FB 4.5×25 - Section Properties

FB 4.5×25 has a mass of 0.88 kg/m, tabulated to EN 10058. A 12 m length therefore weighs about 11 kg, which is the figure to use for a steel take-off or when checking a lift.

FB 4.5×25 has a depth of 25 mm, a width of 4.5 mm, a wall thickness of 4.5 mm, with a cross-sectional area of 1.13 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 0.703 cm³, and the elastic modulus Wel,y is 0.469 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 0 kNm before any stability check.

The second moment of area about the major axis, Iy, is 0.586 cm⁴, and about the minor axis Iz is 0.019 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 0.72 cm about the major axis and iz is 0.13 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 0.13 cm a 0.0 m effective length already gives lambda = 100. Minor-axis buckling almost always controls unless the weak axis is restrained.