Friction connection vs. ring flange connection in steel towers for wind converters

Abstract

Tubular steel towers are the most commonly used structures to support wind converters. Towers are fabricated in welded segments, complying with the traffic requirements for transportation, and in-situ assembled. Ring flange connections are used to connect two segments. Fatigue endurance of the ring flange to the shell weld, class 71, is often the design criterion and imposes a limit on the shell thickness. Recently studied friction connections with long opened slotted holes, in HISTWIN and HISTWIN2 projects, provides a remedy for this limitation. The main purpose of this paper is to compare performance of the ring flange connection and the novel friction considering connection of a real tubular tower segment 3.37m in diameter and 24mm shell thickness. This cross-section is designed for the ultimate load MEd=45.8MNm and the steel grade S355. Finite Element Method is used to investigate possible failure modes of the connection. Advanced FEA comprise the realistic geometry of the connection, ductile damage material model and element removal using explicit dynamic solver. This allows sophisticated analysis of the behaviour and direct comparison of the results for both connection alternatives. The FEA is validated by down-scaled experiments performed previously within the HISTWIN project. The friction connection is thoroughly examined: geometry of the connection, influence of the shell imperfection in the vicinity of the connection and possible use of higher resistance steel grades. By focusing on key issues of the friction connection recommendations for the design are provided together with a numerical example.