What is it?
A steel beam, often called an RSJ, is a rolled steel section that spans across an opening in a loadbearing wall and carries everything above it. In houses it's usually a universal beam (UB): an I-shaped section with two flanges joined by a web.
Where the opening goes through a cavity wall, the beam has to carry both leaves. That's done with two beams, one under each leaf, or with one beam that has a steel plate welded under it, wide enough for the other leaf to sit on.
What does it do?
The beam picks up the load of the masonry above the opening, along with any floor joists, beams or roof bearing on that wall, and carries it sideways to its two ends. There it sits on padstones, which spread the load into the walls either side.
It replaces the wall that was taken out, so it has to be stiff as well as strong: if it bends too much under load, the brickwork above it cracks.
Where is it used?
Over any opening wider than a standard lintel can span or carry: knock-throughs into extensions, wide patio and bifold door openings, and internal loadbearing walls taken out to open up rooms.
Types
- Universal beam (UB): the usual choice; deeper than it is wide, efficient at spanning. A 203 x 133 x 25 UB is 203 deep, 133 wide and weighs 25 kg per metre.
- Universal column (UC): squarer and heavier, used where the beam has to be shallow.
- Parallel flange channel (PFC): a C-shaped section, often used in pairs or to trim round openings in floors.
- UB with a welded plate: a plate wider than the flange welded under the beam, so one beam carries both leaves of a cavity wall.
- Box sections and flitch beams: for special cases, set by the engineer.
How to specify
The section, its grade (usually S355), the length and the bearings come from a structural engineer's calculations, which building control needs to see. The engineer works out the load from the triangle of masonry above the opening plus anything bearing on the wall, and checks the beam for strength and deflection.
The beam usually needs at least 150 bearing at each end onto solid masonry or a padstone. It arrives cut to length, with any plate welded on and primed.
Other considerations
- Temporary works: the wall above has to be held up (needled and propped) while the opening is cut and the beam goes in.
- Packing: the gap between the top of the beam and the masonry above is packed solid with dry pack mortar, so the wall bears on the beam evenly.
- Fire: in a house, a steel beam is normally protected to give 30 minutes' fire resistance, usually by boxing it in with fire-rated plasterboard.
- Inspection: building control will want to see the beam, its bearings and padstones before they're covered up.
Interacting elements
- Padstone: spreads the load from each end of the beam into the wall
- Brick / Blockwork: the leaves above the opening bear on the beam
- Fire protection: the boarding round the beam that gives it its fire resistance