Load moment interaction curves for concrete filled tubes

Load-moment interaction curves for concrete filled tubes can be generated using a variety of methods including: accurate numerical methods using equilibrium, strain compatibility and appropriate material stress-strain relationships; approximate numerical methods in which the non-linear material stress-strain relationships are simplified; or empirical methods in which equations are fitted to either results from tests or results from numerical methods. Each of these methods are examined and compared with reference to current design codes. It is found that: discretisation of the cross-section influenced the accuracy of numerical methods; the stress-strain relationships for the materials must take account of the insitu conditions of the actual member; care should be taken if empirical or semi-empirical methods are to be extrapolated beyond their calibrated range; and methods in design codes are not necessarily conservative over the full load-moment range.