ST 6X20.4 - Section Properties

ST 6X20.4 has a mass of 20.4 lb/ft (30.4 kg/m), tabulated to AISC Shapes Database v16.0. A 12 m length therefore weighs about 365 kg, which is the figure to use for a steel take-off or when checking a lift.

ST 6X20.4 has a depth of 6 in (152.4 mm), a width of 5.25 in (133.4 mm), a web thickness of 0.462 in (11.7 mm), a flange thickness of 0.659 in (16.7 mm), with a cross-sectional area of 5.96 in² (38.5 cm²). These are the nominal dimensions from AISC Shapes Database v16.0; rolling tolerances apply to the delivered section.

The plastic section modulus about the major axis, Wpl,y, is 126.34 cm³, and the elastic modulus Wel,y is 69.973 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 45 kNm before any stability check.

The second moment of area about the major axis, Iy, is 786.7 cm⁴, and about the minor axis Iz is 280.5 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 4.52 cm about the major axis and iz is 2.69 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 2.69 cm a 0.3 m effective length already gives lambda = 100. Minor-axis buckling almost always controls unless the weak axis is restrained.

The St Venant torsion constant It is 35.05 cm⁴. Closed sections carry torsion in a shear flow around the perimeter, which makes them far stiffer in torsion than an open section of the same weight and is usually why one is specified.