Detailing Redundant Pattern–Induced Failures in Transmission Line Towers: Fid-Bau Portal

Detailing Redundant Pattern–Induced Failures in Transmission Line Towers

Rao, N. Prasad / Balagopal, R. / Rokade, R. P. / Palani, G. S.

Transmission line (TL) towers may fail prematurely during mandatory testing for many reasons, even though they were designed based on the provisions of different standards. Five premature failures observed during full-scale testing at the Tower Testing and Research Station, Structural Engineering Research Centre [Council of Scientific and Industrial Research (CSIR)-SERC], Chennai, India, are grouped into two categories and studied in detail. The first two towers failed due to improper and inadequate redundant patterns and design. The third tower failed due to the detailing not matching the design assumptions. The fourth tower failed due to insufficient bolts in the stitch plates, and the fifth tower due to improper detailing. All the failures are modeled using general-purpose finite-element software. The test and analytical results are compared with the design code provisions. This study emphasizes the importance of detailing and the secondary member design.

The reasons for the premature failure of transmission line towers during prototype testing are discussed. The reasons for failure are categorized as failure due to inadequate redundant patterns in design and failure due to improper detailing. The following remedial measures are discussed to prevent premature failure in full-scale testing of TL towers. The redundant member force from the analysis shall be checked to ascertain if it is within 2.5% or more of the axial force in the primary member or limiting slenderness ratio of 250 as per design code provisions. The guidelines proposed for detailing the gusset plate connections such as the line of action of force from the cross-arm ground wire peak shall meet at the common node for smooth transfer of forces as per design assumption. Suggestions are provided to increase the number of stitch bolts in cruciform connections of a 150- or 200-mm wide flange, and all these stitch bolts shall be designed to cater to the actual leg force.