Q. A beam made of C.I. having section of 50mm external diameter and 25 mm internal diameter is supported at 2 points 4 m apart. The beam carries concentrated load of 100N at the centre of it. Find out maximum bending stress induced in beam. 

Sol.:
  
This problem is case of simply supported beam with the load at its midpoint, and in this case the maximum bending moment = M = WL/4
...(i)
Let  I = The moment of inertia = 

We know that

Maximum stress at the time of Bending moment (at  y = 25mm) = M.y/I
= (100 × 10³ × 25)/287621.4 = 8.69 N/mm²

The maximum bending stress = 8.69 N/mm² .......ANS