RSJ

An RSJ is a structural steel beam made by hot-rolling steel into an I-shaped or H-shaped cross-section. The term is primarily British; in North America, the same thing is called a W-beam (wide-flange), I-beam, or H-beam depending on proportions. The I-section concentrates material in the top and bottom flanges, with a thin web connecting them, which maximises resistance to bending while minimising weight. RSJs are manufactured to British Standard BS 4-1 or its modern equivalent BS EN 10025.

The most common RSJ sections in building work are the Universal Beam ranges: 127×76 mm, 152×89 mm, 203×102 mm, 254×102 mm, and larger. Depths typically range from 100 mm to 600 mm. Each section has a published moment of inertia and section modulus, which engineers use to calculate whether a given beam will safely carry a required load over a given span. An RSJ spanning 6 metres between supports will deflect less under load than a shallower beam of the same steel grade, so depth is selected based on span and imposed load.

RSJs are used as primary load-bearing members in multi-storey steel-frame buildings, as lintels over openings in brickwork, as cantilever supports for balconies and canopies, and to support floor joists or precast concrete units. They are bolted or welded to columns, other beams, and steel plates. Connection design is critical: a poorly detailed connection can concentrate stress and cause failure even if the beam itself is adequate. Corrosion is a common failure mode where RSJs are exposed to weather or damp environments; protection requires paint systems, galvanising, or stainless steel for aggressive locations.

Why RSJ, not just I-beam?

The term rolled steel joist emerged in the 19th century when these sections were first produced by passing steel ingots through a series of shaped roller dies. The rolling process (as opposed to riveting plates together) was a manufacturing innovation. Modern manufacture is still hot-rolling, though tolerances and quality control are tighter. The word joist

Architects and structural engineers must specify not just the section size but also the steel grade (typically S275 or S355 in the UK, equivalent to Grade 250 or Grade 350), length, mill tolerances, and any special requirements such as cambering, end machining, or paint finish. An RSJ ordered without these details will not be acceptable on site. Bearing stiffeners are often welded to the web at support points to prevent web crippling, particularly where heavy point loads are applied or bearing width is limited.