SHS 70×70×7.1 - Section Properties

SHS 70×70×7.1 has a mass of 13.6 kg/m, tabulated to BS EN 10210-2:2006. A 12 m length therefore weighs about 163 kg, which is the figure to use for a steel take-off or when checking a lift.

SHS 70×70×7.1 has a depth of 70 mm, a width of 70 mm, a wall thickness of 7.1 mm, with a cross-sectional area of 17.3 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 40.3 cm³, and the elastic modulus Wel,y is 32 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 14 kNm before any stability check.

The second moment of area about the major axis, Iy, is 112 cm⁴, and about the minor axis Iz is 112 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 2.54 cm about the major axis and iz is 2.54 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 2.54 cm a 0.3 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 185 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.