Basic Rules for the Design of Beams
Reinforced Cement Concrete (RCC) beams play a crucial role in structural design. Proper dimensioning and reinforcement are essential for ensuring structural integrity. Below are the fundamental rules for RCC beam design as per IS 456-2000.
1. Beam Dimension
Effective Depth
The minimum effective depth (d) of the beam is considered as 1/10th of the clear span of the beam.
Breadth
The beam’s minimum breadth (b) should be between 1/2 to 2/3 of the total depth.
Example:
If the span of the beam is 5000mm:
- Effective depth, d = 1/10 × 5000mm
d = 500mm - If the effective cover is 40mm,
D = d + cover
D = 500mm + 40mm = 540mm - Minimum breadth,
b = 1/2 × 540mm
b = 270mm
2. Beam Reinforcement
Minimum Reinforcement
The minimum area of tensile reinforcement is calculated using:
As = 0.85bd / fy
Where:
- As = Minimum area of tensile reinforcement
- b = Breadth of the beam
- d = Effective depth
- fy = Characteristic yield strength of steel
Example:
If fy = 415 N/mm²,
As = (0.85 × 270mm × 500mm) ÷ 415
As = 276.50 mm²
Maximum Reinforcement
The maximum area of reinforcement is calculated as:
As(max) = 0.04bD
Example:
As(max) = 0.04 × 270mm × 540mm
As(max) = 5832 mm²
3. Side Face Reinforcement
If the depth of the beam exceeds 750mm, side face reinforcement is required along the vertical faces.
- The total area should be at least 0.1% of the web area.
- The spacing between the bars must not exceed 300mm.
4. Spacing of Reinforcement
Horizontal Spacing
- Equal diameter bars: Minimum spacing should be equal to the bar diameter.
- Unequal diameter bars: Spacing should be the larger bar diameter.
- Aggregate size influence:
- 20mm aggregate: Minimum spacing = Aggregate size + 5mm
- 25mm aggregate: Minimum spacing = 30mm
Vertical Spacing
- Bars ≤12mm: Minimum spacing is 15mm.
- Two-thirds of aggregate size or 15mm — whichever is higher.
5. Deflection Control
For Span up to 10m
- Cantilever beam: Span/effective depth ≤ 7
- Simply supported beam: Span/effective depth ≤ 20
- Continuous beam: Span/effective depth ≤ 26
For Span Above 10m
The obtained ratio is multiplied by 10/span in meters.
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