As we know that cost of the concrete structure project depends on the amount of steel added and concrete. As steel is much expensive than concrete so its exact amount is much important as one can know beforehand the cost of the entire project. Following are the methodologies which are usually practiced during estimation of steel rebar.
You can make a rough estimate by using thumb rule that about 80 kg of rebar is required per cum. of slab. So if slab size is say 10m x 10m x 0.2m. Concrete = 10 x 10 x 0.2 = 20 cum. Therefore, Rebar = 80 kg x 20 = 1600 kg = 1.6 MT.
This method is often used to get a rough idea of the cost and weight of the steel that is used for the building construction.
Another thumb rule can be used that 1% steel by volume of slab. So for above slab having volume = 20 cum.
Steel = 1% of 20 cum. = 0.2 cum = 0.2 x 7850 kg/cum = 1570 kg = 1.57 MT
The thumb rules for above 2 methods are based on your own experience. It may differ from one building to another. Only after having enough data can you make a correct assumption of the thumb rules. It is better to take a thumb rule from a previously constructed building of similar nature to the one you are calculating now. This is also known as benchmarking.
By measuring the quantity from the Drawings or from a BBS:
This is more detailed method of measurement. 1st you have to measure the total length of the different diameters of bars by including the length of straight bars, lap lengths, bend bars etc. Then multiply the total lengths with the unit weight of the respective bars to get the total weight of steel.
E.g. Total length of 20 mm bars is 100 meters. Then Weight = Length x unit weight of 20 mm dia bar = 100 x 2.47 = 247 kg’s.
By Using Estimation Software:
Using different estimation software’s like Plan Swift and Blue Beam also help us to obtain direct mass of the bar just by providing the area and spacing between the bars. The amount obtain by these software’s is usually exact or near to the exact amount and some waste material usually 5-10% of total weight calculated is applied and you will get the exact amount.
You can use different online calculators to estimate the quantity of steel reinforcement.