However, structural collapses, mainly associated with wind action, are not uncommon. A typical example of this failure was found in the Itaipu electrical power line between the cities of Foz do Iguaçu and Ivaiporã, in the Paraná State, Brazil. This incident occurred on November, 2nd, 1987, where ten steel towers collapsed due to wind forces that reached, at that time, 130km/h (36m/s) [1,2].

Based on the usual methods of structural analysis involved in the design of steel telecommunication and transmission towers, the structure's dynamic behaviour is not considered. However, the main loading to be taken into account in the electrical transmission line tower's structural analysis is produced by the wind, which acts dynamically over the structural system composed by towers and cables. Despite this fact, it is not uncommon for slender steel towers to present disadvantageous dynamic properties, making them vulnerable to wind action [1].

Considering that many accidents associated with this kind of structure occur even for wind velocities below that specified in the design process, it is possible that most of these accidents have been produced by dynamic actions. Therefore, this study develops a structural analysis method that allows for a more accurate evaluation of the electrical transmission tower's dynamic behaviour [1,2].

This paper proposes an analysis methodology based on the use of a finite element model that can accurately simulate the coupled behaviour between the transmission line cables and the suspension structures when subject to the dynamic loading produced by the wind. The three-dimensional model proposed is composed by beam and truss spatial finite elements and accounts for the geometric non-linear effects produced by the large displacements associated with the line cables and insulators. The wind loading is modelled as a random process based on its statistical properties [1].

The conclusions have indicated that the numerical results are very useful to demonstrate the most important aspects of the investigated transmission line structural behaviour. This relates to the electric and lightning cables oscillation, when submitted to the action of the wind forces that excite the steel towers dominant vibration modes. The fundamental period is equal to 6.51s which means that, when exposed to the atmospheric turbulence dynamic effects, the fluctuating response of the low-damped tower-cables coupled system, in along wind and across wind directions can be very significant. On the other hand, it must be emphasized that the amplification dynamic factor value is not expected, when the traditional design practice calculations are utilised.

1

M.I.R. de Oliveira, "Structural Analysis of Transmission Line Steel Towers Subjected to Wind Induced Dynamic Effects", MSc Dissertation, Civil Engineering Post-graduate Programme, PGECIV, State University of Rio de Janeiro, UERJ, Brazil, 2005. (In Portuguese)

2

R.S. Rodrigues, "Mechanics of the Aeroelastic Collapse of Transmission Line Towers", PhD Thesis, COPPE/UFRJ, Rio de Janeiro, Brazil, 2004. (In Portuguese)

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