Designing power line towers using circular hollow sections

Abstract

It has become a challenging exercise to obtain land in order to further develop the electrical infrastructure in South Africa. The reason for this is that high voltage transmission towers visually impacts the surroundings and require a large servitude in order to accommodate these structures. The requirements for low visible towers with small foundation footprints may be achieved with double circuit power line towers. However, the structural loading in tower member's increase drastically as a result of large conductor bundles, higher reliability, smaller foundation footprints and a increase in wind loading because of the taller structures. This limits the further economical use of standard angular hot rolled sections and requires that alternative cross sections are considered in the design of power line towers. The aim of this research is to focus on the practical and cost-effective implementation of circular hollow sections (CHS) in power line towers. The design of a power line system consist of a family of tower structures which include a large number of structural and non-structural members as well as many connections resisting various combinations of loads. The outcome of this research proves that a feasible and practical way exist to implement circular hollow sections in power line tower design using current design software, current design standards and current manufacturing techniques for South African conditions. It is recommended that connections between tower elements should be similar to existing connection practices where possible. This will reduce the requirements for specialized software or connection standards. This will also facilitate the design of hybrid tubular and angular member towers. Hence a review of current angular member and connection design practices are given for the reader. Before the design of a tubular power line tower may be done, various hollow section connections and stability criteria are reviewed. The CIDECT manuals provide an excellent resource for hollow section connections for static and dynamic conditions. It is important to note that it is not the intention of the author to question or improve on the existing hollow section design formulae, but rather to show their ease of implementation in the power line industry. A tubular tower was designed and fabricated in order to combine the theory and practical implementation thereof. In the design of this test tower, the author introduced a novel cross arm design. The new configuration cross arm has only three main chords compared with the conventional cross arm with four main chords. It is envisaged that this new cross arm configuration will reduce overall tower cost as well as construction cost. An analytical and numerical structural analysis was used to design the test tower. An isolated analysis was also performed on the tower cross arm in order to compare and validate the use of less expensive structural software. The comparison considered a full nite element analysis (ANSYS) compared with a beam element analysis (Prokon). The results show that there is an excellent correlation between the two models given that specific, yet simple modeling techniques are used to model the tower elements. In order to conclude the validity of the recommended design approach and the integrity of the test structure, physical testing was done at the Eskom tower test facility. The structure was securely fixed to the base of the test bed and strain gauges were fitted on several of the tower members. Steel wire ropes with load cells were fitted to the cross arms of the structure and three typical load cases were evaluated. Comparing the physical test results with the Prokon model, a 10% variation between member loads were recorded. The loads in the test tower was in most cases higher compared with the Prokon model. In summary, the design process proposed here may successfully be used to design and manufacture CHS power line towers. The design process uses current design software, current design standards and current manufacturing techniques. Further investigation on full scale structures are required in order to study the economics of tubular towers versus angular member towers. This study should include fabricators and construction experts in order to comprehend the impact on the power-line industry. The author suspects that the fabrication cost of CHS towers will be slightly higher but the construction cost will be significantly less.