BBS Height Calculator

Why Contractors Lose Money Here

A height that is off by one cover or one bar diameter repeats in every bar of the same mark: cranks that do not sit on the top steel, chairs that push the top mesh into the cover, and column bars that come up short of the lap.

Real Site Example

150 mm slab, 20 mm top and bottom cover (IS 456 Table 16, mild exposure), 10 mm crank bars, bottom mesh 10 + 8 mm, top mesh 8 + 8 mm: crank height = 150 − 20 − 20 − 10 = 100 mm; chair height = 150 − 20 − 20 − 18 − 16 = 76 mm. Column with 3.0 m floor-to-floor height, 16 mm Fe500 bars in M25: Ld in tension = 16 × 435 / (4 × 1.4 × 1.6) = 777 mm; in compression 777 ÷ 1.25 = 621 mm, more than 24φ = 384 mm, so the lap is 625 mm and each bar is 3,000 + 625 = 3,625 mm (5.73 kg).

Professional Best Practices

Take every height from the member depth on the drawing, deduct the cover on both faces and the bars in between, and write the result on the BBS line so the bar bender does not work it out on site.

Engineering Checklist

• Crank height: slab thickness − top cover − bottom cover − crank bar diameter
• Chair height: slab thickness − both covers − both bottom layers − both top layers
• Covers from the drawing; otherwise IS 456 Cl. 26.4.2 and Table 16 for the exposure
• Column lap: Ld in compression, not less than 24φ (IS 456 Cl. 26.2.5.1(d)), unless the drawing gives a value
• Foundation starter bars and top-floor bars follow the drawing, not the per-floor length

Government & Standards References

• IS 456:2000 — Clause 26.4.2 and Table 16 (nominal cover: 20 mm mild, 30 mm moderate, 45 mm severe, 50 mm very severe, 75 mm extreme exposure)
• IS 456:2000 — Clause 26.4.2.1 (column longitudinal bars: cover not less than 40 mm nor less than the bar diameter) and 26.4.2.2 (footings: at least 50 mm)
• IS 456:2000 — Clause 26.2.1 and 26.2.1.1 (development length; design bond stress, +60% for deformed bars, +25% in compression)
• IS 456:2000 — Clause 26.2.5.1(d) (lap length in compression)
• IS 2502 — bending and fixing of bars for concrete reinforcement

How Experienced Contractors Handle This

Estimators keep one BBS line per bar mark with its shape, dimensions and heights, and total the schedule by diameter for the steel order.