RHS 80×40×3.6 - Section Properties

RHS 80×40×3.6 has a mass of 6.27 kg/m, tabulated to BS EN 10210-2:2006. A 12 m length therefore weighs about 75 kg, which is the figure to use for a steel take-off or when checking a lift.

RHS 80×40×3.6 has a depth of 80 mm, a width of 40 mm, a wall thickness of 3.6 mm, with a cross-sectional area of 7.98 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 20 cm³, and the elastic modulus Wel,y is 15.7 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 7 kNm before any stability check.

The second moment of area about the major axis, Iy, is 62.8 cm⁴, and about the minor axis Iz is 20.6 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.

Its local-buckling ratios are cf/tf = 8.11 for the flange and cw/tw = 19.2 for the web. EN 1993-1-1 Table 5.2 compares these against limits scaled by epsilon = sqrt(235/f_y), so the class depends on the steel grade as well as the geometry - the same section can be Class 1 in S275 and Class 3 in S355. Use the Capacity tab on this page for the classification at a specific grade.

iy is 2.81 cm about the major axis and iz is 1.61 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 1.61 cm a 0.2 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 50.8 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.