Full plastic capacity of equal angle sections under biaxial bending and normal force

Steel angle members are usually designed by elastic methods, which may lead to uneconomical design. In order to make use of the plastic reserves of the section, the knowledge of its exact failure surface is important. In this study, it is shown that idealizing the section with lines of no thickness, which is common in the literature, entails a significant error when the neutral axis intersects a leg at a small angle. A simple interaction formula is proposed, which is shown to be very accurate based on an exhaustive analysis of all common angle sections listed in the tables. Further increase in accuracy is achieved through analytical relations for the cross-sectional properties, both elastic and plastic, that are derived. The proposed analysis forms the base of inelastic design and does not address stability issues.