Interaction Strength of Steel-Concrete Composite Beam-Columns Including the Balance Point

The maximum bending moment capacity of steel-concrete composite column cross sections occurs with concurrently applied axial compression. This is seen in the shape of the interaction diagram, where the bending moment capacity increases with increasing axial compression before reaching the balance point. The size of this bulged region of the interaction diagram can be significant, especially for concrete-dominant sections. However, it is often neglected in design because of two stability-related concerns. First, the simple transformations that are recommended to convert cross-section strength to member strength produce illogical results near the balance point, with member strength exceeding cross-section strength. Second, research has shown that the stiffness reductions used in elastic analyses are not sufficient for highly slender concrete-dominant composite members subjected to high bending moments. This work seeks to address these issues through the development of more advanced transformations and stiffness reductions. These new recommendations will more accurately capture the strength of composite members and allow for more efficient designs.