FB 25×75 - Section Properties

FB 25×75 has a mass of 14.72 kg/m, tabulated to EN 10058. A 12 m length therefore weighs about 177 kg, which is the figure to use for a steel take-off or when checking a lift.

FB 25×75 has a depth of 75 mm, a width of 25 mm, a wall thickness of 25 mm, with a cross-sectional area of 18.75 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 35.16 cm³, and the elastic modulus Wel,y is 23.44 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 12 kNm before any stability check.

The second moment of area about the major axis, Iy, is 87.89 cm⁴, and about the minor axis Iz is 9.766 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 2.17 cm about the major axis and iz is 0.722 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 0.722 cm a 0.1 m effective length already gives lambda = 100. Minor-axis buckling almost always controls unless the weak axis is restrained.