W8x31 — Dimensions, Weight & Section Properties
American wide-flange (W) beam per ASTM A6 / AISC Steel Construction Manual
W8x31 is an American wide-flange beam 8 inches (203.2 mm) deep with a 8 in (203.2 mm) flange, weighing 31 pounds per foot (46.1 kg/m) — that is what the "31" in the designation means. All values below are the standard AISC published section properties used in US structural design, with metric equivalents alongside.
Typical uses: floor and roof framing, columns, headers, and general steel construction. For a quick stiffness check, see the deflection example below or open the beam calculator with your own span and load.
Dimensions
| Depth d | 8 in (203.2 mm) |
| Flange width bf | 8 in (203.2 mm) |
| Web thickness tw | 0.285 in (7.2 mm) |
| Flange thickness tf | 0.435 in (11 mm) |
Weight & Cross-Section
| Weight per foot | 31 lb/ft (46.1 kg/m) |
| 20 ft bar | 620 lb |
| 40 ft bar | 1,240 lb |
| Area A | 9.13 in² (58.9 cm²) |
Section Properties (strong / weak axis)
| Moment of inertia Ix | 110 in⁴ (4,579 cm⁴) |
| Section modulus Sx | 27.5 in³ (451 cm³) |
| Radius of gyration rx | 3.47 in |
| Moment of inertia Iy | 37.1 in⁴ (1,544 cm⁴) |
| Section modulus Sy | 9.27 in³ (152 cm³) |
| Radius of gyration ry | 2.02 in |
Compared with the neighbouring sizes
| Shape | d × bf (in) | lb/ft | Ix (in⁴) | Sx (in³) | Δ weight | Δ Ix |
|---|---|---|---|---|---|---|
| W8x28 | 8.06 × 6.54 | 28 | 98 | 24.3 | −10 % | −11 % |
| W8x31 (this page) | 8 × 8 | 31 | 110 | 27.5 | — | — |
| W8x35 | 8.12 × 8.02 | 35 | 127 | 31.2 | +13 % | +15 % |
Deflection example
A simply supported W8x31 spanning 13 ft with a 10 kip point load at midspan deflects 0.25 in (E = 29,000 ksi, Ix = 110 in⁴). The L/360 serviceability limit for this span is 0.43 in — the shape passes this quick check.
Calculate with your own span and load →
Frequently asked questions
How much does a W8x31 beam weigh?
W8x31 weighs 31 pounds per foot (46.1 kg/m). The number after the "x" in the designation is always the weight in lb/ft. A 20 ft bar weighs 620 lb and a 40 ft bar 1,240 lb.
What are the dimensions of a W8x31?
W8x31 is 8 in (203.2 mm) deep with a 8 in (203.2 mm) wide flange. The web is 0.29 in thick and the flanges are 0.44 in thick. Note the actual depth differs from the nominal W8 label — always design with the actual value.
What are the dimensions of a W8x31 in mm?
W8x31 is 203.2 mm deep with a 203.2 mm wide flange; the web is 7.2 mm and the flange 11 mm thick. In metric section properties: Ix = 4,579 cm⁴, Sx = 451 cm³.
How much does a W8x31 weigh in kg?
46.1 kg per meter. A 6 m bar weighs 277 kg and a 12 m bar 554 kg — handy when ordering US shapes from metric suppliers.
What is the moment of inertia of a W8x31?
About the strong axis, Ix = 110 in⁴ (4,579 cm⁴) with section modulus Sx = 27.5 in³. About the weak axis, Iy = 37.1 in⁴ and Sy = 9.27 in³. Use the beam deflection calculator to turn these into deflection and stress for your span and load.
W8x31 or W8x35 — what does one size up change?
W8x35 is about 13% heavier (35 vs 31 lb/ft) but about 15% stiffer (Ix 127 vs 110 in⁴). Going one step up within the same depth group is usually the cheapest way to cut deflection.
Values are entered from the governing standard’s published tables and cross-checked programmatically against its mass and section formulas before publication. Spotted an error? Write to advertising@metalabout.net — verified corrections ship within days. About