Every year, about 17,000 kilometers of new transmission lines above 110kV level will be built in China, and about 450,000 tons of steel will be used to construct transmission towers every year. With the rapid development of China's electric power industry, more and more transmission lines with large capacity, long distance and high voltage are used, and the load of poles and towers is increasing.
At the same time, due to the shortage of land resources and the improvement of environmental protection requirements, the problem of line corridor and demolition is becoming more and more serious. In order to reduce the line corridor, reduce the amount of demolition and the occupation of land and foundation earthwork excavation, multi-loop tower with the same tower and large slope will be used more and more. With the development of tower scale and large-scale, the specifications of tower profiles become larger and larger, which makes the commonly used low-strength hot-rolled angle steel difficult to meet the application requirements. It is necessary to seek new forms and varieties of steel profiles.
With the development of China's iron and steel industry and the continuous improvement of scientific and technological level, high-strength grade steel and other forms of steel products are emerging more and more. Cold-formed thin-walled steel has been widely used in construction, transportation, municipal, industrial and civilian industries due to its advantages of light weight, high strength, high material utilization and convenient construction. In this paper, the feasibility of using high strength cold-formed thin-walled steel in transmission tower is discussed, in order to promote the use of cold-formed thin-walled steel products in power industry.
Development Status of Transmission Line Towers
At present, the main materials used for transmission line towers in China are hot-rolled angle steel, supplemented by a small number of steel pipes. The main types of steel are Q235 and Q345. The towers are hot-dip galvanized and anticorrosive, and their service life is about 50 years. With the continuous development of iron and steel industry, the steel used in the tower has also been improved in material and type. The concrete manifestations are as follows:
Use of high strength steel. Compared with the advanced countries outside the park, the steel used in transmission tower structure in China is single in variety, low in strength and small in choice. When the load of tower is large, only the composite section can be used to make up for the shortage of low strength of material, which increases the workload and investment of design and processing. Ten kinds of material standards are recommended in the Guidelines for Design of Transmission Towers in the United States for designers to choose. The yield strength of high strength steel reaches 448 MPa. The strength grade of steel listed in the Code for Design of Steel Structures in Russia has reached 578 Mpa. In recent years, with the continuous development of China's metallurgical industry, the production of high-strength steel is no longer difficult. The quality of high-strength structural steel in China has been improved rapidly and steadily, and the supply channels are becoming more and more smooth, which makes it possible to use high-strength steel in transmission line poles and towers. In the early scientific research project of 75OkV transmission line, the State Electric Power Company's Electric Power Construction Research Institute has studied the connection structure, design parameters of components, supporting bolts and economic benefits of high-strength steel when it is used. It is believed that high-strength steel has fully met the requirements for use in iron towers in terms of technology and use, and the use of high-strength steel can be used. Reduce tower weight by 10%-20%. But at present, the biggest obstacle to the extensive use of high strength steel is the supply of section steel. There are many kinds of steel needed for the tower, and the demand for various specifications in batches is not large. At the same time, the forming of high-strength steel is difficult, the cost of rolling die is high, and the adjustment of rolls is difficult, which makes the enthusiasm of the steel plant to produce high-strength steel is not high and the supply is difficult to guarantee.
Use of steel pipe towers. In the 1980s, when many countries in the world developed UHV transmission lines, steel tube profiles were applied to the tower structure, and steel tube towers with steel tube as the main material appeared. In Japan, steel tubular towers are almost used in 10OOkV UHV lines and towers. They have thorough research on the design technology of steel tubular poles.
Drawing lessons from foreign experience, steel tube profiles have been used in the double-circuit tower of 5OOkV and the four-circuit tower of the same tower in China, reflecting its good performance and benefits. Its advantages are as follows: firstly, it can reduce the wind pressure on the tower body (the shape coefficient of the member is almost double that of the angle steel); secondly, under the condition of equal cross-section area, the rotary radius of the round pipe is about 20% larger than that of the angle steel; thirdly, it can improve the bearing capacity of the structure, generally speaking, the consumption of the steel tube tower is reduced by 10% - 20% than that of the angle steel tower; at the same time, it can reduce the number of members, shorten the construction period and is easy to build the tower. The structure is diversified. However, the use of steel tube also has the shortcomings of limited specification, poor external quality and high price. At the same time, the joint structure between steel tubes is complex and the processing efficiency is low.
Use of cold-formed thin-walled section steel. Cold-formed thin-walled steel has been used abroad for decades. It is understood that the countries that have used cold-formed thin-walled steel in transmission towers are Italy, the United States, Canada, the United Kingdom, the former Soviet Union, Mexico and so on. According to foreign statistics, cold-formed thin-walled section steel can save 10% - 18% of steel compared with traditional hot-rolled angle steel. The U.S. Guidelines for the Design of Transmission Towers contain descriptions for the bearing capacity calculation of equal-limb crimped angle steel and other section steel except angle steel, which indicates that the American Association of Civil Engineers has already noticed the application of cold-formed section steel components in the tower. However, in China, the use of cold-formed thin-walled steel in transmission towers is still blank. With the large-scale development of tower structure, the profile specifications of the tower are becoming larger and larger, which makes the commonly used low-strength hot-rolled section steel difficult to meet the use requirements, and the specification of hot-rolled section steel is limited, and the supply of goods is insufficient. Although the shortcomings of profiles can be partly compensated by the special treatment of composite sections and joints, the design of profiles can be improved.