The design of reinforced concrete structures is generally based on linear structural analyses: To determine the reinforcement including the serviceability limit state design, the internal forces are determined linearly; afterwards the cross-section analysis is performed. However, this procedure considers the cracking that is typical for reinforced concrete with the corresponding nonlinear material rules of reinforced concrete only at the cross-section level.
By including the nonlinear behavior of reinforced concrete in the determination of internal forces, you get realistic states of stresses and therefore distributions of internal forces that, in statically indeterminate systems, differ significantly from the linearly determined internal forces due to stiffness redistributions. For the serviceability limit state design, this means that the nonlinear material behavior of the reinforced concrete must be taken into account to realistically calculate deformations, stresses, and crack widths.