Longitudinal force on transmission towers due to non-symmetric downburst conductor loads

Many failure incidents for transmission line structures have been observed in various locations around the globe as a result of downburst wind events. The localized nature of such downbursts imposes a load case on transmission towers that does not exist under large-scale events such as hurricanes. This load case happens when the downburst location leads to an uneven and unequal distribution of wind velocity along the conductor spans located at opposite sides of a tower. Hence, a difference in the tension forces will develop between the two spans of the conductors attached to the left and right sides of the tower. This difference in tension forces will lead to a net longitudinal force acting on the tower cross arm, which is believed to be the reason for the failure of a number of towers during downbursts. The estimation of this force requires conducting a nonlinear analysis of the conductors under the variable loading conditions resulting from the most critical downburst location, while taking into account a number of key parameters including the flexibility of the insulators and the conductor's pretension force. The objective of this paper is to develop a simple procedure that can be used by practitioners to estimate such a force. A study is first conducted to determine the most critical downburst configuration expected to produce the highest values of the longitudinal forces. A parametric study is then conducted to assess the variations in the longitudinal forces with various geometric and material parameters influencing the behaviour of the conductors. Results of this parametric study are then used to develop a set of charts that can be used to estimate the longitudinal forces using three-dimensional linear interpolation. An example is provided to illustrate the application of the developed procedure. (C) 2016 Elsevier Ltd. All rights reserved.