Tower Design and Manufacture - Small Wind Turbines: Analysis, Design, and Application - page 218

Fig. 10.8 Jig to make 3 m
long sections of a tubular
lattice tower at Kijito
Windpower, Kenya. The top
section of a tower is in the jig
and one of the three base
plates for the bottom section
is shown by the white arrow

wind mills and so no optimisation was attempted. It was fortunate that the
maximum FEA stress was 156 MPa which is only slightly lower than that in the
monopole tower described in the previous section and corresponds to a CF of
0.612. The FEA was done in two parts. First, the overall simple model of the
structure shown in Fig. 10.7 was used to find the maximum stresses and the
linear buckling factor. Then the base and connection plates with their gussets
were modeled in detail, similar to the base of the monopole tower shown in
Fig. 10.4 .
The remaining task is to assess buckling strength. This was done in the FEA by
noting the linear buckling factor, LBF, whose maximum value was 3.53 at the
bottom of the back leg in Fig. 10.7 . However, it is sensible to check this value
against empirical rules available in several forms. Geometric models used in FEA
are usually idealised whereas correlations such as those used in Sect. 10.2 , include
effects of manufacturing defects and inaccuracies. For the circular members of the
example tower, either the [ 13 ] guidelines or Eurocode 3 [ 5 ] can be used. The
former use equations similar to those given above for octagonal sections. For axial
compression, the limiting stress is

F y for d = t 26203 = F y

F a ¼ ð 10 : 26 Þ

0 : 75F y þ 6550t = d for 26203 = F y \d = t 82745 = F y

For bending:

F y for d = t 41372 = F y

F b ¼

ð 10 : 27 Þ

0 : 7F y þ 12411t = d for 41372 = F y \d = t 82745F y