Degree Name

Doctor of Philosophy


Department of Civil and Mining Engineering


An experimental and theoretical study concerning the ultimate load behaviour of curved steel struts infilled with higher strength concrete has been carried out. As well, a nonlinear finite element model for investigating the elasto-plastic behaviour of such elements has been carried out. The analysis accounts simultaneously for both the geometrical and material nonlinearies. Different stress-strain relationships of the material are assumed to take into account strain hardening as well as residual stress. This study involves the structural characteristics of the composite sections under compressive axial loads.

A total number of 78 composite as well as 11 hollow section curved stmts have been tested for the different structure forms. Material and composite curved strut tests have been performed on circular sections of 60.4 mm outside diameter and wall thickness 2.3 m m ER W curved struts, (with a low strain hardening ratio, and with radii of curvatures equal to 2000mm, 4000mm and 10000mm), 60.4 mm outside diameter and wall thickness 3.9 m m seamless composite curved stmts (with a high strain hardening ratio and radii of curvatures equal to 2000mm and 4000mm). The struts were tested as pinned ended struts and loaded concentrically.

The composite curved steel struts subjected to compressive load were analysed both by a established theoretical method by assuming the initial deflected shape to be part of a sine wave and also by using a non-linear finite element method. The theoretical method greatly simplifies the analysis in comparison with other methods, and gives results which in most cases are close to the maximum loads. All computational procedures of the theoretical method are programmed on a computer and also a finite element package ( Nastran ) has been used to investigate the behaviour of stmt over the elasto-plastic part of stress-strain diagram. In this case the results are close to the maximum loads obtained experimentally.

The ultimate load capacity of curved composite stuts is extensively investigated by numerical experiments. The computed results show that the maximum load is influenced mainly by the slenderness and initial deflection at mid-height of tube, but the steel strength, concrete strength and diameter to thickness ratio are also found to be significant.

By comparing the theoretical results (intersection of elastic and plastic curves) with experimental results, it is shown that the theoretical method can predict with reasonable accuracy the experimental maximum loads. The error was -12% to +13% for ERW struts with 2000 mm initial radius of curvature, -2% to +3% for ERW struts with 4000 mm initial radius of curvature, -9% to +9% for ERW with 10000 mm initial radius of curvature, 0 to +11% for seamless struts with 2000 mm initial radius of curvature and 4% to 16% for seamless struts with 4000 mm initial radius of curvature.

The theoretical load-deflection behaviour of the as-received curved struts obtained from Nastran compared well with the experimental results. The residual stress effect due to initial curvature is taken into account by using different material properties (stressstrain) across the cross-section of the curved struts. In addition, the interaction of the concrete core and the steel tube have been modelled by the utilisation of gap elements to form an analytical model for the composite sections. The differences between the maximum loads obtained from the finite element method and experimental results is 5% to +6%.

Design methods and various ultimate load design formulae are investigated and it is found that no single formulae gives accurate results over all ranges of the significant parameters.