FB 9×300 - Section Properties

FB 9×300 has a mass of 21.2 kg/m, tabulated to EN 10058. A 12 m length therefore weighs about 254 kg, which is the figure to use for a steel take-off or when checking a lift.

FB 9×300 has a depth of 300 mm, a width of 9 mm, a wall thickness of 9 mm, with a cross-sectional area of 27 cm². These are the nominal dimensions from EN 10058; rolling tolerances apply to the delivered section.

The plastic section modulus about the major axis, Wpl,y, is 202.5 cm³, and the elastic modulus Wel,y is 135 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 72 kNm before any stability check.

The second moment of area about the major axis, Iy, is 2,025 cm⁴, and about the minor axis Iz is 1.823 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 8.66 cm about the major axis and iz is 0.26 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 0.26 cm a 0.0 m effective length already gives lambda = 100. Minor-axis buckling almost always controls unless the weak axis is restrained.