Column Confinement Zone Calculator
Confinement length, hoop spacing and bar size for each detailing class.
Inputs
Result
Formula
| End zone length | l = max(long side hc, Hn/6, 500 mm) |
| Ground storey base | not less than Hn/3 |
| Rigid floor | 500 mm above and below |
| Max spacing | C1 min(6d,100); C2 min(8d,100); C3-4 min(8d,150) |
| Min diameter | C1 10 mm; C2-3 8 mm; C4 6 mm |
| Full height | corner, transfer, low shear span, Hn/hc ≤ 4 |
500 mm section, 3600 mm clear height, class 2, 20 mm bars: max of 500, 600 and 500 is 600 mm, so the end zone is 600 mm; max hoop spacing is min(160, 100) = 100 mm, minimum diameter 8 mm.
FAQ
Why confine the ends and not the middle?
Bending moments peak at the ends and plastic hinges form there, so hoops must confine the concrete to keep it from crushing while it dissipates energy. Near mid-height the moment is small and the column is close to pure axial load, so hoops only hold the bars and resist shear - spacing can be relaxed. It is why post-earthquake photos show crushed ends and intact middles.
Why take the largest of three instead of just Hn/6?
Because the governing factor changes with proportions. A stocky 800 mm column has a plastic hinge zone proportional to its depth, so the section governs; a slender tall column is governed by Hn/6; a shallow plant room storey is caught by the 500 mm floor. Each limit guards a different case, so the largest applies.
Where do the 100 mm and 6d limits come from?
They prevent local buckling of the longitudinal bars. Each bar acts as a strut and the hoops are its lateral supports: the closer the supports the less the bar can bow. The 6d limit scales with bar size, 100 mm is the absolute cap, and class 1 uses 6d rather than 8d because those members are expected to reach deeper plastic deformation.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.