Experimental behaviour of very high-strength concrete-encased steel composite column subjected to axial compression and end moment

Abstract

A type of cost-effective and very high strength concrete (VHSC) with 28-day compressive strength of 100-150 MPa is developed for applications in concrete-encased steel composite column constructions. This paper experimentally investigates the structural behaviour of VHSC encased steel composite columns based on a series of pure compression and eccentric compression tests. It is found that such high-strength composite column exhibits brittle post peak behaviour and low ductility but with acceptable compressive resistance. Throughout the tests, the main failure of VHSC encased column under compressive load is brittle spalling of concrete followed by local buckling of the reinforcement bars. The splitting and slippage may occur between concrete and steel section due to bending downward action. The confinement effect by the shear stirrups designed based on normal reinforced concrete codes may not be sufficient. Composite column subjected to initial flexural cracking due to end moment load with large eccentricity may experience degradation in the stiffness and ultimate resistance so that plastic design resistance may not be achieved. Analytical studies show that the N-M interaction model based on current design codes may over-predict the combined resistance of the composite columns. Therefore, a modified elastic-plastic design approach based on strain compatibility is developed to evaluate the compressive resistance of concrete encased composite columns. The validation against the test data shows a reasonable and conservative estimation on the combined resistance of VHSC encased composite columns.