RAM Concept uses a post-tensioning steel stress-strain curve base on a standard "power formula" that has been used in various forms for 25 years:

f_p = ε_p {A + B/[1 + (C ε_p)^D]^1/D} ≤ f_pu

Where A, B, C, and D are coefficients chosen to best fit the experimental stress-strain curve data.

RAM Concept uses coefficients A, B, C, and D based on an analysis of prestressing steel stress-strain curves included a paper by Develapura and Tadros [Develapura, R. K. and Tadros, M. K.,"Critical Assessment of ACI 318 Eq. (18-3) for Prestressing Steel Stress at Ultimate Flexure", ACI Structural Journal, V. 89, No. 5, September-October 1992, pp. 538-546]. RAM Concept ’s values are:

A = 0.0311 E_p

B = E_p - A

C = 0.958 E_p/f_py

D = 7.36

These values provide exact correspondence with the recommended parameters for 270 ksi (1860 MPa) strand with f_py of 0.9 f_pu. For other prestressing materials, there may be small differences (a few percent) from the theoretical curves in the region between the start of yield and ultimate strength.