The secondary moment also known as parasitic bending moment are additional moments induced at a section of a statically indeterminate structure due to redundant reactions developed as a consequence of prestressing structure and variation of secondary moment is a two span continuous beam prestressed by a straight eccentric tendon.

In continuous post tensioned girders, curved cables can be suitably positioned to resist span and support moments, continuity of members in framed structures leads to increase stability, in framed structures leads to increase stability continuous girders are formed by segmental construction using precast units connected by prestressed cables.

The computation of collapse or ultimate load is influenced by degree of redistribution of moments in continuous structure cables positioned to cater for secondary moments are not generally suitable to provide required ultimate moment under a given system of loads.

A transformation profile is any tendon profile consisting of straight lines between rigid supports and having zero eccentricity at simple end supports and a tendon following such profile will produce support reactions and uniform longitudinal compression but bending moments.

However a highly conservative estimate of ultimate resistance of structure is obtained by assuming collapse to occur when moment, calculated by linear elastic ory first reaches a value equal to flexural strength at any section.

Stresses due to secondary moments can be eliminated by selecting suitable tendon profile which do not induce secondary moments and it is also possible to provide for secondary stresses in design and if under reinforced sections are used, redistribution of moments will be more or less complete resulting in higher collapse loads and se loads could be estimated by suing well established plastic ory as applied to structural steel members.

It is important to note that analysis of indeterminate structures supporting equivalent loads directly yields resultant moments consequently, secondary moments, if required are obtained as difference of resultant and primary moments at every cross section of structure.

In design of unidirectional members like continuous beams, axial contraction due to effect of prestressing is: Neglected as it has very little influence on force and moments in structure but in case of two dimensional structures, such as portal frames and bents, axial contraction of members significantly influences primary and secondary moments in structure.

The redistribution of moments is permitted under following conditions: equilibrium between internal forces and external loads is maintained for approximate load combinations, at sections where moment capacity after redistribution is less than that from elastic maximum moment diagram following relationship should be satisfied ((x_u/d) + ?m/100)) < 0.8, x_u = depth of neutral axis, d = effective depth, ?_m = percentage reduction in moment.

The overall cross sectional dimensions are fixed using permissible compressive stresses in concrete, fc by equation
Z = bh²/6 = My/f_c assuming a suitable values of b = 0.4-0.5h, overall depth this obtained so as obtain range of moments which generally determines cross sectional dimensions of member.