How steel I-beams are made

An I-beam puts steel where bending stress is highest and leaves out the middle, where it would do almost nothing.

stations: 6
things upstream: 25
raw sources: 12
steps from the ground: 5

The short version

Most I-beams are rolled in one piece. A hot steel bloom is run back and forth through a structural mill, where grooved rolls squeeze it step by step into a shape with a vertical web between two flanges. Very large sections are instead fabricated by welding steel plates together into a plate girder.

Cast a bloom

Molten steel is cast into a bloom, a thick block that is the usual starting point for large structural sections. Steel for beams must be consistent all the way through, since a beam carries load in its flanges.

Goes in
Steel
Soak it to rolling heat

The bloom is held in a furnace until it is hot all the way through. If it is rolled while cold in the middle, it can crack and tear.
Break it down in the blooming mill

Heavy reversing rolls squash the bloom, pass after pass, rolling forward and back. The cross-section shrinks and the piece gets longer, ready for the shaping mill.
Form the web and flanges

In the structural mill, grooved rolls squeeze the steel into a shape like the letter I, a thin upright web joined at top and bottom to wide flanges. Each pass cuts a slightly more finished profile.
Cut to length

The beam is trimmed to the length ordered. Because a beam is rolled hot, uneven cooling can leave stresses in shapes whose thickness varies, so it is left to cool carefully.
Weld up the giants

Where a span needs a section bigger than any mill can roll, plates are welded, and sometimes bolted or riveted, into a plate girder. It has the same I-shape, assembled from flat steel.

Goes in
Steel

End of the lineSteel I-beam Next it goes into skyscraper, locomotive.

Worth repeating

The first rolled I-beam, made from one piece of wrought iron, was patented in 1849 by Alphonse Halbou of Forges de la Providence in Belgium.
In a beam, the web resists shear forces and the flanges resist most of the bending.