The deflections of symmetrically loaded and simply supported beam at mid span point are directly obtained from second moment area orem since tangent is horizontal at this span, In most of cases of prestressed beams tendons are located with eccentricities towards soffit of beam to counteract sagging bending moments due to transverse loads.

The French code limits deflection of members carrying masonry and partition walls to span/500, which is total increase in deflection due to short term and sustained loads over instantaneous deflection due to self weight.

Large deflections under dynamic effects and under influence of variable loads may cause discomfort to users, members of structural concrete are designed in such a way that it possess adequate stiffness to control deflections or it may leads to a very great impact on serviceability conditions and strength of a structure at working loads.

The deflection including effect of temperature, creep and shrinkage occurring after erection of partitions and application of finishes should not normally exceed span/350 whichever is less, creep is directly proportional to stresses but above this value it increases more rapidly.

In both pre and post cracking stages effect of creep and shrinkage of concrete is to increase long term deflections under sustained loads, which is estimated by using empirical methods that involve use of effective (long term), modulus of elasticity or by multiplying short term deflections by suitable factors.

According to Neville and ACI committee report, creep curvature is obtained due to prestress based on a very simplified assumption that creep is induced into concrete by average prestress acting over with respect to given time.

A draped tendon with a trapezoidal profile considering bending moment diagram deflection at centre of beam is obtained by taking moment of area of bending moment diagram over one half of span A = ?Pe/6EI(2l₁²+6l₁l²+3l₂²).

The FIP-1984 recommends following suitable deflection limits for floors, roofs and or horizontal members in building limits for deflection that occurs after addition of partitions up to:
Span/500 to Span/1000.

The computations of deflections in this stage are made by considering moment curvature relationships which involve section, properties of cracked beam, based on prestressing forces and live loads deflections of prestressed concrete members can be calculated if longitudinal distribution of curvatures and magnitude of beam can be known at that particular time.

The revised American code considers bilinear character of load deflection characteristics by incorporating a suitable effective value of flexural rigidity in unilinear formula, In Unilinear method, deflections are computed by a simple equation of form
A = ?L²M/E_cI_t, a = maximum deflection, L = effective span, M = maximum moment in beam, E_c = modulus of elasticity of concrete, I_c = second moment of area equivalent cracked moment, ? = constant.