Suppressing Torsional Buckling Effects of Angle Members : Application on lattice towers

Abstract: Wind towers are a today under a global development and many countries put more focus on this environmentally friendly way of producing electricity. The performance requirement increase and at the same time the wind towers should be economical. One way of achieving better performance is to build higher towers which increase the harvesting efficiency. One way of achieving high towers is to use a lattice structure. High lattice towers require more material and have a more demanding structural design. By using cold form steel angles as columns for the lattice tower the aim is to achieve a high utilization ratio of the steel angels. Angle members are susceptible to torsional buckling, which is often the critical mode. It is thus essential to enhance their torsional response. The four columns in a lattice tower are restrained against sideway displacements by the braces and diagonals, which limits their flexural bucking length. In contrast, restraining the torsional rotations is challenging. As angels are susceptible to torsion and the flexural buckling length is decreased, the flexural-torsional interaction becomes significant. The objective of this study is to investigate thin walled angle members under compression. The idea for this project is to increase the torsional properties of an angle by lacing together the free ends of its two legs. If the lacing acts like a plate the angle columns can behave similar to a closed section.  The aim will be to increase the buckling resistance of the angle column. The design is assessed through GMNIA investigations in the FEM program Abaqus and compare laced columns to their unlaced counterparts. Using this method the result will show how the buckling resistance and buckling mode is affected by the lacing. The results of this study showed that lacing had a positive effect on columns in cross-section class 4. Columns in these two classes reached a higher buckling resistance and the buckling mode shifted from torsional buckling to localized buckling. The result showed increased effect by using a higher density of lacing.