HSS 18X18X7-8 - Section Properties

HSS 18X18X7-8 has a mass of 197.24 lb/ft (293.5 kg/m), tabulated to AISC Shapes Database v16.0. A 12 m length therefore weighs about 3522 kg, which is the figure to use for a steel take-off or when checking a lift.

HSS 18X18X7-8 has a depth of 18 in (457.2 mm), a width of 18 in (457.2 mm), a web thickness of 0.814 in (20.7 mm), a wall thickness of 0.814 in (20.7 mm), with a cross-sectional area of 54.3 in² (350.3 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 5,669.9 cm³, and the elastic modulus Wel,y is 4,785 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 2013 kNm before any stability check.

The second moment of area about the major axis, Iy, is 109,469 cm⁴, and about the minor axis Iz is 109,469 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 17.68 cm about the major axis and iz is 17.68 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 17.68 cm a 1.8 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 175,650 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.