Large deflections and wide cracks are observed in compression face and this is because area of concrete provided is unable to resist resultant internal compression forces and member also fails in flexure due to crushing of concrete considerable warnings are observed before impending failure.

For under reinforced sections, failure of prestressing steel is assumed to take place at a maximum tensile strain of 0.5% however method can be considerably simplified under reinforced sections, in which stress in tensile stress at collapse stage is more or less equal to characteristic tensile strength of tendons.

The percentage of steel provided, both tensioned and un tensioned taken toger should be sufficient so that when concrete in pre compressed tensile zone cracks, steel is in a position to bear additional tensile stresses transferred to it by cracking of adjacent fibers of concrete, reby preventing a sudden failure of beam due to fracture of steel in tension.

If cross section is provided with an amount of steel greater than minimum, failure is characterized by an excessive elongation of steel followed by crushing of concrete and as bending loads are increased excessive elongation of steel raises neutral axis near compression face at critical section of beam.

The required section modulus z from equation Z= M_L/F_C fc is permissible stresses for concrete, but section modulus, Z=bd²/6 where d- depth of beam and beam should be equal to L/20 or L/25, b-width of beam, which is given as, b=6z/d².

The points a, b, represents points of application of forces P and C respectively, now if beam is subjected to live load in addition to dead load and prestress n point of application of force c reach point a from point a point of application of force p and magnitude of force C remain unchanged, if 0 represents centroid of beam section, n distance ob is k_b and oa? is k_t, if force C acts at lower kern point bottom fibers of beam are subjected to maximum permissible stresses and top fibers are subjected to minimum permissible stress.

If neutral axis of section lies outside flange, n ultimate moment of resistance of flanged section is calculated by combining moment of resistance of web & flange portion
Where Ap_w = area of prestressing steel for web (A_p-Ap_f), Ap_f = area of prestressing steel for flange (0.45fck (b-b_w) (d_f/f), f = characteristic tensile strength of prestressing steel, d_f = flange thickness, b = width of beam, b_w = width of web after considering are cases in tendon, effective prestress fpc should be greater than 0.45f.

The resistance of concrete in tension is neglected in strain compatibility method at end of section, maximum compressive strain in concrete at failure reaches a particular value are assumptions made in strain compatibility method.

Many codes have recommended simplified procedures for calculating flexural strength of concrete sections which are reinforced with high tensile steel in tension zone; use of strain compatibility method, generally applicable for both under & over reinforced sections is illustrated.

The shape of profile depends on reaction of cable and se are obtained by replacing forces acting on concrete with tendons, a prestressed concrete beam with bent tendons has extreme fiber stress, f = P/A+M/z and P/A-M/Z, prestressed concrete beam with parabolic tendon has a net downward force acting on beam as F = W-W_e.