An I-beam is a structural steel beam whose cross-section looks like a capital letter I: two horizontal plates, called flanges, joined by a vertical plate called the web.
Parts of an I-Beam
- Flanges: the top and bottom plates. Their width is bf and thickness tf.
- Web: the vertical plate between the flanges, with thickness tw.
- Depth: the overall height, d.
- Fillets: the rounded transitions where the web meets the flanges, measured by the k distance.
Why the I Shape Works
Most of the steel sits in the flanges, far from the center of the section, which gives a high moment of inertia for the amount of steel used. Depth matters most: a W16x26 weighs 26 lb/ft with Ix = 301 in⁴, while a heavier W8x31 weighs 31 lb/ft with Ix = 110 in⁴.
Types of I-Beams
| Type | Description | Shapes in AISC database |
|---|---|---|
| W (wide flange) | Parallel flanges; the most common beam today | 289 |
| S (American Standard) | Narrow, sloped flanges | 28 |
| M (miscellaneous) | Light shapes that are not W or S | 16 |
| HP (bearing pile) | H-shaped with nearly equal flange and web thickness | 22 |
Common Uses
I-beams are used as floor and roof beams, girders, lintels and, in wider H-shaped sizes, as columns. The size for any load-carrying use must be chosen by a licensed structural engineer.
Next: how to read steel beam sizes and H-beam vs I-beam.
Source: AISC Shapes Database v16.0, consistent with the AISC Steel Construction Manual, 16th Edition. Last updated: September 2026. Reference information only, not engineering advice. Disclaimer.