FB 8×65 - Section Properties

FB 8×65 has a mass of 4.08 kg/m, tabulated to EN 10058. A 12 m length therefore weighs about 49 kg, which is the figure to use for a steel take-off or when checking a lift.

FB 8×65 has a depth of 65 mm, a width of 8 mm, a wall thickness of 8 mm, with a cross-sectional area of 5.2 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 8.45 cm³, and the elastic modulus Wel,y is 5.633 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 3 kNm before any stability check.

The second moment of area about the major axis, Iy, is 18.31 cm⁴, and about the minor axis Iz is 0.277 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 1.88 cm about the major axis and iz is 0.231 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 0.231 cm a 0.0 m effective length already gives lambda = 100. Minor-axis buckling almost always controls unless the weak axis is restrained.