Installation of Steel Support Structure (by Welding) in a Cut Opening

A steel support structure is never sized by eye. The structural engineer sets the size of the sections, the anchoring method and whether a lintel is enough or a full frame is needed. This is done in a structural calculation or design. The engineer looks at the type of wall (reinforced concrete panel, solid brick, aerated concrete), the load coming from the floors above and where the opening sits in relation to panel edges and other openings.

If you live in a flat, contact the building management or the owners' association before any work starts. Cutting into a load-bearing wall affects the whole building, so they will usually want to see the engineer's documents. Building rules differ from country to country and sometimes from one municipality to the next. Check with your local building authority whether your alteration needs a notification, a permit or another form of approval. Do not assume that a simple notice will be enough.

Welding means open flame, heat and sparks. A qualified welder should do it. Indoors, fire safety needs real attention: remove anything combustible, keep a fire extinguisher close by and keep checking the area for a while after the work has finished. Some buildings and insurers also require a written hot work permit.

The exact sequence comes from the structural engineer. With concrete panels the steel is often fitted after the opening has been cut. In other walls the lintel goes in first and the wall below it is removed afterwards. A typical sequence looks like this:

1. Mark out the opening and the position of the steel sections according to the design, and check for pipes and cables in the wall.
2. Prop the ceiling or the wall above the future opening with adjustable props if the engineer specifies it.
3. Cut the opening with a diamond wall saw or wire saw, and finish the corners with core drilling so that cracks do not start from overcut corners.
4. Clean the edges of the opening, removing dust and loose pieces of concrete.
5. Fit the steel sections on one or both sides of the wall and align them level and flush.
6. Drill holes for the anchors or through bolts and fix the sections as designed.
7. Weld the sections together at the corners to form a frame and remove the slag from the welds.
8. Fill the gaps between the steel and the wall with non-shrink mortar or grout so that the section bears on its whole surface.
9. Remove the props only once the grout has cured and the work has been checked.
10. Apply anti-corrosion paint to the steel, especially the welds, and prepare the surface for plaster or cladding.

Steel sections

Lintels and frames are made from hot-rolled steel sections. Channel sections (for example UPE or UPN) are very common in concrete panel walls, because they can be placed on both sides of the wall and clamped together with bolts. I and H sections (such as IPE or HEB) are chosen for wider spans and heavier loads, where a stiffer beam is needed. Angles are used more for small openings or as secondary parts. The type and size of section come from the calculation, not from whatever happens to be in stock.

Anchoring

The sections are fixed to the wall with chemical anchors, mechanical anchors or threaded rods passing through the wall and tightened on both sides. Chemical anchors hold well in concrete and solid masonry, but they need properly cleaned drill holes and the curing time given by the manufacturer. In hollow bricks, mesh sleeves are used, otherwise the resin simply runs into the cavities.

Welds and corrosion protection

Welding turns the individual sections into one rigid unit. Where the design calls for continuous welds, they must be continuous, not just tack welds. After welding, the joints are cleaned and painted, because the weld is usually the first place where rust starts.

Two openings that look similar can mean quite different amounts of work. The job takes longer and costs more mainly because of:

• A wider opening and heavier sections that have to be carried in and held in place during fitting
• A thick reinforced concrete wall with dense reinforcement
• Propping needed through several floors, or props that have to stay in place for longer
• Tight space, such as a small flat with no room to manoeuvre or a lift that the sections will not fit into
• Pipes or cables in the wall that have to be rerouted first
• A complicated frame shape, sloping or irregular reveals

On the other hand, the job goes more smoothly with a clear structural design, a cleared and covered room, easy access for materials right up to the work area and agreed hours when noisy work and welding are allowed.

• A lighter or different section than the engineer specified, used just because it was available
• Cutting the opening without propping, or running the saw cuts past the corners, which weakens the panel and causes cracks
• A section that does not bear on the wall over its full surface because the gap was never grouted
• Anchors set in uncleaned holes or loaded before the resin has cured
• Tack welds instead of continuous welds, or frame corners that are not fully welded
• Welding without protecting the surroundings, leading to scorched floors and smoke in neighbouring flats
• Missing anti-corrosion paint, especially on welds and on parts that will later disappear under plaster
• Working in an unheated space regardless of temperature, so that mortar and resin do not set properly

Go through these points at handover, ideally before the steel is covered with plaster:

• The sections match the structural design in type, size and position
• The lintel or frame is level and flush with the wall
• Anchors or bolts are in the specified number and properly tightened
• Welds are continuous, free of cracks and cleaned of slag
• The gap between steel and wall is filled and the section does not stand off anywhere
• All steel, including the welds, has anti-corrosion paint
• There are no new cracks in the wall above or around the opening
• Props were removed only after the grout had cured and with the approval of the responsible person
• The area was checked after welding, with no smouldering residue or burnt spots
• You have the structural design or engineer's report and a record of the work carried out