StructuralEngineer.ie's short answer on RSJ vs lintel: "RSJ" (rolled steel joist) is the trade name for a steel beam. Today it's usually a universal beam (UB) or column (UC) in S275 or S355 steel. A lintel is a shorter, lighter member over a door or window, typically precast concrete or a proprietary steel lintel. Use a lintel for a normal opening with modest load, and a designed steel beam where a wall carries a floor across a wide opening.
The terms
- RSJ. Originally a specific older section shape, now used loosely for any rolled steel beam. Builders and homeowners say RSJ, and engineers specify UB or UC sizes.
- Universal beam (UB). The I-section steel most used for domestic beams. It’s deeper than it is wide, so it’s efficient in bending.
- Universal column (UC). Squarer section, used for posts and sometimes for shallow beams where headroom is tight.
- Precast concrete lintel. Standard sizes for openings in blockwork. Cheap and simple for short spans.
- Proprietary steel lintel. Galvanised, often insulated, for window and door heads in cavity walls, used from the manufacturer’s load tables.
When each is used
| Situation | Usual choice |
|---|---|
| New window or door in a wall with little load above | Precast or proprietary lintel |
| Knock-through of a load-bearing internal wall | Designed UB on padstones |
| Wide rear-wall opening for sliding doors | UB or a goalpost frame (beam and posts) |
| Tight headroom | UC, a deeper flitch, or a beam set into the floor zone |
| Very heavy loads or long spans | Larger UB, posts down to new foundations |
What the engineer decides
The engineer works out what sits on the opening (floors, walls, roof and chimneys), then picks the lightest member that’s strong enough and stiff enough, checks the bearings and padstones, and specifies fire protection. Deflection often governs, because a beam that’s strong enough but too flexible cracks the ceiling or jams sliding doors.
Bottom line
Lintels are for ordinary openings with modest loads. A load-bearing wall coming out needs a designed steel beam, whatever you call it. For beam calcs with a fixed fee, call 01 270 9764.
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Frequently asked questions
Is a UB stronger than an RSJ?
It isn't a question of strength by name. A universal beam is simply the modern rolled section most people now call an RSJ. The engineer chooses the size and grade from the calculated loads.
Can I use a catalogue steel lintel for a knock-through?
Proprietary steel lintels are designed for window and door heads in cavity walls with modest loads. A wall carrying a floor across a wide opening usually needs a designed beam.
What is a padstone?
A concrete or engineering-brick block under each end of a beam that spreads its concentrated load into the wall below. Its size comes from the bearing check in the calcs.
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