Redundant Induced Bracing Failures in Transmission Line Towers

Transmission line towers designed based on the provisions of different standards may fail during mandatory testing for many reasons. Two different types of premature failures observed during full-scale testing at Tower Testing and Research Station, Structural Engineering Research Centre, Chennai (CSIR-SERC), are studied in this paper. These towers failed prematurely due to improper design assumptions secondary to the primary bracing connection. The failures are modelled using general-purpose finite element software. The test and analytical results are compared with various codal provisions. Hip bracing connected to primary bracing with a single bolt acts as a hinge and fails to provide adequate restraint. The assumption that hip bracing provides restraint to the primary bracing against the in-plane deformation is invalid, and it can only prevent the primary member's displacement along the hip's axis due to its axial stiffness. This study recommends a double bolt connection for redundant members.