CF CHS 88.9×4 - Section Properties

CF CHS 88.9×4 has a mass of 8.38 kg/m, tabulated to BS EN 10219-2:2006. A 12 m length therefore weighs about 101 kg, which is the figure to use for a steel take-off or when checking a lift.

CF CHS 88.9×4 has a depth of 88.9 mm, a width of 88.9 mm, a wall thickness of 4 mm, with a cross-sectional area of 10.7 cm². These are the nominal dimensions from BS EN 10219-2:2006; rolling tolerances apply to the delivered section.

The plastic section modulus about the major axis, Wpl,y, is 28.9 cm³, and the elastic modulus Wel,y is 21.7 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 10 kNm before any stability check.

The second moment of area about the major axis, Iy, is 96.3 cm⁴, and about the minor axis Iz is 96.3 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 3 cm about the major axis and iz is 3 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 3 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 193 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.