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# Interaction diagram FAQ

## How is the interaction surface generated?

Answer: The axial-force / biaxial-moment interaction surface for a reinforced-concrete section is generated according to strain compatibility. A plane of linear strain is rotated in three dimensions relative to the cross-section area. Maximum concrete strain occurs at extreme fiber locations, and varies linearly through the strain-profile depth for each realization of interaction angle.

To design reinforcement, the Section Designer generates interaction surfaces for a range of code-prescribed reinforcement conditions, then coordinates design according to capacity principles.

Additional information on the column design process is available through Documentation > Concrete Frame Design, then under the column-design chapter of the code-specific manual.

## 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.