Tension reinforcement leads to increasing deflection in concrete structures?

In BS8110 a modification factor is applied to span/depth ratio to take into account effect of tension reinforcement. Actually deflection of concrete structure is affected by stress and amount of tension reinforcement. To understand their relationship let us consider the subsequent equation relating to beam curvature:

Curvature = 1/r = e/ (d-x)

Where r = radius of curvature

  e = tensile strain in tension reinforcement

  d = effective depth

  x = depth of neutral axis

Provided that tensile strain in tension reinforcement remains constant, curvature of concrete structure increases with depth of neutral axis. It is perceived that depth of neutral axis rises with tension steel ratio. Consequently curvature of concrete section is directly proportional to the tension steel ratio. Additionally, largervalue of the depth of neutral axis enhances increased area of concrete compression so that effect of creep on deflection appears to become significant.