WT 3X12.5 - Section Properties

WT 3X12.5 has a mass of 12.5 lb/ft (18.6 kg/m), tabulated to AISC Shapes Database v16.0. A 12 m length therefore weighs about 223 kg, which is the figure to use for a steel take-off or when checking a lift.

WT 3X12.5 has a depth of 3.19 in (81 mm), a width of 6.08 in (154.4 mm), a web thickness of 0.32 in (8.1 mm), a flange thickness of 0.455 in (11.6 mm), with a cross-sectional area of 3.67 in² (23.7 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 27.53 cm³, and the elastic modulus Wel,y is 14.519 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 10 kNm before any stability check.

The second moment of area about the major axis, Iy, is 95.32 cm⁴, and about the minor axis Iz is 355 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 2 cm about the major axis and iz is 3.86 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 3.86 cm a 0.4 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 9.53 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.