UB 686×254×170 - Section Properties

UB 686×254×170 has a mass of 170.2 kg/m, tabulated to BS EN 10365:2017. A 12 m length therefore weighs about 2042 kg, which is the figure to use for a steel take-off or when checking a lift.

UB 686×254×170 has a depth of 692.9 mm, a width of 255.8 mm, a web thickness of 14.5 mm, a flange thickness of 23.7 mm, with a cross-sectional area of 217 cm². These are the nominal dimensions from BS EN 10365:2017; rolling tolerances apply to the delivered section.

The plastic section modulus about the major axis, Wpl,y, is 5,630 cm³, and the elastic modulus Wel,y is 4,920 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 1999 kNm before any stability check.

The second moment of area about the major axis, Iy, is 170,000 cm⁴, and about the minor axis Iz is 6,630 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.

Its local-buckling ratios are cf/tf = 4.45 for the flange and cw/tw = 42.4 for the web. EN 1993-1-1 Table 5.2 compares these against limits scaled by epsilon = sqrt(235/f_y), so the class depends on the steel grade as well as the geometry - the same section can be Class 1 in S275 and Class 3 in S355. Use the Capacity tab on this page for the classification at a specific grade.

iy is 28 cm about the major axis and iz is 5.53 cm about the minor axis. Column buckling is governed by the SMALLER value: the slenderness is lambda = L_cr / i, so with iz = 5.53 cm a 0.6 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 308 cm⁴, and the warping constant Iw is 7.42 dm⁶. Open sections carry torsion mainly by warping rather than uniform torsion, which is why Iw appears in the lateral-torsional buckling resistance - both values feed M_b,Rd.