Behaviour and design of steel-concrete composite columns incorporating high-performance materials

Abstract

Composite columns with high-performance materials have gained increasing attention over the past decades. The use of high-performance materials in composite columns can minimise a cross-section area and self-weight for structural components, with consequent savings in materials and labour work. The major scope of this thesis is to investigate the structural behaviour of steel-concrete composite columns incorporating high-performance materials, which can promote the adoption of such composite columns in construction industry. Two major parts are included in this thesis: the first part is to define the section slenderness limits and local and post-local buckling behaviour of high-performance composite columns; the second part is to investigate the load bearing capacities of concrete-filled steel tubular columns subjected to axial compression, flexural bending as well as combined axial and flexural loads. The first part of the thesis reports a test programme consisting of 32 hollow and composite short columns, of which half are fabricated with high-strength steel (S690) and the other half with ultra-high-strength steel (S960). The primary parameter considered in the test programmes is the slenderness (b/t) of steel plates, specifically, the component plates of square sections and flange outstands of I-sections. The prepared composite columns were tested under axial compression, which was applied to steel sections only. The concrete infills, if any, only restrained the component plates from inward buckling. Failure modes, load-axial shortening response and axial strain development were analysed. Non-linear finite element models were developed and validated against the experimental results, which was used to investigate the section slenderness limits and post-local buckling behaviour of high-performance columns. The developed numerical models allowed for residual stresses and initial geometric imperfections being considered. With the 2 obtained experimental and numerical results, existing international design provisions for steel and composite columns were evaluated. Accordingly, recommendations for the section slenderness limits and post-local buckling strength of high-performance columns were suggested. The second part of the thesis elaborates an experimental program on the axial and flexural behaviour of concrete-filled steel tubular (CFST) columns utilising ultra-high-strength steel (S960) and high-strength concrete (C70 and C100). The effects of concrete strength, load eccentricity and member slenderness (Le/r) on axial and flexural resistance, initial stiffness and ductility were assessed. With the obtained experimental results, finite element models were developed and validated, which accurately predicted the axial and flexural behaviour of the CFST columns. A series of parameters, including steel strength, concrete strength and plate slenderness, were evaluated through parametric studies. A commentary on existing standard provisions for CFST columns was provided based on the acquired experimental and numerical results. Based on the results obtained from this thesis, the slenderness limits and optimal approaches for post-local buckling strength of hollow and composite columns have been recommended, which are applicable to the steel grades ranging from 250 to 960 MPa. With the presence of concrete infills, the slenderness limits and post-local buckling strength are significantly improved. In addition, existing standard provisions for hollow and composite columns have been evidenced to be safe for the design of high-performance hollow and composite columns if the recommended slenderness limits and design curves can be adopted.