I want to calculate the quantity of reinforcement steel required for an RCC slab, beam, column or footing. Please explain the formula, number of bars, cutting length, unit weight and provide a worked example.
How to Calculate Reinforcement Steel Quantity in RCC
Reinforced cement concrete (RCC) contains steel bars to improve the tensile strength of concrete. civilengpro.com
The final steel quantity should be calculated from the structural drawing or Bar Bending Schedule (BBS).
Formula
\[ \text{Steel weight (kg)}= \text{Total bar length (m)}\times\frac{d^2}{162.2} \]
Where:
- \(d\) = bar diameter in millimetres
- \(d^2/162.2\) = unit weight of the steel bar in kg/m
Calculation Steps
- Identify the bar diameter and spacing from the structural drawing.
- Calculate the number of bars:
\[ \text{Number of bars}= \frac{\text{Clear width}}{\text{Spacing}}+1 \]
- Calculate the cutting length of each bar.
- Calculate the total bar length:
\[ \text{Total length}= \text{Number of bars}\times\text{Cutting length} \]
- Calculate the steel weight using:
\[ \text{Weight}= \text{Total length}\times\frac{d^2}{162.2} \]
Example: RCC Slab
Assume:
- Slab size = 4 m × 5 m
- Reinforcement = 10 mm diameter bars at 150 mm centre-to-centre
- Concrete cover = 20 mm
- Reinforcement provided in both directions
Bars spanning 4 m:
- Cutting length = \(4 – 0.04 = 3.96\) m
- Number of bars = 35
- Total length = \(35 \times 3.96 = 138.6\) m
Bars spanning 5 m:
- Cutting length = \(5 – 0.04 = 4.96\) m
- Number of bars = 28
- Total length = \(28 \times 4.96 = 138.88\) m
Total steel length:
\[ 138.6+138.88=277.48\text{ m} \]
Unit weight of 10 mm bar:
\[ \frac{10^2}{162.2}=0.616\text{ kg/m} \]
Steel quantity:
\[ 277.48\times0.616=171.1\text{ kg} \]
Therefore, the theoretical reinforcement quantity is approximately 171 kg. Adding 5% wastage:
\[ 171.1\times1.05\approx180\text{ kg} \]
Important Note
For beams, columns, footings and slabs, include main bars, distribution bars, stirrups, hooks, bends, laps and anchorage lengths. A thumb rule may be used for preliminary estimation, but the accurate quantity must come from the structural drawing and BBS.