## The Nigres Towers

2.1 Construction and geometry

The structure consists of five hyperboloids sections with decreasing diameters (34.0 m, 25.8 m, 19.4 m, 14.0 m, 10.0 m, 6.0 m) stacked upon each other. The sections are 24.9 m tall, except for the highest one, which measures 24.3 m. The three transmission lines were supported at 128.0 m. With the trussed outrigger at the top, the structure reaches an overall height of 130.2 m.

Figure 2. Plan view of the 5 hyperboloid sections.

The first three drums are made of 40 members (steel angles), the fourth and fifth one of 20 members. Thus, the first, second and third hyperboloids are divided in plan in 9° segments, whereas the two upper ones are divided in 18°. In plan view, the angle p between the lower and upper end of each vertical expresses the twisting of the hyperboloid. The angle p is constant for the three lowest sections (36°) and becomes more pronounced towards the top of the structure (54° and 72°). Due to the rather subtle twisting of the verticals only the two upper sections display the typical "waisted" hyperboloid form. The two sets of inversely running verticals intersect each other 4 times in the lowest three sections and the highest one, and 3 times in the fourth sections. (See fig. 2).

Between the hyperboloid segments, horizontal lattice girders act as stiffening elements (See fig. 3) The two sets of inversely arranged verticals are slightly offset and joined with two rivets at each "intersection". The 10 mm distance between the faces of the steel angles is hereby bridged with a shim plate.

Each drum is stiffened by ten horizontal ring elements, spaced 2.26m on centre vertically. The connections between the verticals and the horizontal ring elements do not coincide to allow for more simple connection details. (fig. 4)

The connection between the brackets, which are cantilevering of the verticals, and ring elements is bolted. All other connections of the structure (e.g.

Figure 3. View of the steel structure from inside with the lattice girders, which separate the hyperboloid sections.
Figure 4. Steel structure of the first hyperboloid section, showing the slightly offset verticals and the horizontal ring elements. The vertical to the right has been cut by looters.
 Drum Verticals