UB on a drawing means Universal Beam — the hot-rolled steel I-section used for beams in Australia, New Zealand and the UK, written like 310UB40.4.
UB stands for Universal Beam: a hot-rolled steel I-section that is deeper than it is wide, with relatively narrow flanges. That shape is efficient in bending, which is why the UB is the standard floor and roof beam in Australian, New Zealand and UK steelwork.
In AU and NZ the product comes from AS/NZS 3679.1, the standard for hot-rolled structural steel bars and sections, and is designed to AS 4100. In the UK the section dimensions and masses now come from BS EN 10365, with material to BS EN 10025. The older BS 4-1 has been withdrawn, though the 305 x 165 x 40 style of designation it established is still what everyone writes.
How UB appears on a drawing
The AU/NZ designation is nominal depth in millimetres, then UB, then mass per metre in kg/m. A beam mark on a framing plan:
B3: 310UB40.4 GR300, 2/M20 8.8/S EA END - SEE 4/S21
Read: beam mark B3 is a Universal Beam of 310 mm nominal depth weighing 40.4 kg per metre, grade 300 steel, with two M20 grade 8.8 bolts in snug-tightened bearing type at each end, and the connection is detail 4 on sheet S21.
Here is the part that catches people: the first number is nominal, not actual. A 310UB40.4 measures 304 mm deep and 165 mm wide. Every member of the 310UB family shares the label but has its own real depth. If you are setting a soffit level, checking headroom or chasing a clash in a model, take the depth off the section tables, not off the label.
You will also see 310 UB 40 with spaces and the decimal dropped, or a grade appended as 310UB40.4/300. Mill markings commonly read G300 or 300PLUS.
Terms it gets confused with
- UC — Universal Column. Roughly as wide as it is deep, with thicker flanges. A 250UC is not a substitute for a 250UB or the reverse. The trap is that the depths are close — a 250UB37.3 is 256 mm deep and a 250UC72.9 is 254 mm — so they can look interchangeable on a section. The width is where they part company, roughly 146 mm against 254 mm, along with flange thickness, mass and capacity.
- WB / WC — welded beam and welded column, the Australian three-plate sections for deeper spans, e.g. 1200WB455. Not rolled, not in the UB tables.
- PFC — parallel flange channel. Half an I-section in profile, entirely different behaviour.
- TFB / TFC — taper flange beam and channel, the older tapered-flange sections.
- RSJ — a UK legacy term you will still hear on site for any steel beam. Strictly it is the older tapered-flange rolled steel joist, not a modern parallel-flange UB.
- U/S — underside. A different abbreviation entirely, and easy to misread in a hand-lettered note.
AU/NZ versus UK versus US
- AU and NZ — two numbers: depth then mass per metre.
360UB50.7 - UK — three numbers: depth x width x mass per metre, then UB.
305 x 165 x 40 UB. The nearest UK section to a 310UB40.4 is that 305x165x40 at 303.4 mm deep. Close, but not the same section — do not swap them inside a fabrication package. - US — there is no UB. The wide-flange W-shape does the job, named as nominal depth in inches by weight in lb/ft:
W12x26, typically ASTM A992 and sized from the AISC tables. A US detailer will usually work out what you mean by UB, but the section will not be in their database. - Grades — 300 in AU/NZ, S275 or S355 in the UK, A992 (50 ksi) in the US.
A quick tell: if the callout has three numbers you are on a UK-convention set; two numbers means AU/NZ.
Where to check on your own set
- The beam schedule on the structural sheets. The mark is the authority; the plan callout is a label.
- Structural general notes, for steel grade and the section standard being used.
- Published section property tables — your supplier’s handbook or the AS/NZS 3679.1 tables — for actual depth, flange width and root radius.
- The fabricator’s shop drawings, if you are checking what was actually built.
UB, UC, PFC and the rest of the section shorthand are collected in the A-Z of engineering drawing abbreviations.
Other meanings of UB
Outside drafting, UB is the University at Buffalo, the band UB40, the UB-04 US medical claim form, and “upper bound” in mathematics. On US structural drawings UB is not a section designation at all — W is what you want there.