Real and Apparent Direction of Inertia in the Ultimate Limit State in Doubly Symmetrical Reinforced Concrete Sections

The principal direction of inertia in the ultimate limit state under axial load and biaxial bending of a doubly symmetrical reinforced concrete section is not the same as the direction of the principal axis of symmetry. If a hyperbolic stress-strain relationship is used to describe the behavior of concrete in compression, then, to some extent, the maximum capacity direction deviates from the apparent main axis of inertia (the main axis of symmetry). This study explores the real maximum capacity direction of bending of two reinforced concrete sections with a variable amount of steel using two different axial compression loads and two different stress-strain relationships for concrete (parabolic-rectangular and hyperbolic). The results are presented in a collection of interaction diagrams. (C) 2019 American Society of Civil Engineers.