PolyU develops innovative welding technology for ultra-high strength S960 steel making its debut in Hong Kong public works project

S960 steel is lighter and stronger than commonly used S355 steel, making it an effective material for bridge construction. However, the welding process can reduce its mechanical properties, limiting its applications. A research team from the Department of Civil and Environmental Engineering at The Hong Kong Polytechnic University (PolyU) has developed a welding technology for efficient applications of ultra-high strength S960 steel. By precisely controlling the heat input during welding, the mechanical properties of the steel are maintained. This innovation has been adopted in recent footbridge construction in the Fanling North New Development Area, marking Hong Kong’s first public works using S960 steel, and facilitating innovative application of high-strength steel in bridge construction. 

S960 steel is an efficient structural steel known for its remarkable strength. It meets stringent requirements stipulated in many material specifications and structural design codes, making it widely applicable in the construction of high-rise buildings and long-span structures such as sports and exhibition centres. However, the mechanical properties of S960 steel may undergo significant changes during welding, resulting in reductions of both strength and ductility of up to 20% to 30%.

Led by Prof. Kwok-fai CHUNG, Professor of the Department of Civil and Environmental Engineering, and Director of the Chinese National Engineering Research Centre for Steel Construction (Hong Kong Branch) (CNERC) of PolyU, the research team has developed an innovative solution to maintain the mechanical properties of the S960 steel, by examining effects of heat input energy during welding onto the S960 steel through experimental investigations and numerical simulations. In collaboration with a local steel fabricator, the research team concludes that there are optimal ranges of heat energy for different weld joint designs for S960 steel of various thicknesses which minimise or even eliminate adverse effects of welding.

As demonstrated in a recent project undertaken by the Civil Engineering and Development Department of the Government of the HKSAR, this solution has been adopted in two segments of a footbridge in the first phase of the Fanling North New Development Area – Fanling Bypass Eastern Section (Shek Wu San Tsuen North to Lung Yeuk Tau). The two segments were designed as stiffened box girders made of the S960 steel and were fabricated in a factory with effective control of the welding processes.  Site assembly of these two segments of the foot bridge was completed in September this year. The new welding technology enables the use of thinner and fewer steel materials, thereby reducing the self-weight of the footbridge, and hence, the number of foundation piles required. This, in turn, has led to decreased carbon emissions and enhanced sustainability.

Prof. Chung said, “We are dedicated to conducting impactful research on the scientific understanding, engineering principles and design guidance needed to facilitate engineers in adopting high-strength S690 and S960 steel in construction, setting a precedent for modern steel construction. The Fanling North project is a good example of using S960 steel for construction in Hong Kong and provides valuable reference for formulatingrelevant technical guidelines and specifications for future use.”

The technology developed by Prof. Chung’s research team for the effective application of the S690 steel has been implemented in various infrastructure projects, including the Double Arch Steel Bridge of the Cross Bay Link in Tseung Kwan O, the long-span roof structures of the Kowloon Tsai Swimming Pool, and the steel roofs of both the East and West Stands of the Yuen Long Stadium (which is currently under construction).