Behaviour of pretensioned bolts in friction connections towards the use of higher strength steels in wind towers

Sammanfattning: During recent years wind energy has established as an alternative to common energy sources. To advance its competitiveness, the costs for the construction of a wind tower have to be reduced. One possible option is the use of friction grip joints instead of flange connections to join various tower segments in a tubular steel tower. Additionally, the time necessary for installation and maintenance of the bolts in these connections can be decreased, not only for implementation in tubular towers but also in lattice towers. Four different bolt types have been investigated with respect to the ease of installation and maintenance on the one hand and structural applicability on the other hand. The latter one is mainly defined by the behaviour of the pretension force in the bolts. Various influences on the reduction of clamping force are experimentally checked, such as the type and thickness of coating, the thickness of the clamping package and external loading. The slip factor, which plays an important role in friction connections, is experimentally achieved in a test on a double shear lap joint. The experiment is thoroughly examined by a finite element analysis, which models the interaction between bolts and plates. In various numerical analyses the influence of steel grade and possible assembling tolerances on the resistance of a friction joint is investigated both for single and double shear lap joints. Shortages of EN 1993 parts 1-8 and 1-12 for the use of slip critical joints are identified.