Ductile detailing of columns, confining steel

Columns must stay stable while the beams yield, so IS 13920 asks for good confinement at the column ends, where plastic hinges can form.

General requirements

• Minimum dimension 200 mm (300 mm if the unsupported length exceeds 4 m or the beam span exceeds 5 m); $b/D \ge 0.4$.
• Longitudinal steel 0.8% to 4% (IS 13920; IS 456 allows up to 6%, but 4% is the practical limit for ductile columns). Lap splices only in the central half of the column height, designed as tension splices, with hoop spacing $\le 150$ mm over the lap; not more than 50% of the bars spliced at one section.

Special confining reinforcement

• Provided over a length $l_o$ at both ends of each column, measured from the joint face, where $l_o \ge$ the largest of: the larger lateral dimension, $h/6$ (clear height) and 450 mm.
• Hoop spacing in this zone: $s \le \tfrac14 b_{min}$, but not less than 75 mm and not more than 100 mm.
• Hoops are closed with $135^\circ$ hooks, tail $\ge 6\phi$ (not less than 65 mm). Minimum diameter 8 mm (10 mm for bars over 32 mm).
• Area of the hoop bar for a rectangular hoop:

$$A_{sh} = 0.18\, s\, h \,\frac{f_{ck}}{f_y}\left(\frac{A_g}{A_k} - 1\right)$$

where $A_{sh}$ is the cross-sectional area of the hoop bar, $h$ is the longer dimension of the rectangular confining hoop measured to its outer face (not more than 300 mm), $A_g$ is the gross area and $A_k$ is the area of the confined core measured to the outside of the hoops. For circular spirals, $A_{sh} = 0.09\, s\, D_k \frac{f_{ck}}{f_y}\left(\frac{A_g}{A_k}-1\right)$.

• Where a column supports a discontinued stiff member (such as a wall), the confining steel runs over the full height of the column.
• Outside $l_o$ the spacing follows IS 456 (not more than the least lateral dimension, $16\phi$ or 300 mm).

   Joint ---------------
     ^     | # # # # # |  s = 75-100 mm
     | lo  | # # # # # |  (closely spaced hoops)
     v     | # # # # # |
   ------- |#  #  #  # |  
     h-2lo | #    #    |  normal spacing (IS 456)
   ------- | # # # # # |  close spacing again