Clause 7.2 defines the transverse reinforcement requirements for columns in ductile frames. Special confining reinforcement is required over a length lo from each joint face and on both sides of any section where flexural yielding may occur. The length lo is the largest of: the column lateral dimension, 1/6 of the clear height, or 450 mm. Within the lo zone, the spacing of hoops must not exceed the minimum of B/4 (where B is the minimum column dimension) and 75 mm.
Key Requirements
•Transverse reinforcement shall consist of closed-loop hoops with 135° hooks
•Special confining reinforcement shall be provided over a length lo from each joint face, where lo is the largest of: (a) column lateral dimension at the section, (b) 1/6 of clear column height, (c) 450 mm
•Maximum spacing of hoops in the lo zone shall not exceed the minimum of B/4 and 75 mm, where B is the minimum column dimension
•Outside the lo zone, spacing of hoops shall not exceed the minimum of: (a) half the least lateral dimension, (b) 8 × smallest longitudinal bar diameter, (c) 300 mm
•When a column terminates into a footing or mat, special confining reinforcement shall extend at least 300 mm into the footing
•Lap splices in columns shall be provided only in the central half of the member length
•At lap splice locations, hoop spacing shall not exceed the minimum of B/4 and 100 mm
Reference Tables
Column Hoop Spacing — Confining Zone vs Mid-Height
Column Size (mm)
B/4 (mm)
Hoop Spacing in lo Zone (mm)
Hoop Spacing Outside lo (mm)
300 × 300
75
75
150
300 × 450
75
75
150
300 × 600
75
75
150
400 × 400
100
75
200
450 × 450
112
75
225
600 × 600
150
75
300
In the lo zone, the 75 mm cap almost always governs for columns up to 300 mm. For larger columns, B/4 may be less than 75 mm only if B < 300 mm (not permitted for most seismic buildings).
Formulas
lo = max(D, Hclear/6, 450)
Length of special confining reinforcement zone at each end of the column
lo = Length of confining zone in mmD = Larger lateral dimension of the column in mmHclear = Clear height of the column in mm
Practical Notes
✓For a typical 300 mm column with 3 m clear height: lo = max(300, 3000/6, 450) = 500 mm. So confining hoops extend 500 mm from each joint face.
✓The 75 mm maximum spacing in the lo zone is very tight — it means 6–7 hoops per 500 mm zone. Ensure this is shown clearly in drawings and BBS.
✓At lap splices (central half only), the spacing tightens further to min(B/4, 100 mm). This creates a very dense cage that is difficult to pour — use self-compacting concrete or ensure adequate vibrator access.
Common Mistakes
⚠Not extending confining reinforcement 300 mm into footings — the column-footing junction is a critical zone that is often under-reinforced.
⚠Placing lap splices near the top or bottom of the column (within the lo zone) instead of in the central half of the clear height.
⚠Using the mid-height hoop spacing throughout the column instead of the tighter lo zone spacing near joints.