Effective use of high-strength S690 steel in construction

With advances in metallurgy and steel-making technology, structural steel with yield strengths up to 690N/mm2 is produced regularly in many modern steel mills in different countries, such as China, France, Germany, India and Japan. The yield strength of S690 steel is almost twice that of the commonly used structural S355 steel. High-strength steels are used in a wide range of structures, such as heavy-duty lifting equipment in mines and ports, frames and members in wind turbines, as well as container trailers and passenger trains. Use of high-strength steels is highly advantageous for these moving structures because they can carry increased payloads and hence use less energy during operation. For civil engineering and building structures, the use of S690 steel allows a significant reduction in the self-weight of a structure and also a reduction in the loads to be imposed on supporting structures and foundations. In recent years, the cost per tonne of S690 steel in many parts of the world has decreased steadily, which has improved its competitive position with respect to S355 steel. In China, the unit cost of S690 steel has varied typically from 1.25 to 1.35 times that of S355 steel over the past five years. It may be argued that owing to the increased strength of S690 steel, only half of the steel tonnages are needed, compared with S355 steel. Hence, the steel material cost becomes roughly 1.3 times 0.5, or 0.65, of that of S355 steel, leading to a significant saving in material costs. Research work on mechanical properties and structural behaviour of welded sections made from S690 steel conducted at the Chinese National Engineering Research Centre for Steel Construction (Hong Kong Branch) has shown that provided the welding processes are properly controlled, it is possible to minimise or even eliminate any reduction in strength and ductility which might occur due to microstructural change. Extensive research and development work have also been conducted to promote application of high-strength steel for construction. A steel bridge with two arches using 4400 tonnes of S690 steel in Hong Kong, and a steel bridge with mega trusses using 1625 tonnes of Q690 steel in Macau are briefly described. In addition, the initiative of the Development Bureau of the Government of Hong Kong SAR in promoting the use of high-strength S690 steel is highlighted.