Estimating steel bars can be done in many ways like per area in square meter, in volume or simply by direct counting whichever is convenient to you.
There are steel bars for footings, columns, beams, slabs, walls using CHB, or walls that are cast in place and will differ in sizes of rebars to be used plus the designated spacing, required development lengths, location of splices etc.
To get close to actual number of bars needed is to base from structural plans with given schedule of bars and add some contingencies then make adjustments on actual constructions if needed.
FOOTING REBARS
Most arrangements for footings are grid type which you just need to determine if the design requires for hooks (bends) or simply straight but more or less common practice in the country applies the one with hooks.
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| footing rebars |
Consider also the concrete cover from edge of concrete so your steel bars won't be exposed to soil and water which is needed to be deducted when estimating for the overall length.
DIRECT COUNTING
Sample:
Footing Size: 1.20m x 1.20m x 0.35m
No. of sets: 7
Concrete cover: 0.075m (75mm) all sides from top to bottom
Hook/bend: 16d (where d = diameter of bar)
Steel diameter: 16mm
Steel length to be used: 6 meters
Spacing of steel: 7 rebars both ways (or distributed 6 spaces on each side)
Solving for steel bar length needed:
1.20m - (0.075 x 2) + 2 x (16 x 0.016) = 1.562m length for one piece only
therefore:
14pcs @ 1.562m length for one footing only.
From a 6 meter length you can only get 3pcs usable (1.562m x 3 = 4.69m with 1.31m excess)
To be able to supply 14pcs per footing it requires 4.67pcs of 16mm diameter x 6 meters length
14pcs/3pcs usable = 4.67 needed
Total of 33pcs 16mm @ 6m length for 7 sets of column.
AREA METHOD
We can derive from our previous computation and convert into area method by dividing the quantity we computed with the area of footing itself, knowing that a 1.20m x 1.20m = 1.44sqm
therefore:
4.67pcs/1.44sqm = 3.24pcs/sqm
You may add contingency as desired if doing a quick estimate on how many pieces needed.
VOLUME METHOD
Following the process you made with area method you just have to add the thickness to your computation to get pcs of rebars per volume.
therefore:
1.2m x 1.2m x 0.35m = 0.504cu.m
4.67pcs/0.504cu.m = 9.27pcs/cu.m
Same process add any contingency to your estimate if needed.
Note that calculations are for a 6 meters length of steel bar, if using other lengths then results may vary as per number of usable pieces vs wastage.
EXPRESS IN KILOGRAMS
If you don't want to express results in pieces, you may convert to kilograms per area or per volume. Refer to sample table in this post for different sizes and as per steel length.
A 16mm diameter of 6 meters length have 9.465 kgs
Whatever quantity of pieces that will come out from your computation you simply multiply to designated kilograms in table.
Going back to our "area method" instead of 3.24pcs you convert to kilograms first which gives you 30.67 kgs per sqm.
Whereas in "volume method" instead of 9.27pcs you convert to kilograms which gives you a result of 87.74 kgs per cu.m
Reverse the process if you want to convert kilograms to pieces by dividing the total kilograms with the weight of steel bar given in table.
