This page is devoted to frequently asked questions (FAQ) related to design.
How can I find the critical curve of an interaction diagram?
Extended Question: Which of the 24 curves within the 3D interaction surface is the most critical? Should the resultant of M2 and M3 be obtained? And can design-force coordinates (Pu and Mu) be plotted on the interaction surface to indicate whether or not it is outside the curve?
Answer: The interaction surface is defined by many points within a 3D coordinate system, and each of these points represents values of P, M2, and M3 which combine to induce section yielding. Since the critical forces which act on the section include those plotted (P, M2, and M3), the interaction surface provides the most complete description of section behavior.
To simplify interaction-surface data, users may fix values, or ratios between values, to consider 2D curves which represent section cuts through the 3D surface. As suggested in the question, if a critical P-M curve is of concern, the resultant M2-M3 angle should be found such that a 2D curve within this plane corresponds to the desired P-M curve.
Users may determine whether or not coordinates (Pu, Mu) are outside the curve by reviewing the cross-section capacity ratio on the Concrete Design Data menu. If this ratio is greater than 1.0, then the point falls outside the curve, indicating that demand exceeds capacity.