Bending and shape codes
Minimum bend radius, mandrel and end projection by bar size (BS 8666)
- where
- minimum scheduling radius (mm)
- minimum mandrel (former) diameter (mm)
- minimum straight end projection (mm)
- bar diameter (mm)
- length deducted from A + B for one 90° bend (mm)
Explain this equation
Gives the BS 8666 minimum bend radius, mandrel size and shortest straight leg for a bar size, and how much one 90° bend takes off the sum of the outside dimensions.
r is the scheduling radius the bending machine uses, M the mandrel (former) diameter the bar is bent round, and P the shortest straight leg that can be gripped and bent accurately. δ is the length lost when two outside dimensions meet at one standard bend.
r, M and P are table values, not calculated: up to 16 mm bars bend round 4d, larger bars round 7d, to avoid cracking the bar. The bend deduction 0.5r + d is the simplified allowance BS 8666 uses for a 90° bend, because outside dimensions overstate the centreline length round the corner.
Sense check: a 12 mm bar has r = 24 mm, M = 48 mm, P = 125 mm and loses 24 mm per 90° bend. A 25 mm bar has r = 87 mm and loses 68.5 mm.
Watch out: these are minimums for B500 bars. The designer may ask for larger radii where bars are highly stressed or will be rebent.
Gives the BS 8666 minimum bending radius, mandrel size and straight end projection for a B500 bar, and how much a standard 90° bend takes off the sum of the outside dimensions.
Enter your values to see the result.
Good to know
Bars up to 16 mm bend round a mandrel of 4d and bars of 20 mm and over round 7d, so the scheduling radius is about 2d or 3.5d. P is the shortest straight leg the bending machine can hold: shorter legs cannot be bent accurately. Schedule dimensions are outside dimensions, so the cut length is less than A + B; for one 90° bend at the minimum radius BS 8666 takes off 0.5r + d (24 mm for a 12 mm bar). Larger radii must be scheduled as shape 12 or with the radius stated.
Common mistake
Cutting L-bars to the sum of the outside dimensions, which leaves every leg long, or asking for a tighter bend than the mandrel allows, which cracks the rib roots.
Limits
BS 8666:2020 Table 2 values for grade B500 bars to BS 4449. Stainless bars, bars to be rebent, and bends in highly stressed zones may need larger radii set by the designer under Eurocode 2 8.3. The values are scheduling minimums: actual radius is slightly larger because of spring-back.
Sources
- BS 8666:2020 Scheduling, dimensioning, cutting and bending of steel reinforcement for concrete. Specification (Table 2 minimum radii and end projections; Table 5 standard shapes and length formulae)
- BSI preview of BS 8666:2020 (foreword: supersedes BS 8666:2005; Table 2 gains spring-back allowance; Table 5 formulae and limits updated, shapes 48 and 52 added)
- KB Rebar, BS 8666:2020 shape codes sheet (Table 2: r, M, P, links P, hook diameter q = 3d + 2r rounded up to 5 mm)
- Heaton Manufacturing, UK rebar shape codes (total length formulae; minimum radius, former and end projection table)
- Collins Reinforcements, BS 8666 shape codes (dimension rules: free dimensions in brackets, A and end legs not less than P; minimised links 51 and 63)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Always work to the engineer’s drawings and bending schedules, BS 8666, BS EN 1992, BS EN 1090-2, BS 5531/the BCSA National Structural Steelwork Specification and manufacturers’ data. Lifting must be planned under LOLER by a competent person.