Marios-Zoïs BEZAS PhD

Design of lattice towers made of large hot-rolled high strength steel angles.

 

Angles profiles have been used since the very beginning of steel construction due to their easy transportation and on-site erection. They are extensively used in lattice towers and masts for telecommunication purposes or electric power transmission, as well as in a wide range of civil engineering applications such as buildings and bridges. They are also used to strengthen existing structures. Recent developments in transmission towers, like the installation of new 380-kV lines or high-voltage direct current (HVDC) lines, lead to the need for a wider application of large angle sections made of high strength steel. In Europe, lattice towers are designed to different standards (i.e. EN 1993-3-1, EN 1993-1-1, EN 50341-1). However, in all preceding codes, rules concern angles subjected to pure compression and bending effects due to eccentric connection in one angle, being considered through a modification of the buckling length. Eurocode 3 proposes a so-called “general method for lateral and lateral torsional buckling of structural components” which could possibly be applied to angle columns under compression and bending. In contrast, American codes have issued design specifications for single angle members subjected to general loading conditions including compression and bending.

Experimental and numerical investigations have been performed to study strength and stability of angles subjected to combined loading conditions. Experimental investigations refer to column buckling tests on large angle sections from high strength steel. The experiments have been limited to high strength steel only, given the fact that a number of compression tests on angles with lower steel grades were already available. Numerical investigations refer to geometric and material nonlinear analyses including imperfections (GMNIA analyses) with models duly calibrated on the tests that will extend the parameter range of experimental tests. Through the experimental and numerical studies, design rules appropriate for introduction in European Codes will be developed, concerning the classification criteria and the characteristic resistances of the angle cross-sections, as well as the resistances of individual members.

The stability of lattice towers has been also studied through numerical models, considering relevant imperfections as well as geometrical and material non-linearities. The existence of instability modes not presently covered by the norms is highlighted. An assessment of the structural response of an electric transmission tower made of angle section members and designed under gravity and wind loads according to available European norms, has been finally performed, and design recommendations for analysis of lattice towers are developed.