Failure intervention of transmission line tower primary legs using bi-angled cruciform retrofitting

Retrofitting has been suggested by researchers as an intervention measure against failure of transmission towers which needs to be done with minimal disruption. Retrofitting of the primary leg members of transmission line towers by bi-angled cruciform arrangement has been shown in literature to be effective. However, the influence of stresses and deformations existing in the leg member prior to retrofitting on the capacity of the final retrofitted member is not known. Therefore, this study assesses the effectiveness of bi-angled cruciform sections under preload conditions through experiment followed by a numerical study. The level of preload in the primary leg member with respect to its theoretical capacity, slenderness ratio and width-thickness ratio of the angle section were the parameters. The results suggest that bi-angled cruciform retrofit is more effective at lower slenderness and width-thickness ratios and the reduction in capacity of retrofitted member is more noticeable till 50 % preload after which the changes are not significant. While the former two parameters lead to lower load share in the retrofitting member, higher preloads however lead to more equal load share. The application of this retrofitting pattern is demonstrated numerically with a full-scale tower model simulated under code specified load cases. Critical load cases leading to tower failure before design loads were identified and the retrofitting was incorporated into the model for these load cases. The analysis showed that the final loads attained in these load cases improved by upto 70 % thus confirming the potential of this retrofitting technique in preventing tower failure.