The extra top & extra bottom reinforcements are provided as per the required area of the reinforcement of the beam as per design.
In general, in continuous frames beams, undergo hogging moments at the top face (max. in L/4 region from the face of the column) & sagging moments at the bottom middle region & thus the concentration of bending moments is greater in the aforesaid regions.
For further clarification & better understanding of above, kindly refer attached image.
Now while designing you have got two alternatives-
- Either to provide the same steel throughout the top & bottom face.
- Or to provide steel required for extra hogging/sagging moments at respective faces as extra.
choosing option 2 gives economy & it fulfills design requirements as well. Therefore, extra steel at top & bottom face is provided as per design requirements to cater the extra moments.
I hope this article will help you to understand why these extra top and bottom bars are provided in beams. You may also want to see my other post from my Blog. If I have missed anything here, please let me know about that in the comment below this post.
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