Effective use of high strength S690 to S960 steel in construction

By Ir Prof K F CHUNG

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Chinese National Engineering Research Centre for Steel Construction (Hong Kong Branch) (CNERC) was established in October 2015 at The Hong Kong Polytechnic University under the Innovation and Technology Bureau of the Government of the Hong Kong SAR. It received endorsement from the State Ministry of Science and Technology of the People’s Republic of China.

 

Funded by the Innovation and Technology Bureau, CNERC is the only designated Research Centre to promote technological innovations and engineering applications in construction in Hong Kong. The primary objectives of CNERC

are:

 

• to promote sustainable infrastructure development in Hong Kong and various parts of China, especially the Greater Bay Area; and

 

• to promote advances in structural engineering on modern steel construction, in particular effective use of high strength steel in construction.

 

Through technical collaboration with the Development Bureau of the HKSAR Government, the Construction Industry Council and various professional institutions in Hong Kong, CNERC supports the Hong Kong Construction Industry to work with:

 

• the Chinese iron and steel industry to demonstrate effective use of high-quality Chinese steel materials overseas; and

 

• the Chinese steel construction industry to participate in Hong Kong and international construction projects which are designed and specified to international engineering and management practice.

 

Hence, through the work of CNERC, a level playing field is established according to prevailing international practice as well as innovative engineering technology so that high quality Chinese steel materials are readily adopted for construction projects specified to international standards. As such, the Chinese steel construction industry is able to compete technically in the international construction markets.

 

In 2020, out of a total 1.8 billion metric tonnes of steel produced worldwide, 1.0 billion metric tonnes were produced in China (https://www.worldsteel.org/), comprising 56.5% of world production (Figure 1). The average unit price of Chinese steel in 2021 was estimated to range from HK$8,000 to HK$10,000 per tonne. Together with an estimated workforce of about 20 million people in China, both the Chinese iron and steel industry and the Chinese steel construction industry have been considered major contributors to the National Strategic Development Plan of the Chinese Central Government for many years.

 

Figure 1: World steel production in 2020

 

High strength S690 to S960 steel

With an ever-increasing demand for infrastructure development in the world, engineers are regularly tasked to design and construct multi-storey buildings and long span bridges with reduced materials, time and money. In general, high strength constructional materials offer very attractive solutions with direct savings in reduced quantities of materials used.

 

In the 1990s, high strength S690 to S960 steels were produced in a number of countries with advanced steel-making technology, such as Germany and Japan. These high strength steels are manufactured as engineering steel plates with typical thicknesses ranging from 6 mm to 30 mm. They are commonly used as key members in large lifting equipment and machinery in ports and mines. Table 1 summarises key mechanical properties given in the European steel materials specification EN 10025 Part 6 ¹. For many years, these high strength steels cost at least 1.7 to 2.5 times that of S355 steel, and their use has become prohibitively expensive in construction when large quantities are used.

 

With advances made in steel-making technology in China during the 2010s, a number of leading Chinese steel producers now regularly produce high strength S690 to S960 steel plates with thicknesses ranging from 6 mm to 80 mm. Table 2 summarises key mechanical properties given in the Chinese steel materials specification GB/T 1591 ². It is clear that both the chemical compositions and the mechanical properties stipulated in EN 10025 and GB/T 1591 are highly compatible and consistent. In general, these high strength Chinese steels are manufactured with specific compositions of special metallic elements through well-controlled heat treatments. As well, they possess excellent mechanical properties, in particular strength and ductility, as shown in Figure 2. Many modern steel producers in China have been accredited to produce high strength S690 steel to EN 10025 and GB/T 1591 with full certification of their quality assurance systems. Therefore, many produce Class 1 materials with cM at 1.0 under the Code of Practice of the Structural Use of Steel of the Buildings Department of the Government of the Hong Kong SAR ³.

 

Table 1: Mechanical properties of structural steel to EN 10025 -1 to 6 and EN 1993-1

 

Table 2: Various requirements of structural steel to GB/T 1591 and EN 1993-1

 

Figure 2: Typical measured stress-strain curves of various steels

 

High strength S690 to S960 steel are highly efficient structural steel, and they possess a high level of ductility which satisfy all requirements as stipulated in various materials specifications and structural design codes.

 

It should be noted that EN 10025 Part 6 stipulates a different set of ductility requirements for S690 steel when compared with that of S355 steel. These requirements should be considered as a balance between cost competitiveness

and structural adequacy for effective use of high strength steel. In general, deformations at fracture of steel members and connections are commonly found to be at merely 5% in practical members.

 

Structural and cost efficiency

In recent years, the unit cost of high quality Chinese S690 steel materials has been typically 1.25 to 1.35 times that of normal strength S355 steel despite a 100% increase in strength. Thus, for a given structure under strength design, only one-half of the steel quantity is needed at Grade S690 compared to that of Grade S355. Hence, the total cost of the S690 steel materials is only about 1.3 x 0.5 or 0.65 that of the S355 steel materials, giving a direct saving on material costs of about 35%. Associated savings on fabrication also exist, notably on reduced materials, effort and time for welding. Other direct savings include reduced costs for handling, transportation and erection as well as reduced loads on supporting structures and foundations.

 

Similarly, the unit cost of high quality Chinese S960 steel materials is about 1.7 times that of normal strength S355 steel despite an increase of about 200% in strength. Therefore, the total cost of the S960 steel materials is only about 1.7 x 0.3 or 0.51 that of the S355 steel materials, giving a direct saving on material costs of about 49%.

 

Consequently, many engineers are eager to exploit the effective use of high strength S690 to S960 steel in construction because of excellent strength-to-self-weight ratios as well as cost benefits. However, with lack of scientific understanding on their mechanical properties and structural behaviour, the potential benefits of these high strength steels in construction have not readily materialised so far.

 

What are the problems?

Even though high strength S690 to S960 steels were commercially available in the 1990s, structural engineers have had technical concerns about adopting them in construction because of conflicting reports on their structural behaviour over the past 30 years. It is noteworthy that the high strength steels are manufactured under a highly-controlled heat treatment process known as “Quenching and Tempering” to produce fine grain crystalline forms with high strength and toughness. However, a heating/cooling cycle induced during welding may initiate phase transformation, re-crystallisation and grain growth in microstructures of the welded sections.

 

Figure 3: Microstructural changes in Q690-QT steel after welding

 

Significant reduction occurs in mechanical properties if both the heat input energy during welding and the cooling rates after welding are not properly controlled ⁴, particularly, in those steel plates with practical thicknesses in construction, i.e. 6 mm to 80 mm thick. It is highly desirable to assess any reduction in various mechanical properties of these welded sections, and, more importantly, to eliminate such reduction in welded sections.

 

Moreover, most modern structural steel codes, such as European Structural Steel Code EN 1993-1-1 ⁵, were published in the 2000s, with their design methods primarily based on test results of normal strength steels, i.e. S275 and S355. Some of these methods have since been found to be overly conservative when applied to S690 to S960 steel owing to lack of sufficient materials data and test results, as stipulated in EN 1993-1 ^{6, 7}. More importantly, little technical information exists in the literature on welding for design and construction engineers to specify as well as to appraise welding procedures.

 

Steel research at CNERC

In order to promote effective use of high strength S690 to S960 steel in construction, CNERC has conducted a number of research and development projects to achieve the following:

 

• Raise technical awareness and acceptance of effective use of high strength steels, as well as their potential advantages in engineering applications using high quality Chinese steels.

 

• Generate scientific knowledge and engineering data on mechanical properties of high strength steel plates and welded sections, and their structural behaviour under various loading conditions in practice.

 

• Develop efficient design rules for adoption of these high strength steels and their welded sections in construction projects, and to obtain acceptance from regulatory agents.

 

• Develop reliable and economical welding procedures for thick high strength steel plates (up to 80 mm) with established quality assurance.

 

• Work with structural engineers to develop innovative engineering applications of high strength steels in construction projects.

 

• Work with steel producers to improve various mechanical properties of high strength steels for seismic resistant structures, and to develop engineering applications on new steel products under coastal and marine conditions.

 

Major equipment at CNERC includes:

 

• 2,500 tonne compression testing system for structural behaviour of S690 to S960 stocky columns and piles (Figure 4);

 

Figure 4: The 2,500 tonne electro-hydraulic servo-controlled compression test system Matthew 2,500 in Laboratory Y001

 

• Robotic welding system for improved welding quality and repeatability to establish suitable welding procedures (Figure 5a);

 

• High frame rate infrared video camera for real-time temperature measurements during welding (Figure 5b); and

 

 

Figures 5a and 5b: The robotic welding system for developing welding procedures specifications in Laboratory W001

 

• Digital Imaging Co-relation (DIC) system for non-contact measurements of displacement and strain fields of structural members, joints and connections (Figure 6).

 

Figure 6: Precision measurements on a T-joint between S690 CFCHS for experimental stress and strain analysis. Dantec DIC system includes two cameras

 

Key achievements of various steel research at CNERC include:

 

• publishing over 100 technical papers on high impact journals and conference papers for international conferences on structural engineering and modern steel construction technology;

 

• establishing effective use of Chinese steel through equivalent steel design to Australian, American, European and Japanese steel materials specifications ⁸;

 

• contributing to development of technical design guides, codes of practice and construction specifications in Hong Kong and China;

 

• providing scientific understanding and experimental results on structural behaviour of high strength steel members and their welded sections for design codification to
  • the Technical Committee of the European Structural Steel Code EN 1993; and
  • the Technical Committee of the Code of Practice for the Structural Use of Steel of the Buildings Department of the Government of the Hong Kong SAR;

 

• conducting an engineering programme at master’s level on structural steel design (steel structures as well as steel-concrete composite structures) to European Structural Steel Code EN 1993 using high quality Chinese steel materials ⁹; and

 

• conducting professional courses for both design and construction engineers on practical structural steel design using Chinese steel materials.

 

Specific tasks completed

The following tasks have been completed:

 

• full range true stress-strain model for S690 and S960 steel undergoing non-uniform deformations up to fracture;

 

• thermomechanical numerical simulation of practical welding process for S690 welded sections to determine transient temperature distribution history during welding and residual stresses after welding;

 

• determination of mechanical properties of S690 and S960 welded sections with various welding parameters under tension;

 

• hysteretic deformation characteristics of S690 and S960 welded sections with various welding parameters under cyclic actions of different target strains and loading frequencies;

 

• mechanical properties and structural behaviour of S690 and S960 welded H-sections under different heat treatments and microstructural changes;

 

• structural behaviour of stocky and slender S690 welded H-sections and cold-formed circular and square hollow sections under (i) compression, and (ii) combined compression and bending;

 

• resistance assessment of partially restrained S690 welded I-sections under lateral loads;

 

• advanced numerical simulation on transverse bending and longitudinal welding in cold-formed circular and square hollow sections;

 

• deformation characteristics of T-joints between cold-formed circular hollow sections under (i) brace axial forces, and (ii) brace in-plane bending;

 

• advanced thermomechanical simulation analysis on profile welding on T-joints between cold-formed circular hollow sections;

 

• design rules and parameters for structural design of columns and beams of S690 welded sections according to European Structural Steel Code EN 1993-1; and

 

• welding technology for S690 and S960 steel plates, and complementary certified welding procedure specifications for various joint details.

 

Through advanced welding technology, it is possible to minimise and even eliminate any reduction in mechanical properties of these high strength steel due to welding.

 

As such, all the steel research at CNERC is considered highly valuable to generate scientific understanding, engineering data and design guidance to facilitate engineers adopting high strength S690 to S960 steel in construction. A number of technical papers, practical design guides and typical welding procedures for construction using S690 to S960 steel have become available as definitive reference documents to pave the way for general acceptance of these high strength steels in construction. Selected projects are described below.

 

Figure 7: Strength reduction and softening in S690 welded sections under tension

 

Figure 8: Stocky columns of S690 welded H-sections with splices under compression

 

Figure 9: Slender columns of S690 welded H-sections under compression

 

Figure 10: Partially restrained beams of S690 welded I-sections under lateral point loads

 

Figure 11: Stress-strain curves of heat-treated coupons of S690 steel with homogeneous microstructures

 

Innovative engineering applications

The high strength S690 steel has an excellent strength-to-self-weight ratio, so is highly efficient for use in heavily loaded structures. Typical applications include piles and columns in buildings and supporting members in bridges.

 

An exemplary application of high strength S690 steel is the construction of the Double Arch Steel Bridge of the Cross Bay Link at Tseung Kwan O by highly skilled design and construction engineers of AECOM, China Road and Bridge Construction, and Civil Engineering and Development Department of the Government of the Hong Kong SAR.

 

Construction of the steel bridge is a record breaker in Hong Kong as well as in China, being the first long span of 214 m with double arches using S690 steel crossing a sea bay. After complete fabrication, the bridge was towed over a sea voyage of about 900 nautical miles for installation. Hence, this innovative engineering solution of high strength S690 steel enabled the delivery of the steel bridge to be completed with significant saving in construction cost and time.

 

High strength steel construction provides a niche for both Hong Kong and China to export their construction materials and professional services to the international construction markets.

 

Figure 12: The complete steel bridge: double arches, cables and bridge decks (Courtesy of CRBC)

 

Other examples of effective use of high strength S690 to S960 steel are:

 

• steel trusses for load transfer within buildings to achieve high structural efficiency

 

• footbridges in congested sites to achieve streamlined and speedy erection

 

• noise barriers in highways near residential buildings to reduce steel tonnages

 

• steel piles in groups to improve buildability and reduce demand on machinery

 

It should be noted that the CNERC was engaged by the Development Bureau in early 2022 to provide expert advisory services on a number of large scale steel structures to exploit benefits of adopting S690 steel in construction.

 

Looking into the future

CNERC will work closely with various stakeholders in the construction industry in Hong Kong and both the iron and steel industry and the steel construction industry in China to support:

 

• Hong Kong establishing itself as the International Design Centre for Infrastructure development; and

 

• China becoming the International Construction Centre for infrastructure development.

 

The Double Arch Steel Bridge of the Cross Bay Link at Tseung Kwan O is a vivid demonstration of the high-level capabilities of:

 

• an international bridge consulting firm in Hong Kong designing to European standards and of a leading Chinese contractor in Hong Kong constructing the bridge; and

 

• a high-quality Chinese fabricator using high quality Chinese S690 steel to fabricate the bridge to international standards.

 

High strength steel construction provides a niche for both Hong Kong and China to export their construction materials and professional services to international construction markets.

 

Chinese National Engineering Research Centre for Steel Construction (Hong Kong Branch)

(http://www.polyu.edu.hk/cnerc-steel/en/)

 

The CNERC for Steel Construction (Hong Kong Branch) is established to promote sustainable infrastructure development through effective use of constructional steel materials and modern technology in structural engineering. The core objectives of the CNERC are:

 

• To establish a high level technological platform to enable effective design and construction of modern building and civil engineering structures to promote sustainable infrastructure development in Hong Kong.

 

• To advance technological capabilities of the Hong Kong Construction Industry in design and construction of super high-rise buildings, long span bridges and buildings of large enclosure using high performance materials in Hong Kong and overseas.

 

The CNERC is dedicated to promoting technological developments and internationalisation of the steel construction industry in both Hong Kong and China. It is actively engaged with international as well as national exchanges in research and development of steel construction.

 

Professional publications by CNERC

Professional Guide Selection of Equivalent Steel Materials to European Steel Materials Specifications

 

This Professional Guide aims to provide technical guidelines and design information on effective use of American, Australian, Chinese, European, Japanese and Russian steel in construction through systematic comparisons

on chemical compositions and mechanical properties. High quality Chinese steel is readily adopted as equivalent steel which satisfies European steel materials specifications for construction in Hong Kong. Supporting organisations to this

Professional Guide include:

 

• Chinese National Engineering Research Centre for Steel Construction (CNERC) 國家鋼結構工程技術研究中心
• China Steel Construction Association 中國鋼結構協會
• Central Research Institute of Building and Construction 中冶集團建設中央研究院
• Metallurgical Corporation of China (MCC) 中國冶金科工股份有限公司
• China Iron and Steel Association 中國鋼鐵工業協會

 

Technical Guide Effective Design and Construction to Structural Eurocodes: EN 1993-1 Design of Steel Structures - S235 to S690

 

This Technical Guide provides detailed guidance on design and construction of structural steel using European steel materials and products.

 

More importantly, it provides specific guidance on effective use of Chinese steel materials, allowing engineers to select suitable steel materials and products, yet meet specific project requirements on materials and structural design.

 

This Guide enables engineers to exploit new opportunities in international construction markets, striving for enhanced economic development of the construction industry in Hong Kong as well as in the region with the support of Chinese steel materials and fabricators.

 

References

1. BS EN 10025-6: Hot rolled products of structural steels – Part 6: Technical delivery conditions for flat products of high yield strength structural steels in the quenched and tempered condition (2019). European Committee for Standardization (CEN).
2. High strength low alloy structural steels (2018). National Standard of the People’s Republic of China. GB/T 1591.
3. Code of Practice for the Structural Use of Steel (2021). Buildings Department. Government of Hong Kong SAR.
4. Chung K.F., Ho H.C., Hu Y.F., Wang K., Liu X., Xiao M. & Nethercot D.A.. (2020). Experimental evidence on structural adequacy of high strength S690 steel welded joints with different heat input energy. Journal of Constructional Steel Research. 204:110051.
5. BS EN 1993-1-1: Eurocode 3: Design of steel structures - Part 1-1: General rules and rules for buildings (2005). European Committee for Standardization.
6. BS EN 1993-1-12: Eurocode 3: Design of steel structures - Part 1-12: Additional rules for the extension of EN 1993 up to steel grades up to S700 (2007). European Committee for Standardization.
7. prEN 1993-1-1:2018, Eurocode 3: Design of Steel Structures - Part 1-1: General Rules and Rules for Buildings (Final document) (2018). European Committee for Standardization.
8. Selection of equivalent steel materials to European steel materials specifications (2022). Professional Guide PG-001, Chinese National Engineering Research Centre for Steel Construction (Hong Kong Branch), Third Edition.
9. Technical Guide on Effective design and construction to Structural Eurocodes: EN 1993-1-1 Design of Steel Structures (2021). The Construction Industry Council, Second Edition.