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1) Extra top bar are provided to increase tensile strength of beam ( to control negative moment) at support.
2) To resist diagonal tension stress and in order to prevent inclined cracks to occur by action of maximum shear stresses which concrete fails to resist
A beam with monolethic joint(like in concrete) is having positive moment between supports and negitive moment at supports.
Concrete is having high strength in compression but very week in tension.
Positive moment (bending towards bottom side) creats tention at the bottom and compression at top in between the supports.
On the other hand negtive moment (bending towards top side) creats tention at the top and compression at bottom on the supports.
Bottom bars are provided to increase tensile strenght of beam (to control positive moment) at bottom side in longitudinal direction in between the supports.
Top bars are provided to increase tensile strenght of beam (to control negative moment) at top side in longitudinal direction at the supports.
