Bar Bending Schedule (BBS) is a crucial document in construction projects. It includes details like shape, size, and cutting length of reinforcement bars. Cutting length is calculated based on the geometry of the bar and is vital for accurate reinforcement placement. Below are examples of cutting length calculations for common bar shapes.
For a straight bar, the cutting length is equal to the total length of the bar without any modifications.
Formula: Cutting Length = Total Length
Example: A straight bar of 6000mm length.
Cutting Length: 6000mm
Bent-up bars are used to provide additional strength in slabs and beams. They involve straight segments and an additional length for the bend.
Formula: Cutting Length = L1 + L2 + (π × d × θ / 180)
Where:
L1 = Length of the first straight segment
L2 = Length of the second straight segment
d = Diameter of the bend
θ = Angle of the bend (in degrees)
Example: A bar with L1 = 1200mm, L2 = 1500mm, d = 12mm, and θ = 90°.
Cutting Length: 1200 + 1500 + (π × 12 × 90 / 180) = 2718.85mm
Stirrups are used to hold main reinforcement bars together and resist shear forces. The cutting length includes the perimeter and allowances for hooks and bends.
Formula: Cutting Length = 2 × (A + B) + 2 × Hook Length - Bend Deductions
Where:
A = Length of one side
B = Length of the adjacent side
Hook Length = 9 × Bar Diameter (standard value)
Example: A rectangular stirrup with A = 400mm, B = 600mm, and bar diameter = 10mm.
Hook Length: 9 × 10 = 90mm
Cutting Length: 2 × (400 + 600) + 2 × 90 = 2180mm
Cranked bars are used to provide additional support in beams and slabs. The cutting length considers the additional length due to the crank.
Formula: Cutting Length = Total Straight Length + Number of Cranks × Crank Length
Where:
Crank Length = √(d² + h²)
d = Depth of crank
h = Horizontal length of crank
Example: A bar with total straight length = 5000mm, d = 200mm, h = 300mm, and two cranks.
Crank Length: √(200² + 300²) = 360.56mm
Cutting Length: 5000 + 2 × 360.56 = 5721.12mm
Circular bars are typically used for circular beams or columns. The cutting length is the circumference of the circle, adjusted for overlaps or hooks.
Formula: Cutting Length = π × Diameter + Overlap Length (if any)
Example: A circular bar with diameter = 1000mm and no overlap.
Cutting Length: π × 1000 = 3141.59mm
Cutting length calculations are critical for avoiding wastage of material and ensuring the proper fit of reinforcement bars. Each shape requires a specific formula based on its geometry, ensuring precision in reinforcement detailing and construction efficiency.