Column design, 1350 kN, 3.4 m

Data and assumptions

• Column 400 mm (width $b$) × 400 mm (depth $D$); $P_u = 1350$ kN; $f_{ck} = 25$ MPa, $f_y = 500$ MPa. Clear cover 40 mm (IS 456 cl. 26.4.2.1), 8 mm ties.
• Moments: about major axis (x-x) $M_{ux}$ = 150 kN·m; about minor axis (y-y) $M_{uy}$ = 100 kN·m.
• Both ends fully restrained for displacement and rotation: $l_e = 0.65L = 0.65 \times 3.4 = 2.21$ m (IS 456 Table 28).
• Section size is not given and is selected by trial.
• Sizes are given as $b \times D$ with $D \ge b$; moments $M_{ux}$ act about the major axis (depth $D$) and $M_{uy}$ about the minor axis (depth $b$).

Slenderness

Minimum eccentricity (cl. 25.4)

$$e_{min} = \max\left(\frac{L}{500} + \frac{D}{30},\ 20\right)$$

• About x-x: $e_x = 3400/500 + 400/30 = 20.1$ mm (min. 20 mm); adopt 20.1 mm; $M_{ux,min} = P_u e = 27.2$ kN·m
• About y-y: $e_y = 3400/500 + 400/30 = 20.1$ mm (min. 20 mm); adopt 20.1 mm; $M_{uy,min} = 27.2$ kN·m

Design moments

• Design moments: $M_{ux} = 150.0$ kN·m, $M_{uy} = 100.0$ kN·m (not less than the minimum-eccentricity values).

Longitudinal reinforcement

$\dfrac{P_u}{f_{ck} b D} = \dfrac{1350000}{25 \times 400 \times 400} = 0.338$ Bars are chosen so that $p$ lies between 0.8% (minimum) and 4% (practical maximum; IS 456 cl. 26.5.3.1 allows 6%). The uniaxial capacities $M_{ux1}$, $M_{uy1}$ at $P_u$ are read from the SP-16 interaction charts for rectangular sections; here they are computed by strain compatibility with the IS 456 stress block and the design stress-strain curve of steel (same basis as the charts), with effective cover $d' = 40 + 8 + \phi/2$.

Check

$P_{uz} = 0.45 f_{ck} A_c + 0.75 f_y A_{sc} = 0.45 \times 25 \times 156073 + 0.75 \times 500 \times 3927 = 3228$ kN $\dfrac{P_u}{P_{uz}} = \dfrac{1350}{3228} = 0.418$, so $\alpha_n = 1.36$ (IS 456 cl. 39.6; linear between 1.0 at 0.2 and 2.0 at 0.8).

$$\left(\frac{M_{ux}}{M_{ux1}}\right)^{\alpha_n} + \left(\frac{M_{uy}}{M_{uy1}}\right)^{\alpha_n} = \left(\frac{150.0}{212.9}\right)^{1.36} + \left(\frac{100.0}{212.9}\right)^{1.36} = 0.977 \le 1.0$$

The interaction condition is satisfied; the section is safe.

Provide 8-25 mm bars ($A_{sc} = 3927$ mm², $p = 2.45\%$), within 0.8% to 4%.

Lateral ties

• Diameter $\ge \max(\phi_{max}/4,\ 6) = \max(25/4, 6) = 6$ mm → provide 8 mm.
• Pitch $\le \min$(least lateral dimension, $16\phi_{min}$, 300 mm) $= \min(400, 400, 300) = 300$ mm → provide 8 mm ties @ 300 mm c/c.

Detailing

CROSS-SECTION 400 x 400
    +-------------------------+
    |                         |
    |   o        o        o   |
    |                         |
    |                         |
    |                         |
    |   o                 o   |
    |                         |
    |                         |
    |                         |
    |   o        o        o   |
    |                         |
    +-------------------------+
   o = 25 mm bars, ties 8 mm @ 300 c/c

Answer: 400 × 400 mm column with 8-25 mm longitudinal bars ($p = 2.45\%$) and 8 mm ties @ 300 mm c/c.