An old name for a steel beam, still in daily use
RSJ stands for rolled steel joist. Rolled because it is shaped by being passed hot between profiled rollers rather than welded together, and joist because that was the word for a beam when the name was coined.
It is the phrase almost every British builder uses for a steel going over an opening. It is also, strictly, the name of a section that stopped being the standard choice a long time ago.
The actual difference is the flange
Look at the end of the beam. On a true RSJ the inside faces of the top and bottom flanges slope, so each flange is thicker where it meets the web and thinner at its tip. On a universal beam those faces are parallel, and the flange is the same thickness all the way out.
Parallel flange universal sections superseded tapered ones as the everyday structural steel decades ago, and the word never caught up. Genuine RSJs are still rolled in the smaller sizes, and an older building may well have real ones in it, which is worth knowing if you are trying to match or extend something existing.
The taper has one practical consequence that shows up immediately: a bolt head or a nut will not sit flat on a sloping flange, so fixing to a genuine RSJ needs tapered washers. Anything clamped to the flange has the same problem.
UB, UC, and which one is which
Two families cover most of what gets used. A universal beam is deeper than it is wide, putting steel far from the centre where bending needs it, and it goes in horizontally. A universal column is roughly square in proportion, which resists buckling equally in both directions, and it goes in vertically.
A UC does get used as a beam where there is no headroom for a deeper section. It works and it uses more steel to do the same job, which is a trade worth making knowingly.
Getting one specified
Nothing on this site will tell you what size beam your opening needs, and that is not caution for its own sake. The answer depends on the span, on everything bearing on it including whatever is above, on the deflection limit, on restraint along the length and on the bearing at each end. It is a design output and it belongs to whoever signs it.
What you can do before that conversation is arrive with the right information, which is what the engineer brief tool is for. Once a size is specified, the UK section properties table gives the real dimensions behind a serial size, which are not the same as the designation and catch people setting out. The wide flange beam sizes table is the American equivalent, and metal weight tells you what it weighs, which is the number that decides how it gets into the building.
Common questions
What does RSJ stand for?
Rolled Steel Joist. Rolled describes how it is made, squeezed between shaped rollers while hot rather than welded up from plate, and joist is the old word for a beam in this context. The name comes from a time when it was the standard structural section, and it has stayed in everyday speech long after the section itself stopped being the usual choice.
Is an RSJ the same as a universal beam?
Not technically, and yes in practice for most conversations. A true RSJ has tapered flanges, meaning the inner faces slope, while a universal beam has parallel flanges. Parallel flange sections replaced tapered ones as the standard decades ago, so when a builder or an architect says RSJ they nearly always mean a universal beam. Small RSJs are still rolled, and older buildings are full of genuine ones.
Does the difference actually matter on site?
It matters when you are fixing to it or matching an existing one. A sloping flange face means a bolt head or a nut does not sit square, so tapered washers are needed, and anything being clamped or bolted to the flange has to account for the angle. It also changes the section properties, so a nominally similar RSJ and universal beam are not interchangeable in a calculation. Where it does not matter is ordinary conversation about a steel going over an opening.
What is the difference between a UB and a UC?
Proportions and intended job. A universal beam is deeper than it is wide, which puts material where bending needs it, so it is used horizontally. A universal column is roughly as wide as it is deep, which makes it stable against buckling in both directions, so it is used vertically. A UC does appear as a beam where headroom is tight and a shallow section is needed, and it is a less efficient use of steel for that job.
What is the American equivalent?
Two of them, and the mapping is unusually tidy. The S-shape, formally the American Standard Beam, has sloped inner flange faces and is the direct counterpart of an RSJ. The W-shape, the wide flange, has essentially parallel flanges and is the counterpart of a universal beam. Just as here, the W-shape is what almost everything gets built from and the S-shape is the older survivor. Americans and Canadians say I-beam colloquially, in much the same catch-all way that RSJ is used in Britain.
How do I know what size I need?
You do not work it out from a table, and no page on this site will tell you. The size depends on the span, on everything the beam is carrying including anything above it, on how much it is allowed to deflect, on how it is restrained along its length, and on how it bears at each end. Getting that wrong is not a cosmetic error. What is worth doing before you speak to anybody is gathering the information they will ask for, which turns a vague enquiry into a quick one.