2×PFC 180×75×20 Back to Back - Section Properties

2×PFC 180×75×20 Back to Back has a mass of 40.7 kg/m, tabulated to BS EN 1993-1-1:2005 / BS 4-1:2005. A 12 m length therefore weighs about 488 kg, which is the figure to use for a steel take-off or when checking a lift.

The plastic section modulus about the major axis, Wpl,y, is 352 cm³, and the elastic modulus Wel,y is 304 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 125 kNm before any stability check.

The second moment of area about the major axis, Iy, is 2,740 cm⁴, and about the minor axis Iz is 2,470 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 7.27 cm about the major axis and iz is 3.68 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 3.68 cm a 0.4 m effective length already gives lambda = 100. Minor-axis buckling almost always controls unless the weak axis is restrained.