Joints between tower sections

Towers are normally fabricated in several sections for transport reasons, so joints are required. Welding on site is an expensive operation, so bolted joints are almost always used, although sleeved joints, in which each tapered tower section is threaded over the one beneath and forced into place by jacking, have been used successfully.

The structurally most effective joint is made with friction grip bolted splice plates oriented vertically and sandwiching the walls of the abutting tower sections between them. Provided the grip force is adequate, the joint will not slip even under the extreme load, with the result that the bolts are not subject to fatigue loads. Unfortunately, apart from the effect of splice plates on the external appearance, there are practical difficulties of joint assembly, because bolting requires the provision of some form of personnel access on the outside of the tower. Nevertheless splice plates are used on some towers.

The most popular bolted arrangement is the internal flanged joint as illustrated in Figure 7.42. The flanges are butt welded to the ends of the mating sections, with the flange outer edge flush with the tower wall. Alternatively the flange may be formed with a stub section of tower wall already attached. Such flanges, which are termed weld neck flanges, provide a smoother transition from wall to flange (as illustrated in the lower half of Figure 7.42) and result in a higher butt-weld fatigue category.

After assembly, each bolt is torqued or tensioned to induce a preload between the flanges in order to minimize in-service bolt fatigue stresses. The bolt should be initially sized to resist the prying force induced by the extreme tower-wall tensile stresses - taking the fulcrum adjacent to the flange inner edge - and then checked for fatigue.

The fatigue calculation for the bolts in a flanged joint depends on the relationship between the bolt load and tower wall stress, which only remains linear while contact is maintained over the full flange width. The VDI Guideline (Verein

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Renewable Energy Eco Friendly

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