Non-linear

The non-linear (FIB) model code defines a schematic stress-strain curve. This is used in BS 8110-2, EN1992-1 and IRC:112.

This has a peak stress $f_{c F I B}$.

This is defined as

$$\frac{f}{f_{c F I B}}=\frac{k \eta-\eta^2}{1+(k-2) \eta}$$

with

$$k=\alpha \frac{E_c}{f_c / \varepsilon_c}$$

Where the factor $\alpha$ is code dependent.

Code	$f_{c F I B}$	$\alpha$
BS 8110-2	$0.8 f_c$	1.4
EN 1992-1	$f_c+8 \mathrm{MPa}$	1.05
IRC:112	$f_c+10 \mathrm{MPa}$	1.05