Partially and Fully Prestressed Concrete Sections under Biaxial Bending and Axial Load

Abstract

This paper presents a general method that determines the ultimate strength capacities and interaction diagrams for any orientation of the neutral axis of a prestressed concrete section of any shape under axial load (tension or compression) and bending about two axes. Explicit expressions of the nominal strength capacities are developed using the Gauss integral method for equilibrium, a nonlinear stress-strain relationship for the concrete, a multilinear elastic-plastic relationship for the conventional reinforcement, and a modified Ramberg-Osgood function for the prestressed steel. The proposed method can be used in the study of the effects of creep, shrinkage, and confinement of the concrete, as well as relaxation in the prestressed steel (bonded and/or unbonded) on the nominal strength and failure mode of partially or totally prestressed concrete sections under biaxial bending and axial load. The nominal biaxial interaction diagram and failure mode of concrete sections with arbitrary shapes can be obtained directly using a computer and, as expected, both depend on the cross-section characteristics and reinforcement layout; the constitutive stress-strain characteristics of the concrete and different reinforcements, including the effects of creep, shrinkage, and confinement on the concrete and relaxation on the prestressed steel; and the type, duration, and intensity of applied axial load and orientation of the flexural moments. Five examples are presented to verify and show the effectiveness of the proposed method.