Interaction diagrams for ambient and heated concrete sections

Bending moment axial force interaction diagrams are a commonly used tool in any design office. When designing for fire conditions, the large axial forces which develop place an additional importance on the consideration of the interplay between axial forces and moments. This paper presents a new method for calculating the biaxial bending moment/axial force capacity for a general section through the use of the sectional tangent stiffness. A beam–column section subject to fire is assessed, and comparisons made with simplified design tools. It is concluded that derivation of the interaction surface from the tangent stiffness matrix is possible, and that current simplified methods for fire design cannot be assumed conservative.