ESTIMATING THE RELIABILITY OF TRANSMISSION LINE TOWERS

Tangent suspension lattice towers are the most numerous and also the most competitively designed elements of a transmission line. These towers have a small degree of static indeterminacy and failure of any one of a few critical members leads to tower failure. First member failure can be treated as the tower failure, and the tower system simplified as a purely series system for reliability analysis. Loads on the tower consist of wind, wire breakage, temperature and dead load effects. Traditionally, only wind on the tower and dead load are modelled probabilistically while the other major load case of wire breakage is treated deterministically. This study looks at the relevance of this assumption by modelling the joint effect of wind, temperature and wire breakage probabilistically. Wire failure is assumed to occur once tension in a wire reaches its ultimate strength under the combined action of dead, wind and temperature stresses. The failure of the tower is due to force in any member crossing its limit state of strength. Appropriate models have been used for loads on the tower and member resistances. A program has been developed to model the tower, interface with the structural analysis and perform the reliability analysis. A commercial finite element package (NASTRAN ® ) is used in this study to perform structural analysis. Monte Carlo Simulation results are found to compare well with first order reliability estimates. Using the software, reliabilities of a couple of standard towers are evaluated under critical load combinations