High strength steel

Introduction 

High strength steels (HSS), with yield strengths over 460 MPa, are attracting increasing attention from structural engineers in recent years owing to their potential to enable lighter structures while being more sustainable and economic. Compared to conventional carbon steels, the structural application of HSS is still rather rare, which could be attributed to the lack of understanding and limited research into structural HSS. The current available design rules in the world are all conceived as a simple extension to the traditional design rules for ordinary carbon steels. Consequently, the potential of HSS is limited by the use of design criteria that are not optimised for the characteristics of the material. 

To provide a better understanding of the structural response of HSS and contribute towards the development of optimum design criteria for these structures, the Steel Structures Group at Imperial College London has been carrying out extensive research activities on high strength steel structures. The main observers on this topic involve Professor Leroy Gardner, Dr Marios Theofanous (graduated, currently working at University of Bermingham), Dr Sheida Afshan (graduated, currently working at Brunel University London), Dr Jie Wang (graduated, currently working at Imperial College London), Xin Meng.

 A recently accomplished high strength steel project – HILONG (founded by the Research Fund for Coal and Steel) is briefly introduced herein. The project can be divided into two parts: Part I is related to the design of basic structural members made of high strength steels, at the material level, the cross-section level and the member level; Part II aims at maximising the material efficiency of HSS structures, focusing on the practical design of a novel structural form of prestressed high strength steel trusses.

Part I – Design of HSS structural members

A comprehensive experimental and numerical programme hot-finished S460 and S690 SHS and RHS members has been carried out. The programme covered different structural aspects at material level (tensile and compressive material coupon tests), cross-section level (stub column, beam and combined axial plus bending tests) and member level (column tests). Based on full scale experiments, validated numerical simulations and statistical verifications, the current structural HSS design provisions specified in EN 1993-1-1 [1] and EN 1993-1-12 [2] have been evaluated and modified if necessary [3-5].

HSS1
Fig.1 Typical failure modes of HSS specimens in different types of experiments. (From left to right, top to bottom: 1) tensile flat coupon tests; 2) tensile corner coupon tests; 3) stub column failed by local buckling; 4) stub column failed by global buckling triggered by local buckling; 5) stub column failed by elephant-foot buckling; 6) full cross-section test under tension; 7) 4-point bending beam tests; 8) stub column under combined axial plus bending failed by material yielding; and 9) stub column under combined axial plus bending failed by local buckling).

Part II – Design of prestressed HSS trusses

In this part, a novel structural form- HSS trusses with a prestressing cable housed in the bottom chord has been studied to exploit the combined benefits of light-weight high strength steel and prestressing technique in long span structures. Four large scale experimental tests have been carried out, showing a significant improvement in the strength and extend of the elastic range of the structure by simply introducing a prestressed cable. Fig 2 shows the set-up of the truss tests, where the truss was loaded at five central bottom chord joints and pinned supported at both ends, with the lateral deflection of top chords prevented by restraint cables. The experimental results have demonstrated the performance gains in prestressed HSS trusses, revealing their high material efficiency and the potential for long span applications. Additionally, design rules of individual prestressed tubular members have also been developed to facilitate the practical design of the proposed structural form [6]. 

HSS2
Fig.2 Set-up of full-scale truss tests.

References

[1] EN 1993-1-1: 2005. Design of steel structures – Part 1-1: General rules and rules for buildings. 2005

[2] EN 1993-1-12: 2007. Design of steel structures – Part 1-12: Additional rules for the extension of EN 1993 up to steel grades S 700. 2007

[3] Wang J., Afshan S., Gkantou M., Theofanous M., Baniotopoulos C. and Gardner L. Flexural behaviour of hot-finished high strength steel square and rectangular hollow sections. Journal of Constructional Steel Research (2016) 121: 97–109.

[4] Wang J., Afshan S., Schillo N., Theofanous M., Feldmann M. and Gardner L. Material properties and local buckling behaviour of high strength steel square and rectangular hollow sections. Engineering Structures (2016) 130: 297-315.

[5] Wang J. and Gardner L. Flexural buckling of hot-finished high strength steel square and rectangular hollow section columns. Journal of Structural Engineering ASCE (2017). Available online on Feb 17 2017.

[6] Wang J., Afshan S. and Gardner L. Axial behaviour of prestressed high strength steel tubular members. Journal of Constructional Steel Research (2017) 133: 547–563.