A design tells you 2T20. A bar bending schedule tells the bender-and-fixer how many bars to cut, how long, what shape, and where they go — and tells the quantity surveyor how many tonnes of steel to buy. It is the document that turns a calculation into something that can be built and priced.
The columns, and what each one is for
| Column | What goes in it |
|---|---|
| Member | Which beam, column or slab — e.g. 1B1 |
| Bar mark | The label painted on the bundle. Must match the mark on the drawing, or nobody can find it on site |
| Type & size | T20, R8 — grade and diameter in one |
| No. of members | How many identical beams this schedule covers |
| No. in each | Bars of this mark in one member |
| Total no. | The two multiplied — what is actually cut |
| Length of each | The cutting length, rounded to 25 mm |
| Shape code | The standard shape, from BS 8666 |
| A, B, C, D, E / R | The leg dimensions and bend radius that shape needs |
Cutting length is not the same as bar length
Steel stretches slightly around a bend, so a bar bent to a 600 × 400 L-shape is not 1000 mm of bar. The cutting length is the sum of the legs minus a bending allowance at each bend, and BS 8666 gives the formula for each shape code alongside the shape itself.
Shape 11 one 90-degree bend
Shape 21 two bends, opposite hands
Shape 51 closed link, four bends plus hooks
Every shape's own length formula and its required dimensions
come from the table in BS 8666. Look it up rather than adding
the legs — that is where the bending allowance hides.
Lengths are rounded to the nearest 25 mm, and a schedule that gives a length to the millimetre is a schedule written by someone who has never watched bars being cut.
Turning bars into kilogrammes
Steel weighs 7850 kg/m³, and putting that through the area of a round bar gives the rule everyone actually uses:
T8 = 64/162 = 0.395 kg/m
T12 = 144/162 = 0.889 kg/m
T16 = 256/162 = 1.580 kg/m
T20 = 400/162 = 2.469 kg/m
T25 = 625/162 = 3.858 kg/m
The exact figure is π/4 × d² × 7850 × 10-6 = 0.006165 d², and 1/162 = 0.006173. The difference is a tenth of a percent, which is why the shortcut has outlived every calculator it was invented for.
Worked example: scheduling one beam
The beam designed on the beam design page: 225 × 450, 4.0 m span, 2T20 bottom, 2T12 hangers, T8 links at 225 c/c, 25 mm cover.
Bottom bars
Mass = 2 no. x 3.95 m x 2.469 = 19.5 kg
Links
Inside = (225 - 50) x (450 - 50) = 175 x 400
Perimeter = 2(175 + 400) = 1150 mm
Hooks = 2 x 10d = 2 x 80 = 160 mm
Length ~ 1310 mm before the bending allowance is deducted
Mass = 18 x 1.31 x 0.395 = 9.3 kg
Hangers
The schedule
| Mark | Type | No. | Length | Shape | Mass |
|---|---|---|---|---|---|
| 01 | T20 | 2 | 3950 | 00 | 19.5 kg |
| 02 | T12 | 2 | 3950 | 00 | 7.0 kg |
| 03 | T8 | 18 | 1310 | link | 9.3 kg |
| Total | 35.8 kg | ||||
A sense check worth doing: 35.8 kg in 0.405 m³ of concrete is about 89 kg/m³. Beams usually land somewhere between 80 and 150 kg/m³. A schedule that comes out at 20 or at 400 has an error in it — that is a check on your arithmetic, not a design rule.
Mistakes that cost money
- Bar marks that do not match the drawing. The schedule is an index into the drawing; if the marks disagree, neither document is usable.
- Forgetting laps. Bars come in stock lengths — 12 m is typical. Anything longer needs a lap, and the lap is extra steel nobody ordered.
- Scheduling to the millimetre. Round to 25 mm.
- Ignoring the bending allowance on links, where there are four bends and two hooks in a short bar and the error is proportionally largest.
- One schedule for members that are not identical. If two beams differ by a bar, they are two entries.
Let it schedule for you, then check it
Every member Structura designs comes with its BS 8666 schedule already worked out — bar mark, type, size, number, length and shape code — alongside the calculation that produced it, plus the quantities of concrete, formwork and excavation that go with it.
Anchorage and lap assumptions are simplified and stated on the sheet, so you know exactly what to confirm at detailing rather than finding out on site.
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