Structural behaviour of high strength Q690 steel welded H-sections under combined compression and bending

Abstract

Motivation: Nowadays, high strength steels commonly refer to those modern steel products which possess a yield strength equal to or higher than 460 N/mm2. Their yield strengths are two to three times of conventional steel materials, namely Q235 and Q345 steels. Owing to their high strength-to-self-weight ratios, high strength steels can offer effective structural solutions to heavily loaded structures. In the past two decades, high strength steels have been broadly applied in automobile, crane and offshore structures. However, the use of high strength steels in construction sector is limited because of a lack of understanding of their structural behaviour. Compared with those conventional steel materials, high strength steels exhibit a limited degree of strain hardening after yielding, and a smaller elongation at fracture. Moreover, the values of tensile to yield strength ratios are also decreased. These differences in the mechanical properties of high strength steel materials may have significant effects on structural behaviour of steel sections made of high strength steel materials. Some structural steel design codes which are developed on the basis of conventional steel materials have covered or may be readily expanded to high strength steels. In order to promote a wide application of high strength Q690 steel structures, it is necessary to investigate structural behaviour of Q690 steel welded H-sections both experimentally and numerically. It is also necessary to identify or establish supplementary design rules through calibration against test results as well as numerical results. Therefore, a systematical investigation on the structural behaviour of high strength steel sections is needed, and the applicability of the design rules in those design codes on high strength steel materials is also required. Objectives and scope of work: In order to promote a wide application of high strength Q690 steel structures, it is necessary to investigate structural behaviour of Q690 steel welded H-sections both experimentally and numerically. It is also necessary to identify or establish supplementary design rules through calibration against test results as well as numerical results. The scope of work includes: • Task 1: Structural behaviour of stocky columns of welded H-sections - To examine local plate buckling behaviour of Q690 steel stocky columns of welded H-sections under i) compression and ii) combined compression and bending through experimental investigation. • Task 2: Structural behaviour of slender columns of welded H-sections - To examine flexural buckling behaviour of Q690 steel slender columns of welded H-sections under i) compression and ii) combined compression and bending through both experimental and numerical investigation. • Task 3: Design proposals for slender columns of welded H-sections - To make design proposals for Q690 steel slender columns of welded H-sections under i) compression and ii) combined compression and bending through calibration against large amount of numerical results. The areas of interest include: • Section resistances, deformation characteristics and occurrence of local plate buckling of Q690 steel stocky columns of welded H-sections under i) compression and ii) combined compression and bending; • Applicability of current design rules in EN 1993-1-1, GB 50017-2003 and ANSI/AISC 360-16 for section resistances of Q690 steel welded H-sections; • Buckling resistances and deformation characteristics of Q690 steel slender columns of welded H-sections under i) compression and ii) combined compression and bending; • Finite element modeling in which geometrical and material nonlinearity are incorporated; • Various factors that affect flexural buckling resistances of Q690 steel welded H-sections under i) compression and ii) combined compression and bending, including initial out-of-straightness, residual stresses and tensile to yield strength ratios; • Applicability of current design rules in EN 1993-1-1, GB 50017-2003 and ANSI/AISC 360-16 for buckling resistances of Q690 steel welded H-sections. It should be noted that four different cross-sections of welded H-sections, namely Sections H1 to H4 are fabricated with Q690 steel plates of 6, 10 and 16 mm thicknesses. Research Methodology and key findings: Through a series of coordinated experimental and numerical investigation, all the tasks were successfully completed. Key findings of this research project are as follows: • Task 1: Structural behaviour of stocky columns of welded H-sections - A total of 20 stocky columns of welded H-sections were tested under different loading conditions, in which i) 12 welded H-sections were tested under compression ii) 4 welded H-sections were tested under combined compression and major axis bending, and iii) 4 welded H-sections under combined compression and minor axis bending. Section resistances, deformation characteristics and strain variations were carefully obtained, and plastic local plate buckling was observed in all the H-sections. All the measured section resistances are found to be larger than their corresponding compressive resistances provided by three design codes, namely, EN 1993-1-1, ANSI/AISC 360-16 and GB 50017-2003. All these design codes are able to give close predictions to those stocky columns under i) compression and ii) combined compression and major axis bending while fairly conservative predictions to those stocky columns under combined compression and minor axis bending. Apparent enhancement was found in both section resistances and ductility in Sections H1 and H3, which belonged to Class 1 and 2 sections according to EN 1993-1-1. • Task 2: Structural behaviour of slender columns of welded H-sections - A total of seven Q690 steel columns of welded H-sections were tested under compression and a total of eight Q690 steel columns of welded H-sections were tested under combined compression and minor axis bending. All of them failed by overall buckling about minor axes of their cross-sections. Buckling resistances, deformation characteristics and strain variations of all the columns were carefully obtained. All the test resistances are found to be higher than their design resistances in accordance with the design rules in EN 1993-1-12, ANSI/AISC 360-16 and GB 50017-2003. This improvement is attributed to the reduced effects from residual stresses and initial out-of-straightness. Though numerical investigation, it is found that, for Q690 steel welded H-sections under compression, initial out-of-straightness and residual stress are the two main factors which can significantly influence their buckling resistances. For Q690 steel welded H-sections under combined compression and bending, residual stress is the main factor. Compared with those welded H-sections made of Q235 steels, the effects from these factors become less pronounced in Q690 steel welded H-sections with the same non-dimensional slenderness due to increased yield strengths and proportionally reduced ratios of compressive residual stress to yield strength. Moreover, influence of strain hardening may be neglected in determination of buckling resistances of Q690 steel welded H-sections. • Task 3: Design proposals for slender columns of welded H-sections - The assessment of current design rules given in EN 1993-1-1, ANSI/AISC 360-16 and GB 50017-2003 to Q690 steel columns of welded H-sections is performed through the calibration of design buckling resistances against finite element results. It is found that the efficiency of the design rules in EN 1993­1-1 and GB 50017-2003 can be increased by selecting higher column curves regarding with different bending axes. In determination of design buckling resistances of Q690 steel welded H-sections under combined compression and major axis bending, the design rules in EN 1993-1-1 using curve a, the design rules in ANSI/AISC 360-16 using the single curve and the design rules in GB 50017-2003 using curve a are proposed. In determination of design buckling resistances of Q690 steel welded H-sections under combined compression and minor axis bending, the design rules in EN 1993-1-1 using curve c, the design rules in ANSI/AISC 360-16 using the single curve and the design rules in GB 50017-2003 using curve b are proposed. Key findings and their significance: The major academic merits of the research work are as follows: • Experimental investigation into Q690 steel welded H-sections with different cross-section sizes, specimen lengths and loading conditions was carried out. In these tests, stocky columns failed by local plate buckling while slender columns failed by flexural buckling. Their sectional or member buckling resistances, deformation characteristics and strain variations were obtained. • Finite element models were established, in which geometrical and material nonlinearities were incorporated, and they are found to be able to replicate the structural behaviour of Q690 steel slender columns observed in the experiments. • Through numerical studies, it is found that initial out-of-straightness and residual stresses are main factors that influence the buckling resistances of Q690 steel welded H-sections under compression, and the latter is the main factor that influences the buckling resistances of Q690 steel welded H-sections under combined compression and bending. The effects from these factors are found to be proportionally reduced because of high yield strength and low residual stress ratios • Assessment of design rules in different design codes, namely EN 1993-1-1, GB 50017-2003 and ANSI/AISC 360-16 for designing Q690 steel welded H-sections is conducted. Column curves are proposed regarding with different design codes in order to increase their structural efficiency in designing Q690 steel welded H-sections.