Modeling of retrofitted steel transmission towers

•Simplified 1D model for retrofitted steel transmission towers•Load sharing analysis between original members and reinforcing members•Bolt-slip effect analysis•Pre-loading effect analysis•Parametric study on connector number/location/ stiffness, reinforcing member size, reinforcing panel numbers

To meet the increased design loads from upgraded wind load standards and higher power demands, existing aging transmission towers are often in need of retrofitting. One effective way to increase the load carrying capacity of a tower structure is to reinforce its critical leg members through attaching additional elements with bolted cruciform connections. The effectiveness of the leg retrofitted method has been verified experimentally in both leg segments and tower systems. In this paper, a new modeling method for lattice transmission towers with retrofitted leg members was developed in STRAND 7 through the use of truss-beam elements to represent tower members, and spring elements to simulate bolted connections. Numerical results based on the new modeling method agreed well with those from previous experimental retrofitted tower tests. A parametric study showed that: (a) the cruciform connections have a high load-transfer performance in retrofitted steel lattice towers and thus are effective in enhancing the strength of the whole structure, (b) pre-loading in tower members delays the occurrence of the peak load-sharing rate and excessive pre-loading causes a significant reduction of structural strength, and (c) extra mid-panel connections, cruciform connections with higher slip-resistance and increased retrofitting member size can significantly enhance the structural load carrying capacity.