Steel lattice transmission line towers (TL) are widely used as supporting structures for overhead powerlines. According to their supporting configuration, these free-standing tower structures are classified as: Self-supporting towers and guyed towers. In general, they are designed for conductors’ weight and environmental loads such as ice accretion and wind gustiness. Other exceptional loads, such as cable rupture and ice-shedding effects, are also considered. Due to an overall perception that these structures have a relatively low vulnerability to earthquake loads, the earthquakes effects are usually not considered in TL tower design. The current standards, for instance, do not require a design check for earthquake loads, although a significant percentage of transmission line infrastructure is located on Western and Eastern Canada where the seismic risk is considered high and moderate-to-high.
The main objectives of this research are: i) to assess the sensitivity of typical TL towers to earthquake loads, ii) to propose a simplified static method able to approximate the seismic response of TL guyed towers and iii) to study the dynamic interaction between the overhead powerlines and their supporting guyed towers.
In this study, two guyed towers (37.7 m and 53.1 m height) and two Self-supporting towers (36.7 m and 57.1 m height) designed according to current standard provisions were selected for investigation. Detailed three-dimensional finite element models developed in ANSYS-APDL software were subjected to nonlinear time-history analyses. To study the sensitivity of these TL towers to earthquake loads, two sets of ten seismic ground motions were selected as representative for Western Canada. The first set corresponds to site Class “C” and the second to site Class “D”. It is important to note that the frequency content of these records is close to the natural frequency of the studied towers. Each tower was subjected to the aforementioned sets of seismic ground motions and the responses in term of axial forces triggered in tower members were compared with those resulted from standard load cases used in design. It was found that guyed towers are the most sensitive to seismic ground motions.
To reduce the computation time, an equivalent static method is proposed herein in order to approximate the seismic response of the free-standing guyed towers.
Finally, the dynamic interaction between the overhead powerlines and their supporting guyed towers is evaluated. This is done by carrying out detailed nonlinear transient simulations of the coupled tower-conductor system for a set of earthquake ground motion records of different frequency contents and by comparing the results of these simulations with the ones carried out for the free-standing towers.