Prestressed reinforcement is modeled as using a power formula. The curve is defined by four parameters:

• E_ps = the elastic modulus at zero strain (from materials window)
• F_py = the "yield" stress of the reinforcement (from materials window)
• F_pu = the ultimate stress of the reinforcement (from materials window)
• γ_m = partial safety factor for materials

These four parameters are used to calculate the three parameters needed for the power formula, as described in Post-tensioning Material Stress-Strain Curves . The three parameters are:

E_ps’ = E_ps

F_py’ = F_py / γ_m

F_pu’ = F_pu / γ_m

For strength considerations, (ULS) γ_s is used (γ_s = 1.15 for UK National Annex).

For other considerations (accidental LS or SLS), γ_s is used (γ _s = 1.0 for UK National Annex).