Seismic Analysis of Transmission Towers—Case Study

Transmission tower designers are left with little to no guidance on how to design and analyse towers when considering seismic excitation. There are few valuable published works with suggestions on how calculate and lump a tributary mass of the span of conductors, insulators, etc. to the attachment points on the tower supporting the span. This is normally followed by analysing the tower using simplified, quasi-static, or response spectrum methods to obtain the straining action necessary to perform the design. There are several valuable investigations and data collection on the real-life behaviour of transmission towers after seismic events that suggest the majority, if not all, of the recorded failures are attributed to foundation issues. It is still necessary, however, for a transmission tower designer to conclude with higher level of certainty whether a seismic load case is likely to govern the design of the tower, member of a tower or group of members, or not. As part of BC Hydro’s design plan for a new transmission project including three new, short 500 kV transmission lines, analysing the self-supporting lattice towers on these lines was required. The analysis is performed using several methods: 1. Simplified analysis of the towers with conductors lumped masses; 2. Response spectrum analysis of the towers using conductors lumped masses; 3. Full time history analysis of towers using conductor lumped masses and synthesized earthquake acceleration; and 4. Full time history nonlinear analysis of one of the three lines as coupled system consisting of towers and conductors using synthesized earthquake acceleration from the site design spectrum. This paper presents the results obtained from each of the four methods, comparison between forces obtained in several members as well as a discussion of the level of effort involved with each method.