Ultimate Strength of Biaxially Loaded Composite Sections

A theory is developed for the non-linear or linear analysis of arbitrary composite sections subjected to biaxial loading. The accuracy of the theory is verified by comparison with known uniaxial non-linear analyses of steel wide-flange and channel sections encased in reinforced concrete, with classical linear theory based on principal moments of inertia, and with test results from various authors. Three basic shapes of concrete stress-strain diagram are considered, for two allowable compressive strains and two ultimate strengths. This gives twelve different types of concrete stress distribution which are used to analyse four different composite sections. A comparative study of the results indicates that the factor which most influences ultimate biaxial bending strength and curvature is the allowable concrete compressive strain. The shape of concrete stress distribution has little effect on the section behavior.