The curvature of a cross-section is defined as the inverse of the radius of curvature and can be computed from the strain profile:
\[ \varphi = \frac{\varepsilon_c}{kd} = \frac{\varepsilon_s}{d(1-k)} = \frac{\varepsilon_c + \varepsilon_s}{d} \]
Anwar & Najam (2017), Ch. 6, Eq. 6.4
Procedure: The M-φ curve is generated by incrementing the extreme compression fiber strain from zero to εcu. At each increment, the neutral axis depth is found iteratively to satisfy force equilibrium (Pn = Papplied), then the curvature and moment are computed.
Ductility Ratio:
\[ \mu = \frac{\varphi_u}{\varphi_y} \]
Anwar & Najam (2017), Ch. 6, Eq. 6.6
Effective Stiffness:
\[ EI = \frac{M}{\varphi} \]
Anwar & Najam (2017), Ch. 6, Eq. 6.5
Concrete Models:
- Hognestad (1951) — Parabolic ascending + linear descending (unconfined). Univ. of Illinois Bulletin No. 399.
- Mander, Priestley & Park (1988) — Power-law curve for confined concrete. Confinement increases both strength (f'cc) and ultimate strain (εcu). J. Structural Eng., ASCE, Vol. 114, No. 8.
Code References: NSCP 2015, ACI 318-19 (Whitney stress block, β1 factor, φ factors per Table 21.2.2)