Lattice-girder precast floor (predalles)

A semi-precast floor in which a thin reinforced-concrete plank, factory-made, acts as permanent formwork and collaborates with the topping cast on site. Steel lattice girders rise out of the plank, stitching the two casts together and ensuring composite action; between the girders, lightening blocks cut the weight. It is laid on simple props, with no formwork, and offers a flat soffit ready to be finished.

The lattice-girder plank, known by the trade name «predalle», is a reinforced-concrete slab a few centimetres thick, cast in the factory with the floor's bottom reinforcement and the projecting steel lattice girders inside it. On site it is laid from beam to beam, propped, the reinforcement is completed and a concrete topping is cast over it: plank and topping become a single collaborating section. It is the synthesis of prefabrication (quality and speed) and in-situ casting (monolithic continuity).

The plank plays two roles in sequence. During the pour it is permanent formwork: it carries the weight of the fresh concrete and the workers, passing it to temporary props. Once cured it becomes a load-bearing part of the floor, housing the bottom tension reinforcement. The soffit, smooth and already flat, can be skimmed and painted with no backing plaster, removing an operation and the risk of cover spalling typical of clay-block floors.

The heart of the system is the welded lattice girder: a top chord, one or two bottom chords embedded in the plank and the diagonal bars linking them. Projecting from the plank into the topping, the girder transfers the shear at the interface between the two casts and stops them sliding over each other: this is what makes the section «composite». The surface of the plank is deliberately left rough, to improve the bond of the construction joint.

Between the girders, lightening blocks - polystyrene, clay or void formers - are placed, cutting the self-weight without reducing the structural depth. By varying the depth and the reinforcement, the system spans in the order of 5-8 metres (more with prestressed planks). The critical point is the temporary stage: until the topping has set, the plank is thin and flexible, so the propping must be followed scrupulously to avoid excessive deflection and cracks.