Design a built-up column using single lacings:

Design a built-up column using single lacings to carry an axial load of 1200 kN. The connections are all welded (σ_y = 250 MPa). The effective length of the column is 5 m.

Solution

Select two ISMC-300 channels, back to back as shown in Figure 6.15. The properties of the section are:

Area = 45.64 cm² = 4564 mm²;        r_x = 11.81 cm = 118.1 mm

r_y  = 2.61 cm = 26.1 mm;      c_y = 2.36 cm = 23.6 mm

We know that rx for the double system is same as that for the single channel

∴          r_x = 118.1 mm

And

where r_y′ is the radius of gyration of the single angle; substituting the values

ry = = 116.5 mm which is approximately equal to r_x (r_x  = 118.1).

Since r_y is the smaller radius of gyration.

∴          Slenderness ratio, λ = 5000/116.5 = 42.9.

Corresponding value of σ_ac = 137 MPa.

∴          P _allowable = 2 × 137 × 4564 N = 1250 kN > 1200