We use the same bars( which are available in the market) in all structures.
we do not manufacture different bars for different structural elements.
Diameter of the bar will vary according to load condition.
Bars in slab
like in slab we generally use 8mm, 10mm , 12mm because load is less as compared to beam , column, footing.
note:- 8mm bar generally used in one way slab as distribution bar (secondary steel)
Bars in Beam
In beam we use bars having diameter Greater than 12mm (depends upon load)
Bars in Column
In column we use Greater than 12 mm (depends upon load)
Bars in Footing/foundation
Similarly for footing we use 10 mm or more than that (depends upon loading)
Bars which are available in the market are 8 mm, 10 mm, 12 mm, 16 mm, 20 mm, 25 mm, 28 mm, 32 mm, 36 mm and 42 mm
Greater diameter like 64 mm and 128 mm has to be ordered specially from steel plant for manufacturing
Q 1 :- why we are not using 14 mm bar?
There is no specific reason for not using bar of 14,18, 22 mm etc diameter bar. I have mentioned the bars which are available in the market (India). As i mentioned in the first line of this post we use the bars which are available in the market.
Q 2:- How do we decide number of bars & which diameter of bars to use?
We decide the number of bars & diameter of the bars based on area of reinforced is required & convince of placing concrete.