Background

The Civil Engineering and Development Department (CEDD) has been constructing the Fanling Bypass Eastern Section (FLBP(E)), one of the major road infrastructures connecting the Fanling North New Development Area (FLN NDA) with the existing Fanling Highway, to cope with the anticipated traffic demand arising from the new development. Pedestrian connectivity is also a key consideration for promoting sustainable modes of transport, so two footbridges are being constructed alongside the carriageways.

The two footbridges have distinct articulations. Footbridge F4 is a two-span footbridge across the Ng Tung River, connecting the FLN NDA and Siu Hang San Tsuen. The two spans, each approximately 35m-long, connect monolithically to the viewing platform at the pier supporting the viaducts of FLBP(E) and are supported by bearings on the riverside abutments. Footbridge F6 is a more extensive structure, with a 40m diameter circular deck founded monolithically on the piers at the proposed Lung Yeuk Tau Interchange, and four link bridges with maximum spans of 36m founded on bearings on the supporting piers at the sides of the roundabout. The footbridge is also equipped with cycle tracks and lifts for connecting the cycle tracks of FLN NDA with the existing extensive cycle track network in the North District.

Under the conforming scheme, the proposed use of concrete girders to construct the footbridges across a river and over the existing Sha Tau Kok Road, the main artery of the North District, poses different challenges to the project team. Construction works over a river would inevitably be affected by the highly variable water flow in dry and wet seasons. If falsework were needed for cast in-situ construction, the erection, maintenance and dismantling of temporary structures would pose safety risks to workers. The limited space adjacent to the riverbank also poses challenges for installing precast concrete beams. For the construction works at Lung Yeuk Tau, Sha Tau Kok Road, where most traffic and underground utilities of the North District are carried, the congested site was unfavourable for constructing the substructures of the footbridge.

Location of Footbridges F4 and F6 in FLN NDA

The project team of Contract ND/2019/04 is led by CEDD, and comprises AECOM Asia Co. Ltd. (AECOM) as project manager and the Daewoo – Chun Wo – Kwan Lee Joint Venture (DCK JV) as contractor. In view of these challenges and a tight construction programme due to the impact of the COVID-19 pandemic since the contract commencement in 2020, the project team sought an innovative solution able to significantly reduce the weights of the footbridge superstructures, and the extent of foundation works. The innovative solution should also utilise effective off-site fabrication to enhance productivity and safety, with the view to completing the Interchange in time to tie-in with the population intake at the FLN NDA.

Use of High-Strength steel

The construction challenges were effectively overcome by selecting the high-strength steel as the optimal solution for constructing the two footbridges, given its lightweight nature and suitability for modularised off-site fabrication. In terms of cost, the price increase associated with using high-strength steel is generally less than the proportional increase in yield strengths, and therefore, making it economical to use these high-strength steel in structures where its strength can be fully utilised. Besides cost savings, an increasingly important benefit is the reduced use of materials and resources in the construction of steel bridges. Therefore, there is always an environmental benefit for using these high-strength steel¹. It should be noted that the high-strength steel has been used in bridge construction worldwide for many years. For example, several hundred bridges in Japan were constructed with those high-strength steel with yield strengths of 500 to 600 MPa, and even several with 800 MPa, such as the Akashi Kaikyo Bridge completed in 1998., With the above benefits, what, then, had prevented engineers in Hong Kong from widely adopting high-strength steel in structural? Firstly, the design codes available in the 2010s for structural design in Europe, which Hong Kong follows, covered steel grades up to S700 as merely simple additional rules in EN 1993-1-12: 2007 even though their complementary materials standards covered steel grades up to 960 MPa. Hence, for many years, this lack of detailed structural design rules limited the use of these higher-grade steel in construction². It should be noted that the structural design of the S700 high-strength steel is now included in the second generation of EN 1993-1-1: 2022 while the structural design of the S1000 high-strength steel is being incorporated into EN 1993-1-12 which is scheduled for publication in 2027.

Secondly, before late 2000s, although the manufacturing of normal grade steel in China was already sophisticated, the production of high-strength steel required advanced metallurgical engineering, with precise alloy design, thermomechanical controlled processing and strict quality control to ensure materials consistency and weldability of the highstrength steel. For many years, thousand tons of the highstrength steel were imported into China from Europe, and they were commonly used as main structural members in heavy lifting equipment and large mechanical systems. Their supplies usually came with a premium cost, long lead times in procurement, and uncertainties in the supply chain due to geopolitical risks, hindering the development of their use in construction.

In recent decades, China has emerged as the largest steel producer in the world, thanks to its advancements in steelmaking technology. Since the 2010s, a number of leading Chinese steel producers have regularly manufactured highstrength S690 and the ultra-high-strength S960 steel plates in full compliance with the mechanical property requirements of structural steel EN 10025, supported by complete certification on their quality assurance systems⁴. With these advantages, engineers in Hong Kong have begun to use these high-strength steel extensively in both public and private construction projects.

The Decision

With the ready supply of good quality high-strength steel in China, the feasibility of adopting ultra-high strength S960 steel in the construction of the aforementioned footbridges was explored. On 27 June 2023, the Civil Engineering and Development Department and the Chinese National

Engineering Research Centre for Steel Construction (Hong Kong Branch) at the Hong Kong Polytechnic University (CNERC) signed a Memorandum of Understanding to deepen collaboration in the development and promotion of effective use of ultra-high-strength S960 steel, paving the way for the world’s first use of this novel structural steel in infrastructure construction.

The Signing Ceremony of the Memorandum of Understanding on 27 June 2023

Technical Challenges

In the absence of codified design rules for ultra-highstrength S960 steel in BS EN 1993-1, the pioneering use of S960 steel for Footbridges F4 and F6 required innovative solutions to overcome several technical challenges, including the development of technical guidance for the design and construction specific to the use of S960 steel, and procedures for welding during both shop fabrication and site assembly.

Effective design rules had been established to demonstrate the structural adequacy and safety of the proposed Footbridges F4 and F6. To maintain the appearance of the concrete box girders in the conforming design, these footbridges were designed as steel-plated box girders. With established expertise in effective use of high-strength steel in construction, the CNERC was commissioned to compile an expert technical guidance that would enable the application and extension of relevant design rules in various parts of EN 1993-1. Various clauses in Parts 1, 5, 8 and 12 of EN 1993-1 covering relevant design rules for welded sections subject to local plate buckling and overall member buckling, and provisions on longitudinal and transverse stiffeners were critically reviewed. In addition, a comprehensive review on both experimental and numerical investigations on S960 steel members reported in the literature was also carried out.

Following the drafting of the expert technical guidance, selected plated box girders of S960 steel designed in accordance with the guidance was successfully tested. The result demonstrated that the guidance was applicable to the structural design of the plated box girders for the proposed footbridges using ultra-high-strength S960 steel with a high level of structural safety and efficiency. Being one of the leading experts in high-strength steel construction in China, Prof Y J Shi of Tsinghua University was also invited to offer his valuable insight on the guidance. The finalised guidance was then issued to YWL Engineering Pte. Ltd. as the definitive document for the footbridge design. It should be noted that additional research and design development should be carried out to extend the scope of applicability of the guidance for general use.

Another hurdle was to ensure weldability of S960 steel during welding. It was important to source steel plates with a low Carbon Equivalent Value (CEV) to minimise the risk of post-weld hydrogen embrittlement. Before use, all steel plates were rigorously tested by a HOKLAS-accredited laboratory to confirm their mechanical properties. Selecting appropriate welding consumables was equally critical, and low-hydrogen solid wire electrodes were chosen to guarantee both quality and ductility of the welded joints. S960 steel is also known for its microstructural changes caused by heating and cooling cycles during welding. To minimise reductions in strength and ductility in the welded S960 steel plates, CNERC systematically examined the welding processes through experimental investigations and numerical simulations. Specific welding procedure specifications including optimal ranges of heat-input energy during welding for different joint details in S960 steel plates, were successfully derived.

It was crucial for the project team to address a local skills gap as no welders in Hong Kong had prior experience with the S960 steel. To build this capability, a comprehensive training programme for site welding was implemented in collaboration with CNERC. In addition, robotic site welding was introduced to further enhance efficiency and consistency of welding, and also safety of the workers.

Site welding of S960 steel joints

Finally, due to the complex geometry of the plated box girders, including the use of 10 to 16 mm thick steel plates and the circular layout of Footbridge F6, the project team carefully managed the geometry control and the weld sequence to ensure that the welded structure met the design profile. Safety during welding within semi-confined spaces was managed meticulously through careful planning and dynamic risk assessment. A modularisation strategy was adopted, maximising off-site prefabrication and thereby reducing the extent of site welding in constrained areas. The project team also addressed the crucial issue of distortion in welded thin plates. Precise geometric control was achieved using physical restraints, such as strongbacks and clamps, complemented by a carefully sequenced welding followed by a heat-straightening process that released locked-in residual stresses during fabrication.

Fabrication, Assembly and Installation

Sourcing a competent S960 steel producer and fabricator was the key to the realising this pioneering application. DCK JV successfully partnered with Nanjing Steel Group Co., Ltd. and Daming Heavy Industry in Jiangsu Province, which have extensive experience in manufacturing S960 steel in compliance with European Standards and fabricating heavy equipment using S960 steel, respectively. Fabrication commenced in April 2024 with the first batch of modules for Footbridge F4 delivered to the site in Hong Kong by sea two months later. Fabrication of the final batch of the modules for Footbridge F6 was completed in March 2025.

To maximise the benefits of S960 steel, the project team maximised the sizes of the steel modules to reduce assembly and installation works on site. With a deck width of 4.5 m, the six modules of Footbridge F4, with a total weight of 85 tonnes, were readily fitted into standard trailers for delivery from the container terminal in Kwai Chung to the site in the FLN NDA. Footbridge F6, however, had a width of 8.15 m at its circular deck and up to 7.8 m at its link bridges. Balancing the benefits of minimising site welding of with the risks of transporting large modules on public roads, the project team arrived at an ambitious plan of fabricating all 40 modules with a maximum width of 7.8 m in Jiangsu, and delivering them from the temporary cargo handling area in Tuen Mun to the site in FLN NDA at night.

As some of the modules had widths exceeding 7.3 m, the width of a standard two-lane carriageway, the project team collaborated with the University of Hong Kong to utilise their 3D Swept Path Analysis and Virtual Trial Runs, based on the geospatial data collected by the Lands Department, to simulate and assess the risk of conflict with existing features along the 34 km long delivery route. This innovative approach greatly facilitated the liaison with the Transport Department and the Hong Kong Police Force in processing the necessary approvals required for this unprecedented operation. Thanks to the collaborative efforts, all 40 modules were delivered seamlessly over 12 nights between February and April 2025.

Delivery of oversized modules of Footbridge F6

Fabrication of Footbridges F4 & F6

To cater for the different geometry and aesthetic requirements of the two footbridges, different connection methods were adopted, i.e. bolted connections for Footbridge F4 and welded connections at Footbridge F6.

With careful fabrication and precise detailing of connection plates, the modules of Footbridge F4 were assembled seamlessly on site. An assembly ceremony was held in July 2024, and prominent figures from the local construction industry were invited to witness the final stage of site assembly of the world’s first S960 ultra-high-strength steel footbridge through bolting of the segments. The two spans were placed onto their designed positions over the Ng Tung River with a 350-tonne mobile crane near the time of the 75th Anniversary of the Founding of the People's Republic of China, symbolising success in the country’s technological innovation and the distinctive advantages of Hong Kong in having strong support from the Chinese Mainland and maintaining close connection with the world.

On-site Welding

Even with the optimal welding parameters derived by CNERC, well-trained local welders of DCK JV and effective optimal modularisation to minimise site welding, there were still over 1,500 m of welding operation that had to be completed on site for assembly of Footbridge F6. The main challenges included fitting the modules on site with precise alignment to ensure that the overall geometry was within construction tolerances, precise joint preparation between steel plates of only 10 mm thickness, and managing the effects of hot weather and rains on welding. Successful robotic site welding proved to be crucial for the timely completion of all onsite welding operations.

The project team completed the assembly of the circular deck for the quadruple-span lift using four sets of strand jacks within a few hours in July 2025. Additionally, all the link bridges were installed by September 2025.

Installation of the second span of Footbridge F4 in September 2024

Robotic site welding performed by JCT qualified welding operators for timely completion of Footbridge F6

Quadruple span lifting with strand jacks

The world’s first S960 steel footbridge assembly ceremony in July 2024

Assembly and site welding of the circular deck

Way Forward

Compared with in-situ reinforced concrete structures and normal-strength S355 steel structures, the adoption of S960 steel significantly reduces the extent of foundation works and the use of construction materials, thereby enhancing speed, quality, efficiency, and productivity of the project, ultimately benefiting society and promoting sustainability in construction.

The project team looks forward to wider adoption of high-strength steel in Hong Kong following the successful experience gained from this pilot project. It should be noted that CNERC is finalising a Group Standard entitled “Standard for structural design of S690 to S960 high-strength steel structures” for promulgation by the China Steel Construction Society before the end of 2025. The Group Standard consolidates experience gained from this pilot project together with test data derived from its experimental verification. The Group Standard is formulated to be compatible with the Structural Eurocodes, and hence, it will facilitate the design of high-strength S690 to S960 steel structures by international engineers, and enabling the export of new technologies, materials, and standards from the Chinese Mainland to the rest of the world through Hong Kong.

Moreover, the project team is working with the Building Technology Research Institute to compile technical guidance and specifications for robotic welding on highstrength steel. These documents are being prepared for wide dissemination to the local construction industry, and to provide assurance that the challenges in welding S690 to S960 steel can be readily overcome through innovative technologies.

Through these efforts, and by leveraging collaboration between universities, government, and industry, the project's success demonstrates Hong Kong’s roles as a “Super- Connector” and “Super Value-adder”.

Acknowledgement

CEDD would like to express its gratitude to the project manager, AECOM Asia Co Ltd, the contractor, Daewoo – Chun Wo – Kwan Lee Joint Venture, together with its subcontractors, including the footbridge alternative designer and on-site installer YWL Engineering Pte. Ltd, S960 steel producer Nanjing Steel Group Co Ltd, S960 steel fabricator Daming Heavy Industry, as well as its academic partners – the Chinese National Engineering Research Centre for Steel Construction (Hong Kong Branch) at the Hong Kong Polytechnic University, and Tsinghua University, for their collaborative efforts in turning the concept of adoption of ultra-high-strength S960 steel in footbridge construction into reality. Special thanks also go to Jumbo Construction Technology Co Ltd (JCT) for the execution of robotic site welding and development of welding procedure specifications.

The policy support in adopting high-strength steel from the Development Bureau, and the technical support from the Lands Department, Transport Department, Hong Kong Police Force, and the MiCLab of the University of Hong Kong for the delivery of oversized S960 steel footbridge modules in Hong Kong, are gratefully acknowledged.

About the Authors:

1. Ir Tom W L LEUNG, Civil Engineering and Development Department, Government of the Hong Kong Special Administrative Region
2. Mr Keith W K CHENG, Civil Engineering and Development Department, Government of the Hong Kong Special Administrative Region
3. Ir Raymond C HON, AECOM Asia Company Limited
4. Ir Bear Z C DING, Chun Wo Construction and Engineering Co., Ltd.
5. Ir Prof K F CHUNG, The Hong Kong Polytechnic University
6. Mr M Y LEE, AECOM Asia Company Limited
7. Ir Peggie P K HON, Chun Wo Construction and Engineering Co., Ltd.
8. Ir Dr H C HO, The Hong Kong Polytechnic University
9. Ir Y W LEUNG, YWL Engineering Pte Limited
10. Ir Joyce Y Y LAU, Civil Engineering and Development Department, Government of the Hong Kong Special Administrative Region

References

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2. Gogou, E. (2012). Use of High-strength steel Grades for Economical Bridge Design.
3. Miki, C., Homma, K., & Tominaga, T. (2002). High strength and high-performance steels and their use in bridge structures. Journal of Constructional Steel Research, 58(1), 3–20.
4. Chung, K.F. (2022). Effective use of high strength S690 to S960 steel in construction. Hong Kong Engineers, 50, 10-20.