Structural behaviour of high strength S690 steel cold-formed circular hollow sections

Abstract

Motivation: Compared with normal strength steel (NSS), high strength steel (HSS) has a higher strength-to-self-weight ratio, which is highly efficient in heavily loaded structures. Structural hollow sections have been widely applied in various construction projects all over the world, and a circular hollow section (CHS) is one of the most common types of sections used in steel structures because it has a highly desirable architectural appearance and a large resistance against torsion. In general, structural hollow sections are manufactured through hot rolling or cold forming and welding. These manufacturing processes will affect structural performance of these sections to different extents. At present, the use of high strength cold-formed circular hollow sections (CFCHS) is rather limited due to a lack of understanding on their structural performance. Hence, it is necessary to investigate the structural behaviour of these high strength S690 CFCHS as well as to provide effective design rules for their applications. Objectives and scope of work: In this research project, a systematic experimental and numerical investigation into structural behaviour of high strength S690 CFCHS has been carried out. The scope of work is: Task 1: Residual stresses in S690 CFCHS. To examine residual stresses in S690 CFCHS due to i) transverse cold-bending, and ii) longitudinal welding through experimental and numerical studies. Simplified residual stress patterns will be proposed for subsequent studies. Task 2: Stocky columns of S690 CFCHS. To examine section resistances of stocky columns of S690 CFCHS under axial compression, and to propose relevant section classification rules. Task 3: CHS T-joints under various loading conditions. To examine structural performance of T-joints between S690 CFCHS under i) brace axial compression, ii) monotonic brace in-plane bending, and iii) cyclic brace in-plane bending, and to validate current design rules. Four different cross-sections of CFCHS, namely Sections C1 to C4, were fabricated with S690 steel plates of 6 and 10 mm thick, and a total of 32 CFCHS were prepared for the experimental investigation. Research methodology and key findings: The research work is based on a series of experimental and numerical investigations, and all the above-mentioned tasks have been completed successfully. Task 1: Residual stresses in S690 CFCHS. A total of eight CFCHS were tested as follows: i) surface temperature measurements during welding were conducted on four CFCHS with thermocouples, ii) residual stress measurements using the sectioning method were conducted on two CFCHS and two CFCHS­NW (no welding). Numerical models were established and calibrated with measured temperature history at specified points and measured surface residual stresses through coupled thermomechanical analyses. Simplified residual stress patterns were proposed based on these experimental and numerical results. Task 2: Stocky columns of S690 CFCHS. A total of eight stocky columns of CFCHS were tested, which covered Class 2 to Class 4 sections according to section classification rules provided in EN 1993-1-1. Test results on these stocky columns confirmed that predicted section resistances based on EN 1993-1-1 were readily attained in all these columns. Moreover, the proposed residual stress patterns for S690 CFCHS obtained in Task 1 should be incorporated into numerical models of these stocky columns for accurate prediction of their deformation characteristics. Current section classification rules for S355 CHS provided in EN 1993-1-1 were found to be generally applicable for S690 CFCHS with notable conservatism. Task 3: T-joints under various loading conditions. A total of sixteen T-joints between S355 and S690 CFCHS were tested under the following loading conditions: i) brace axial compression, ii) monotonic brace in-plane bending, and iii) cyclic brace in-plane bending. A digital image correlation (DIC) technique was employed to measure surface strain contours of these T-joints under in-plane bending. Various failure modes were identified, and numerical models were established and calibrated against experimental results. In general, these T-joints were found to have large resistances and ductility under various loading conditions. Current reduction factors provided in EN 1993­1-12 were found to be very conservative when applied to T-joints between S690 CFCHS. Key findings and their significances: Major academic merits of this research project are: This is the first research which systematically investigates residual stresses in CFCHS induced by both transverse cold-bending and longitudinal welding. Experimental studies on residual stresses of high strength S690 steel CFCHS have been conducted, and advanced numerical models have been established to simulate deformation characteristic of CFCHS with different steel grades, and geometrical dimensions. Generalized finite element models are developed to predict residual stresses in CFCHS through coupled thermomechanical analyses. Simplified residual stress patterns for S355 to S690 CFCHS have been proposed. Current design rules for section resistances in EN1993-1-1 have been confirmed to be applicable to S690 CFCHS through experimental and numerical studies. Current design rules for T-joints between CHS under monotonic in-plane bending in EN 1993-1-8 have been confirmed to be applicable to T-joints between S690 CFCHS through experimental and numerical studies. Suitable design parameters have been proposed to improve structural efficiency. Structural performance of T-joints between S690 CFCHS under cyclic in-plane bending is confirmed through experimental and numerical investigations, and they have a high level of resistance and ductility. Application of high strength S690 steels in CFCHS is fully validated through a series of comprehensive experimental and numerical investigations. They are technically ready for wide applications in construction.