Do steel transmission towers in Sri Lanka have an optimum base width?

Structural engineers responsible for designing transmission towers optimize the cost of towers by minimizing the weight of a steel tower. However, with the escalating prices of land and increasing concern on environmental issues, it is necessary to minimize the extent of land area occupied by the towers as well. This paper discusses an extensive study on the design of a typical suspension tower in a 132 kV transmission line. The base widths considered were in the range of 3.0 m to 14.2 m. Different heights of towers were considered by varying body extensions from +0 m to +12 m. The study indicated that the weight of steel tower increased 10% to 16%, whereas the land area increased approximately twenty fold. Thus the impact of land value is considerable. The overall cost of tower was computed to include the cost of steel used in the tower and value of land area. Variation in steel unit costs and land prices were also considered in the study. The optimum base widths were found to be in the range of 4.5 m to 6.0 m for lower values of land and 2.7 m to 4.2 m for higher values of land Base widths of existing towers are in the range of 6.5 m to 9.6 m. This study has thus confirmed that considerable cost savings can be realized by optimizing the cost of steel towers by incorporating the value of land as well. An optimum ratio of 6.7 has been obtainedfor height to base width for the range of towers considered