Effective Length Factor of Leg Member in Latticed Steel Tower

Latticed steel towers have been widely used to support overhead transmission lines worldwide. In a tower, leg members are intended to carry the major loads. It is a common practice to design a leg member based on the assumption that its effective length factor is unity (1.0). While this practice has been working well over time, no report is available in the literature to validate this assumption. In this paper, a nonlinear buckling analysis approach is first proposed by using a geometrically nonlinear finite element (FE) code in conjunction with the concept of trigger load. The proposed approach is able to re-produce the ASCE-10 column curve very nicely, proving its practical usefulness. This approach is then used to rigorously examine the unity effective length factor assumption for the leg members. For this purpose, leg members are modeled as a continuous beam-column with multiple linear elastic springs representing the supports of the associated bracing members. As a result, it is found that the unity assumption is valid only if a leg member is adequately supported on its both ends, and would be risky otherwise. Accordingly, a simple, semi-empirical equation is derived, based on the numerical parametric study, to correlate a leg member’s buckling load with the support stiffness.