Compute the stresses at top and bottom fibers in a simply supported pre-stressed concrete beam with a rectangular cross section, 250 mm wide and 450 mm deep, as shown in Figure, at sections A-A, B-B and C-C, due to the following cases of pre-stressing.
(i) A pre-stressing force of 350 KN concentrically applied.
(ii) A pre-stressing force of 260 KN, eccentrically applied with a constant eccentricity of 60 mm towards the soffit of the beam.
In both the cases, the beam is subjected to its self load and an imposed load of 6 kN/m over the entire span of 5 m. Density of the beam may be taken as 24 kN/m3. Calculate the combined stresses at these sections at top and bottom fibres.