Cyclic performance of reinforced legs in retrofitted transmission towers

In Australia, thousands of aging steel transmission towers need upgrading in order to meet the demands of additional communication devices being placed on them and the increased wind loads according to newly revised design standards. One effective retrofitting approach to increase their load capacity is to attach a reinforcing member to the original leg member through a bolted cruciform connector. This paper addresses the structural behaviour of retrofitted tower leg members under dynamic loading conditions that replicate the in-service conditions due to wind loading on real towers. A series of experimental tests and FEM modelling were conducted. The experimental results and numerical models show that the bolt pretension continuously decreases with the increasing loading cycles and magnitudes, which further reduces the bolt-slip load in cruciform connectors. The bolt-slip phenomenon changes the load-sharing rate between original members and reinforcing members in retrofitted legs. The dynamic loading procedure reduces the structural stiffness of retrofitted leg members due to the surface smoothing and bolt pretension loss. A further parametric study shows an obvious reduction in load-carrying capacity of retrofitted leg segments under long term dynamic loading periods. In addition, retrofitted segments with splice connectors in original members are more sensitive to loading cycle numbers.