Degree Name

Doctor of Philosophy


Department of Civil and Mining Engineering


An experimental and theoretical study concerning the ultimate load behaviour of the circular hollow sections filled with higher strength concrete has been carried out. This study involves the structural characteristics of the composite sections under axial concentric and the eccentric loads.

A total number of 63 experimental tests have been carried out for the different loading situations and structure forms. This study has been divided into the three main parts. The first part concerns the behaviour of the hollow sections, and the second part is a study on the ultimate load behaviour of the circular hollow sections filled with higher strength concrete under concentric loads. In the last part the structural behaviour of the composite sections under eccentric loads has been studied.

The steel grade of the circular hollow sections can be categorised as a high strength steel with the yield stress in the range of 400-450 MPa. The compressive capacity of the concrete is within the range of 60-109 MPa. The maximum outside diameter of the steel tube used is 168.3 mm and the minimum is 114.3 mm . The wall thicknesses for the maximum diameter tube are 4.8 mm and 9.53 mm . Althogh these dimensions are not commonly used for structural purposes but covering the whole range of possible dimensions increase the possible options for a perfect design. A n epoxy coated tubes of 4.8 mm wall thickness have also been considered. The wall thicknesses for the minimum outside diameter are 6.3 mm and 4.8 mm . For the justification of the results obtained in Chapter three, another set of tubes with an outside diameter of 62 mm and wall thickness 2 mm and 2.5 mm , and a set of four steel tubes with outside diameter equal to 114.3 mm and wall thickness equal to 6.0 mm have been studied.

In each part of the thesis the results obtained by the tests have been compared with a number of test results by the other authors, and the available criteria for these studies have been investigated to obtain a better solution for estimation of the load carrying capacities.

In addition, the experimental specimens have been modelled for a non-linear finite element analysis to investigate the intensity of stresses in the different directions. Furthermore, the interaction of the concrete core and the steel tube have been modelled by the utilisation of gap elements to perform a real analytical model for the composite sections.

The results of this study show that some of the available methods, and even codes of practice, have a non-realistic estimation for the axial load capacity of the composite sections. The studies on the ultimate load behaviour of the bare tube show that the relationship between the ultimate strain and the slenderness of the tube can be presented by a linear equation which is in agreement with previous studies.

The results of the finite element analysis conclude to an approximate method for determining the axial load-shortening curve of the composite section which is in good agreement with the finite element results and the results obtained by the experiments.