SQ 55 - Section Properties

SQ 55 has a mass of 23.75 kg/m, tabulated to EN 10059. A 12 m length therefore weighs about 285 kg, which is the figure to use for a steel take-off or when checking a lift.

SQ 55 has a depth of 55 mm, a width of 55 mm, with a cross-sectional area of 30.3 cm². These are the nominal dimensions from EN 10059; rolling tolerances apply to the delivered section.

The plastic section modulus about the major axis, Wpl,y, is 41.59 cm³, and the elastic modulus Wel,y is 27.73 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 15 kNm before any stability check.

The second moment of area about the major axis, Iy, is 76.26 cm⁴, and about the minor axis Iz is 76.26 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 1.59 cm about the major axis and iz is 1.59 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 1.59 cm a 0.2 m effective length already gives lambda = 100. Minor-axis buckling almost always controls unless the weak axis is restrained.