Column Confinement Zone Calculator

How to work it out

1. Compute the confinement length as the largest of the column's long side, net height ÷ 6, and 500 mm. For a 500 mm column at 3.6 m net height, Hn/6 is 600 mm, so 600 governs.
2. Treat the base of ground-floor columns separately: confinement extends at least Hn/3, longer than the general-storey rule, because the column base is where plastic hinges form first.
3. Confine 500 mm above and below rigid ground slabs as well — a provision routinely missed in industrial buildings with hardstanding.
4. Set the maximum link spacing: class 1 min(6d, 100 mm); class 2 min(8d, 100 mm); classes 3 and 4 min(8d, 150 mm), where d is the main bar diameter.
5. Set the minimum link diameter: 10 mm for class 1, 8 mm for classes 2 and 3, 6 mm for class 4.
6. Decide whether full-height confinement applies: corner columns, transfer columns, columns with a shear span ratio of 2 or less, and columns whose net height is no more than four times the long side all require it.

Where it goes wrong

• Using 500 mm as the confinement length. The rule takes the largest of three values, and 500 mm is only the floor. For a 500 mm column at 3.6 m the correct figure is 600 mm; building 500 mm loses a link spacing exactly where it matters.
• Treating the ground-floor base as an ordinary storey. The base requires at least Hn/3, far longer than the general rule; omitting it produces the shear failure at the column base that characterises collapse.
• Applying 100 mm spacing universally. Class 1 is min(6d, 100) — at d = 20 mm that is 100 mm, but at d = 12 mm it is 72 mm, tighter than 100.
• Overlooking the special columns needing full-height confinement. Corner columns take biaxial seismic action, transfer columns carry concentrated transfer-beam loads, and low shear-span columns fail in brittle shear.
• Using 90° hooks or open links in confinement zones. Confinement requires closed links with 135° hooks and a straight tail of at least 10d; an open link confines nothing.

Material and cost

Full-height confinement uses 30-50% more link steel than end confinement alone. A 500×500 column at 3.6 m storey takes about 40 kg of links with end confinement and about 70 kg full height. If only a few columns are involved the extra is small, but misclassifying ordinary columns as full-height adds 3-5% to project-wide reinforcement.

On site

• Confinement is measured from the column end at beam or slab soffit, not from floor level. With a 600 mm deep beam the zone extends below the beam, and drawings are easily misread here.
• Link leg spacing also has limits: preferably no more than 200 mm for class 1 and 250 mm for classes 2 to 4. Correct spacing with too few legs still fails.
• Beam-column joint cores must be confined to the same requirement. This is the hardest area to build and is frequently under-provided.
• Net height Hn is measured from the underside of the beam above to the top of the beam below, not storey height. Using storey height shortens the confinement zone.

Basis

• GB 50011-2010 (2016 edition) Code for seismic design of buildings, column confinement zones and link detailing
• GB 50011-2010 (2016 edition) Code for seismic design of buildings, axial load ratio limits for columns
• GB 50666-2011 Code for construction of concrete structures, reinforcement works