SHS 400×400×20 - Section Properties

SHS 400×400×20 has a mass of 235 kg/m, tabulated to BS EN 10210-2:2006. A 12 m length therefore weighs about 2820 kg, which is the figure to use for a steel take-off or when checking a lift.

SHS 400×400×20 has a depth of 400 mm, a width of 400 mm, a wall thickness of 20 mm, with a cross-sectional area of 300 cm². These are the nominal dimensions from BS EN 10210-2:2006; rolling tolerances apply to the delivered section.

The plastic section modulus about the major axis, Wpl,y, is 4,250 cm³, and the elastic modulus Wel,y is 3,580 cm³. Wpl,y is what sets the moment capacity of a Class 1 or 2 section: M_pl,Rd = Wpl,y x f_y / gamma_M0, so at S355 with gamma_M0 = 1.0 this section reaches about 1509 kNm before any stability check.

The second moment of area about the major axis, Iy, is 71,500 cm⁴, and about the minor axis Iz is 71,500 cm⁴. Iy is the value deflection depends on - for a simply supported beam under a uniform load, the midspan deflection is 5wL⁴/384EIy - so on spans where a deflection limit governs rather than strength, this is the number that decides the section.

iy is 15.4 cm about the major axis and iz is 15.4 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 15.4 cm a 1.5 m effective length already gives lambda = 100. Minor-axis buckling almost always controls unless the weak axis is restrained.

The St Venant torsion constant It is 112,000 cm⁴. Closed sections carry torsion in a shear flow around the perimeter, which makes them far stiffer in torsion than an open section of the same weight and is usually why one is specified.