Steel floor with clay tavelloni

A historic, still-current floor: steel beams (joists) at close spacing, with clay tavelloni resting between the bottom flanges and a concrete topping that ties them together. The steel carries the bending, the clay lightens and forms the soffit, the topping makes the collaborating deck.

The floor with steel joists and clay tavelloni is the steel ancestor of the composite deck: born between the 19th and 20th centuries, it still underpins much of the historic urban building stock. Its logic is clear: the steel beams, at close spacing, carry the loads in bending; the clay tavelloni, laid between the bottom flanges, close the bay and act as permanent formwork; a concrete topping, cast above, spreads the loads and ties the elements together.

In this floor each material does one thing, and does it well. The steel beam (an I-profile, the «putrella») works in bending: the top flange in compression, the bottom in tension. The clay tavellone does not carry the floor's vertical loads: resting between the beams, it closes the deck, lightens it and offers a soffit ready to plaster. The upper topping, often reinforced with a mesh, spreads point loads between adjacent beams and, where designed to collaborate, increases the stiffness of the whole.

Steel is non-combustible but not fire-resistant: above 500-600 °C it rapidly loses strength and stiffness, to the point of deforming. The exposed beams of these floors must therefore be protected to reach the required R class: with plasters or fire boards on the soffit, with intumescent paints that swell in a fire, or with the topping and tavelloni themselves which, screening the top flange, contribute to the protection. Neglecting fire protection is the gravest mistake in the refurbishment of these floors.

On existing floors two themes dominate. The first is corrosion: moisture and leaks attack the steel, especially at the ends bearing in the masonry, where rust swells and reduces the resisting section. The second is collaboration: many historic floors have modest, unreinforced toppings with little composite action; modern strengthening (connectors that make the topping integral with the beams, new reinforcement, added concrete) turns the floor into a stiffer, stronger steel-concrete composite section, often without demolition.