HSS 20X20X3-8 - Section Properties

HSS 20X20X3-8 has a mass of 98.94 lb/ft (147.2 kg/m), tabulated to AISC Shapes Database v16.0. A 12 m length therefore weighs about 1766 kg, which is the figure to use for a steel take-off or when checking a lift.

HSS 20X20X3-8 has a depth of 20 in (508 mm), a width of 20 in (508 mm), a web thickness of 0.349 in (8.9 mm), a wall thickness of 0.349 in (8.9 mm), with a cross-sectional area of 27.1 in² (174.8 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 3,261 cm³, and the elastic modulus Wel,y is 2,851.3 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 1158 kNm before any stability check.

The second moment of area about the major axis, Iy, is 72,424 cm⁴, and about the minor axis Iz is 72,424 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 20.32 cm about the major axis and iz is 20.32 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 20.32 cm a 2.0 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 111,550 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.