Built-up Sections:

Built-up sections are used as compression members for heavy loads and consist of one or more solid rolled steel section with flanges having cover plates. The cover plates are connected to the flanges by rivets or welds. These connections are designed to resist a shear force, which is 2.5% of the maximum compressive load on the column.

Example

Figure shows an ISHB 350 @ 72.4 kg/m with two flange plates 400 × 10 mm section each. Find out the load carrying capacity of the column if the effective length is 4.5 m (f_y = 250 MPa).

Figure

Solution

Area of the ISHB section = 92.21 cm²

I_xx = 19802.8 cm⁴

I_yy = 2510.5 cm⁴

The minimum radius of gyration will be about the minor y-y axis.

Iyy of compound section = 25,105,000 + 2 ×(10 × 400³)/12

= 131, 771, 667 mm⁴

Area of compound section = 9221 + 2 × 400 × 10 = 17221 mm²

∴  r_min = = 87.5 mm

∴  λ = 4500/87.5 = 51.4

Corresponding allowable σ_ac = 130.6 MPa (from Table 2)

∴ Load carrying capacity = σ_ac . A = 130.6 × 17221 N = 2249 kN.