed lattice towers, like
monopoles, often increase in width towards the base to withstand the increasing
bending moment. The most famous example is the Eiffel Tower in Paris.
Figure
10.7
shows an 18 m high triangular tubular lattice designed for the 5 kW
turbine specified in Table
10.2
. Table
10.6
gives the main details.
The basic principles of design of lattice towers are similar to those for mono-
pole towers; the highest stresses occur for the extreme wind when the turbine
blades are stationary, and buckling is an important and often controlling consid-
eration for the tower components. However, the geometric complexity of lattice
towers over monopoles has consequences for the analysis. For example, the wind
load is independent of orientation on a monopole but not for a lattice tower,
and some standards require an assessment of up to eight different orientations to
find the worst case. It may well be that the worst case for one load is in a different
orientation than for another. For example, a three-sided lattice tower will usually
have the maximum compression at the base of the ''back'' leg when the wind is
normal to the line between the ''front'' two legs, i.e. in the positive Z direction in
Fig.
10.7
, but the maximum tension will occur if the wind is in the -Z direction.
The lattice itself is sufficiently complex to require a FEA. Simple correlations for
the natural frequency are unlikely to be generally valid.
Tubular lattice towers can be fabricated accurately and easily. The 18 m
example tower is designed to be manufactured in six 3 m sections using the jig
shown in Fig.
10.8
, assembled on site, and erected using a separate gin pole with
temporary foundations. Table
10.6
shows the thick elements in Fig.
10.8
are made
from 60 mm diameter steel water pipe and the thinner bracing is 12.7 mm
diameter low Carbon steel. A major reason for the choice of the tubular lattice
design rather than a standard lattice tower using bolted angle sections is that the
tower is not galvanised and corrosion often starts at the bolted joints.
The wind load on a lattice tower section is usually found by first calculating the
solidity of the front face. Solidity is the total projected area of the tower members
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