Short column 35x45 cm, 1800 kN

Data and assumptions

• Column 350 mm (width $b$) × 450 mm (depth $D$); $P_u = 1800$ 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) $M_{ux}$ = 175 kN·m; about minor axis (y-y) $M_{uy}$ = 105 kN·m.
• As the question states, the bars are placed equally on two opposite faces: the two 350 mm faces, which are parallel to the major axis x-x and so resist the larger moment $M_{ux}$ most effectively.
• 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

Both ratios are below 12, so the column is short; no additional moment is required.

Minimum eccentricity (cl. 25.4)

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

• About x-x: $e_x = 3100/500 + 450/30 = 21.2$ mm (min. 20 mm); adopt 21.2 mm; $M_{ux,min} = P_u e = 38.2$ kN·m
• About y-y: $e_y = 3100/500 + 350/30 = 17.9$ mm (min. 20 mm); adopt 20.0 mm; $M_{uy,min} = 36.0$ kN·m

Design moments

• Design moments: $M_{ux} = 175.0$ kN·m, $M_{uy} = 105.0$ kN·m (not less than the minimum-eccentricity values).

Longitudinal reinforcement

$\dfrac{P_u}{f_{ck} b D} = \dfrac{1800000}{25 \times 350 \times 450} = 0.457$ 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

$P_{uz} = 0.45 f_{ck} A_c + 0.75 f_y A_{sc} = 0.45 \times 25 \times 152675 + 0.75 \times 500 \times 4825 = 3527$ kN $\dfrac{P_u}{P_{uz}} = \dfrac{1800}{3527} = 0.510$, so $\alpha_n = 1.52$ (IS 456 cl. 39.6; linear between 1.0 at 0.2 and 2.0 at 0.8).

$$\left(\frac{M_{ux}}{M_{ux1}}\right)^{\alpha_n} + \left(\frac{M_{uy}}{M_{uy1}}\right)^{\alpha_n} = \left(\frac{175.0}{286.1}\right)^{1.52} + \left(\frac{105.0}{161.3}\right)^{1.52} = 0.996 \le 1.0$$

The interaction condition is satisfied; the section is safe.

Provide 6-32 mm bars ($A_{sc} = 4825$ mm², $p = 3.06\%$), within 0.8% to 4%.

The two layers are $450 - 2 \times 64 = 322$ mm apart, which is more than the 300 mm maximum spacing along the periphery (IS 456 cl. 26.5.3.1), so also provide one 12 mm nominal bar at mid-depth of each 450 mm face (not counted in the design).

Lateral ties

• Diameter $\ge \max(\phi_{max}/4,\ 6) = \max(32/4, 6) = 8$ mm → provide 8 mm.
• Pitch $\le \min$(least lateral dimension, $16\phi_{min}$, 300 mm) $= \min(350, 512, 300) = 300$ mm → provide 8 mm ties @ 300 mm c/c.

Detailing

CROSS-SECTION 350 x 450
    +-------------------------+
    |                         |
    |    o       o       o    |
    |                         |
    |                         |
    |                         |
    |    x               x    |
    |                         |
    |                         |
    |                         |
    |    o       o       o    |
    |                         |
    +-------------------------+
   o = 32 mm bars, x = 12 mm nominal side bars, ties 8 mm @ 300 c/c

Answer: 350 × 450 mm column with 6-32 mm longitudinal bars ($p = 3.06\%$), 3 on each 350 mm face, plus one 12 mm nominal bar on each 450 mm face, and 8 mm ties @ 300 mm c/c.