Friction connections with slotted holes for wind towers

Sammanfattning: In a constantly increasing and competitive market the efficiency of the whole wind power plant is an issue. The tubular steel towers supporting wind turbines account for about 15 to 20% of the total installation costs and their optimization may lead to substantial savings with regard to costs and use of material. One important aspect of the design is the connection between tower's sections. The currently used solution with bolted ring flanges is prone to low fatigue resistance which imposes limitations that can impair the overall efficiency. Improvement could be achieved by implementing friction connections. This idea is investigated and the principles of a new type of friction connection are presented. Long open slotted holes are used to facilitate assembly and the segments are clamped with Tension Control Bolts which can safely and easily be tightened from within the tower. In the first part, the behaviour of segment specimens is investigated experimentally by mean of static and long term tests as well as numerical analyses. Two types of surfaces are considered: ethyl silicate zinc rich paint and corroded weathering steel. The influence of hole's geometry as well as connection configuration are studied. Particular focus is given to the variations of bolt force and the slip resistance. In the second part, the resistance function of the tested connections is derived from statistical analysis according to Annex D of EN1990. The influence of environmental parameters and workmanship on the pretension achieved by TCBs is also investigated to provide recommendation on optimal handling and design pretension. Finally, a method is proposed to design friction connections for wind towers. It is compared to the design of current L flange connections. One example of a 80m-high tower is shown. It highlights the potential benefits of using friction connections as the design is made simpler and material costs savings of up to 80% could be achieved.