Beam-column behaviour of concrete-filled elliptical hollow sections

Concrete filled elliptical hollow sections (CFEHS) are a relatively new addition to the range of composite cross-sections available to structural engineers. The European design code EN 1994-1-1 (BSI, 2004) provides design rules for composite cross-sections, including concrete filled circular hollow sections (CFCHS) and concrete filled rectangular sections (CFRHS), but CFEHS are not considered in the present design code. In order to contribute to the development of design provisions for CFEHS, a comprehensive experimental and numerical study of their column and beam-column behaviour has been carried out. The testing programme covered a range of member lengths, reinforcement ratios and loading eccentricities and consisted of 27 column and beam-column member tests, 70 concrete cylinder compression tests, 3 reinforcing bar tensile tests and 2 steel tube tensile coupon tests. Numerical models were developed using the nonlinear finite element package Abaqus and validated against the experimental results. Using the validated models, detailed numerical parametric studies of CFEHS members have been conducted addressing three different scenarios: (1) members under axial compression, (2) members under combined axial compression and uniaxial bending and (3) members under combined axial compression and biaxial bending. Based on the combined test and numerical data set, along with previous experimental results reported in the literature, new design rules for CFEHS are proposed. It is shown that the current provisions of EN 1994-1-1 (BSI, 2004) for the design of CFCHS and CFRHS are appropriate for the design of members of elliptical cross-section, using either buckling curve b or c, depending on the level of steel reinforcement for members under axial compression. Cross-section axial load-moment (N-M) interaction curves are generally employed for the design of composite members under combined loading. A numerical approach, developed in Matlab, was used to the generate the N-M interaction diagram for CFEHS, which was found to offer a suitable design basis to be used in conjunction with calculated axial forces and second-order moments. Finally, an assessment of the reliability of the design proposals for CFEHS columns and beam-columns in accordance with Annex D of EN 1990 (BSI, 2002) was carried out.