The aims of this study are to generate experimental data on aluminium alloy continuous beams of square and rectangular hollow section (SHS and RHS) with internal stiffeners, as well as to assess the current design approaches for indeterminate structures using this new cross-section shape. The experimental programme comprised material tests and five-point bending tests (i.e. continuous beams over three supports) with three different loading configurations. Generally, the specimens failed by the formation of a collapse mechanism comprising three plastic hinges. Comparisons of the test results with design strengths were also carried out. The design strengths predicted by the American (2010), Australian/New Zealand(1997) and European (2007) specifications are found to be rather conservative. The plastic hinge method in Annex H of Eurocode 9 (2007) provides improved results, while the Continuous Strength Method (CSM) for indeterminate structures gives the most accurate and consistent predictions. The results reveal that both strain hardening at the cross-sectional level and moment redistribution at the global system level have significant influence on the structural performance of stocky aluminium alloy structures.