RCC columns: short or long, minimum eccentricity and axial capacity

Columns carry load from beams and slabs down to the foundation. IS 456 handles them in two ways depending on how slender they are.

Short or long?

l_exD < 12 and l_eyb < 12 → short column

Otherwise the column is slender (long) and must also be designed for the additional moments caused by its own deflection.

Effective length depends on end restraint (IS 456 Table 28), from 0.65 l for both ends fully fixed up to 2 l for a cantilever column.

Minimum eccentricity

Real columns are never loaded perfectly through their centre. IS 456 requires every column to be designed for at least

e_min = l500 + D30, but not less than 20 mm

Axial capacity of a short column

When e_min does not exceed 0.05 D, the axial load capacity is:

P_u = 0.4 f_ck A_c + 0.67 f_y A_sc

A_c is the net concrete area (A_g − A_sc) and A_sc the longitudinal steel area. The factors 0.4 and 0.67 already include the material safety factors and the effect of minimum eccentricity.

Worked example

A 400 × 400 mm short column with 8 bars of 20 mm. M25 concrete, Fe 500 steel.

A_sc = 8 × 314 = 2513 mm², A_c = 160000 − 2513 = 157487 mm².

P_u = 0.4 × 25 × 157487 + 0.67 × 500 × 2513 = 2417 kN

Detailing rules

Item	Rule
Longitudinal steel	0.8% to 6% of Ag (4% is a practical limit)
Minimum bars	4 in a rectangular column, 6 in a circular one
Minimum bar size	12 mm
Tie diameter	at least ¼ of the largest bar and not less than 6 mm
Tie pitch	least of: least lateral dimension, 16 × smallest bar diameter, 300 mm
Cover	at least 40 mm, or the bar diameter if larger