If creep or shrinkage is defined, the section analysis will perform a calculation of the effects on equilibrium of forces and strains. The parts of strain and curvature that correspond to creep and shrinkage are separated, then added to the frame as an inelastic deflection. When the beam is statically undetermined, the added inelastic deflections generates section forces that are added to section forces from other load-effects. Therefore, the effect of creep and shrinkage must be calculated using iteration (see figure). 

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PRE-Stress Documentation