Abstract
This paper deals with an analysis of the ultimate strengths of concrete filled steel box elements subjected to an arbitrary combination of axial thrust, bending moment and shear force. A significant ultimate shear resitance of this composite is characterized by a stable truss action of both materials such that diagonal compression field develops through the filled concrete, while tension field is formed along the opposite diagonal direction of the steel web plates. An ultimate strength analysis was achieved based on the assumed ultimate stress field which reflects the above material behaviors. The solution is given explicityly in the form of an ultimate strength interaction curve of axial thrust and shear force. A notable feature of the solution is that this can be constructed graphically by use of the individual interaction curves for the filled concrete and the peripheral steel element, where each curve is obtainable independently of the other. Accuracy of the solution was examined by a series of experiment. It is verified that the solution gives, on the whole, a conservative estimate of the actual strengths of the tested specimens. More specifically, (1) the predicted strength is sufficiently accurate for the specimens with shear collapse mode, while for the specimens with bending collapse mode errors increase toward conservative side. (2) The solution is more accurate for the specimens whose steel elements have smaller volume fractions.
