Slab base for ISMB 350, 850 kN service

Data: ISMB 350 @ 52.4 kg/m: $h=350$, $b_f=140$, $t_f=14.2$, $t_w=8.1$ mm. Service load $850$ kN → factored load $P=1.5\times850=1275$ kN. Steel Fe410 ($f_y=250$); concrete M25.

1. Bearing strength of concrete (IS 800:2007 cl. 7.4.1)

$0.45f_{ck}=0.45\times25=11.25$ N/mm²

2. Plate size

$$A_{req}=\frac{P}{0.45f_{ck}}=\frac{1275\times10^3}{11.25}=113333\ \text{mm}^2$$

Projection of about 55 mm all round the column (not less than 50 mm): $L=350+2\times55=460$ mm, $B=140+2\times55=250$ mm. Area provided $=115000$ mm² $>A_{req}$ ✓

$$w=\frac{P}{BL}=\frac{1275\times10^3}{460\times250}=11.09\ \text{N/mm}^2<11.25\ \text{N/mm}^2$$

3. Thickness (cl. 7.4.3.1)

Projections $a=55$ mm and $b=55$ mm (larger and smaller):

$$t_s=\sqrt{\frac{2.5w(a^2-0.3b^2)\gamma_{m0}}{f_y}}=\sqrt{\frac{2.5\times11.09\times(55^2-0.3\times55^2)\times1.10}{250}}=16.07\ \text{mm}$$

It must also be not less than the flange thickness $t_f=14.2$ mm. Provide $t_s=18$ mm.

4. Connections

• The column end is machined to bear on the plate; connect with a nominal 6 mm fillet weld all round (load is transferred mainly by contact).
• Anchor bolts: 4 nos. M20, embedded about 450 mm with a washer plate (for erection and to resist any uplift).

Answer: base plate 460 × 250 × 18 mm, 4 anchor bolts M20, 6 mm weld, pedestal of M25 concrete.