Bar bending schedule (BBS) plays a crucial role in estimating steel quantity for beams, columns, and slabs. It provides details on bar shape, size, length, weight, and bending dimensions. This guide will focus on preparing the bar bending schedule for slabs with practical examples.
Types of Slabs
One-Way Slab
A one-way slab is supported on two opposite sides and carries the load in one direction. The ratio of the longer span to the shorter span is equal to or greater than 2 (L/B ≥ 2).
Two-Way Slab
A two-way slab is supported on all four sides and carries loads in both directions. The ratio of the longer span to the shorter span is less than 2 (L/B < 2).
Bar Bending Schedule of One-Way Slab
Example Calculation
Given:
- Length of slab = 5000 mm
- Width of slab = 2000 mm
- Main bar = 12 mm @ 150 mm c/c
- Distribution bar = 8 mm @ 150 mm c/c
- Clear cover (Top and Bottom) = 25 mm
- Thickness of slab = 150 mm
- Development length = 40d (where d is the diameter of the bar)
Ly/Lx Calculation: Ly/Lx = 5000/2000 = 2.5 > 2
Since Ly/Lx > 2, it is a one-way slab.
Step 1: Calculate the Number of Bars
Formula: No. of bars = (Length of slab / Spacing) + 1
Main Bars: No. of main bars = (5000/150) + 1 = 34
Distribution Bars: No. of distribution bars = (2000/150) + 1 = 14
Step 2: Calculate Cutting Length
Main Bars: Cutting Length = Clear Span + (2 x Development Length) + Inclined Length – (Bend Length)
- Clear span = 2000 mm
- Development length = 40d
- Inclined length = 0.42D
- Bend length = 1d for every 45° bend
- D = Thickness of slab – (2 x Clear Cover) – Diameter of bar
- D = 150 – (2 x 25) – 12 = 88 mm
Length of Main Bar: Ly + 2Ld + (1 x 0.42D) – (1d x 4) = 2000 + (2 x 40 x 12) + (2 x 0.42 x 88) – (1 x 12 x 4) = 2838 mm = 2.838 m
Weight of Main Bars: Weight = (d² x L) / 162 x No. of Bars = (12² x 2.838) / 162 x 34 = 86 kg
Length of Distribution Bar: Lx + 2Ld = 5000 + (2 x 40 x 8) = 5640 mm = 5.64 m
Weight of Distribution Bars: Weight = (d² x L) / 162 x No. of Bars = (8² x 5.64) / 162 x 14 = 31 kg
Step 3: Calculate Extra Top Bars
Formula: No. of top bars = {(Ly/4) / Spacing +1} x 2
Calculation: = {(2000/4) / 150 + 1} x 2 = 9 bars
Length of Extra Bar: Ly – (2 x Ly/4) + 2 x 100 (for both-side lapping of 100 mm) = 2000 – (2 x 2000/4) + 200 = 1200 mm = 1.2 m
Weight of Extra Bars: Weight = (d² x L) / 162 x No. of Bars = (8² x 1.2) / 162 x 9 = 4.266 kg
Click Here To See Simplified Bar Bending Schedule For Two-Way Slabs: A Practical Guide For Engineers
