Connection details: wall, filigree slab, ring beam, balcony

Precast elements connect to one another at four types of joint: wall to foundation or to the slab below, wall to wall at the vertical joint, filigree slab to wall through the ring beam, and balcony and stairs to the structure through a thermal break connector or a corbel. Each of these joints follows a fixed pattern: 20 mm bearing, connecting reinforcement protruding from the element, and a joint or structural topping cast on site. Below we describe them in erection order, together with who is responsible for which stage.

Seating the wall

The wall stands on the foundation or on the slab of the storey below, on a levelling grout bed. Starter bars protrude from the substrate, entering holes or ducts in the bottom edge of the element, and are grouted in. Until the connections are cast, temporary props anchored into the floor hold the element — which is why there must be no underfloor heating or other services beneath the screed at the positions where the props are to stand, as they would clash with the drilling. The bearing surface must be clean and level; the levels are checked before the first element, not after it. The seating drawing is on the solid walls page, in the Details section.

Wall to wall: the vertical joint

Wall elements meet at their side edges, where we leave a recess for the joint. The vertical connection is reinforced with bars up to Φ12 mm in grade B500B, and the joint between the walls is filled with C30/37 concrete and floated flush with the face. That is the standard scope of erection under our General Erection Conditions, unless the contract states otherwise. Ready for finishing, the joint is prepared with an anti-crack system: in expanded clay concrete, a 100 mm strip of glass fleece over the joint, and wallpaper or fleece over the whole wall, so that residual shrinkage does not show as a crack on the finished surface. More on this in the text on expanded clay concrete.

Filigree slab on the wall: bearing and ring beam

The filigree plank bears 20 mm onto the wall. Support reinforcement protrudes from the plank, ring beam bars from the wall, and between them the ring beam reinforcement is placed with U-bars in accordance with the reinforcement drawing. Once the planks are laid, the reinforcement of the joints between planks, the additional bottom reinforcement, the top reinforcement, the downstand beams and the trimming reinforcement around openings all follow, and only then the structural topping, which closes the ring beam and the slab into a single diaphragm. The planks are lifted by hooks engaged at the lattice girder nodes, where the top chord meets a diagonal; a hook on the top chord itself, between nodes, can tear the reinforcement out. The spacing of the temporary props and how long they stay in place are described in the text on installing a filigree slab, and the ring beam drawing is on the filigree slabs page.

Two-layer and sandwich walls: where the ring beam sits

In two-layer and sandwich walls, the slab bearing and the ring beam sit in the structural layer, and the insulation runs continuously on the outer side of the ring beam, so no thermal bridge forms at slab level. In a sandwich wall the facing layer is connected to the structural layer by stainless steel or composite connectors and carries no load from the slab; its edges pass by the ring beam. The slab bearing drawings for both types are on the two-layer walls and sandwich walls pages.

Balcony: the thermal break connector

The balcony slab bears 20 mm onto the wall, but the forces are carried by an Isokorb-type thermal break connector cast into the slab: its bars enter the ring beam of the floor, and the insulating body interrupts the thermal bridge. The support reinforcement in the slab is matched to the type of connector, not the other way round, which is why the connector type is settled in the shop drawings together with the cantilever and the slab thickness. The fixing drawing is on the balconies page.

Stairs: bearing on a corbel

The stair flight bears on a corbel formed in the slab or in the landing, in accordance with the bearing drawing. The corbel and the bearing are dimensioned in the shop drawings, and the flight arrives with its bearing edges finished, so all that remains on site is setting it in position and grouting. The drawing is on the stairs page.

Who does what

Within the standard scope of erection, Formee carries out the additional reinforcement of vertical wall connections and the erection joints between walls, and supplies and installs the props. In-situ work outside the precast scope — casting in-situ structural members, wet-cast columns and beams, in-situ walls, cores and ring beams — is the contractor's responsibility, unless the contract covers it separately. The structural topping of the filigree slab, the ring beam reinforcement and the reinforcement of joints between planks are carried out in accordance with the documentation and the scope agreed in the contract. This division is set out in the General Erection Conditions, available for download on the quotation page.

What is settled before production

All the details described here are resolved at the shop drawing stage in Tekla Structures: bearing lengths, protruding bars, joint recesses, cast-in items and connectors. An element produced with the wrong detail cannot be corrected on site, which is why approval of the shop drawings is the only moment at which the connections can still be changed. Plans and sections are enough for a quotation; the details are produced after the order, together with the element geometry.