W_min/W_ud = (50x2400x9.81�0.125x25x30/50�10⁶x0.85²) = 0.31
Ultimate load excluding factored selfweight = (1.4�2)+(1.6�9) = 17.2kn/m, W_ud = 17.2/1-1.4�0.31 = 30KN/M, W_min = (0.31�30) = 9.3kn/m, Ultimate moment, M_u = (0.125x30x30²) = 3400knm, Ultimate shear, V_u = (0.5x30x30) = 450kn.

M_u = 0.10f_cubd² and if b = 0.5d
D = (42�10⁶x2/0.10�50)^1/3 = 270mm.

The cracked section analysis of a partially prestressed flange section with tensioned high tensile steel and un tensioned reinforcement is carried out under following assumptions: The strain distribution across section is linear, tensile strength of concrete below neutral axis.

12 diameter, stress = 1100n/mm², e = 50, prestressing force = 1000n/mm²
Force in each cable = ((12x50x1100)/1000)) = 660kn.

Area = 36.75mm², e = 580, f_inf = 26.5kn/m, z_b = 99�10⁶
p = (Af_infZ_b/Z_b+A_e) =(367500�26.5x99x10⁶/(99�10⁶)+(367500�580)) = 308x104kn/m².

In dimensioning prestressed concrete flexural members effective depth and breadth of section at compression face are determined solely on basis of ultimate flexural strength requirements and The breadth of compression face may be assumed by considering number of covering a given width of bridge deck of a suitable ratio of b?d being in range of 0.4 to 0.6.

F_cp = (?p/A) = (0.8�182000/31400) = 4.65n/mm²
V_cw = 0.67bh(f₁²+0.8f_cpf_t)^1/2 = (0.67x50x320(1.72+0.8�4.65�1.7)1/2/10³) = 33.2kn
V_cw > V_u hence safe against shear failure.

The value of shear moment arm I/S varies between 0.67 and 0.85h for I sections, ratio f_cp/f_t generally varies between 2 and 3 for small span girders with straight tendons for long span girders with curved tendons, ratio, f_cp/f_t can be taken between 3 and 4 and effective shear as 0.8vu since curved cables contribute to ultimate shear resistance of section.

In excess of cracking moment M_cr just prior to application of moment tensile strain in prestressing steel is ?pe and compressive strain in concrete at tendon level is ?_ce and se strains can be evaluated from prestressing force acting at this stage ?_pe = p/A_pE_p , ?_ce = P/E_c(e²/I_c+1/A_c), P = effective prestressing force, e = eccentricity of tendons, A_c = cross sectional area of concrete section, I_c = second moment of area of concrete section.

The reinforcement can cater for serviceability requirements such as control of cracking and partially for ultimate limit state of collapse which can result in considerable reductions in costlier high tensile steel and saving in prestressing steel contributes to an overall saving in cost of structure.