Prefabrication experiments - 117 - Structures - 8 - Reinforced Concrete waffle slabs

Reinforced concrete embodies architectural variability. Concrete structures can be cast in any shape or size, on or off-site, through precise engineering and manufacturing processes. Concrete surfaces are quick to produce, durable and require minimal maintenance. Furthermore concrete construction simplifies the building process as the shaped surfaces provide exposed flooring or ceilings without any other structural components such as girders or joists as for lightweight steel or timber framing. Curing time and weight are reinforced concrete’s main downsides. Reducing weight while providing interesting architectural expression has been a recurring theme for the quest for greater efficiency in concrete building systems.

Thin shell concrete construction or pleated plate construction makes use of form resistant shapes to decrease concrete’s weight to span ratio. In simpler floor slab construction, ribbing has been explored to provide stiffness and optimal mass distribution to reduce overall weight. Pier-Luigi Nervi's Gatti wool factory's ribbed slab is perhaps one of the most famous ribbed slabs which uses stress-lines to produce a waffle like expressive structural ceiling pattern.  Used in more simple orthogonal compositions, ribs are moulded and the concrete cast into a waffle type formwork such as Louis Kahn’s triangular pattern at the Yale Art Gallery. A matrix of interconnected beam-like elements, the resulting waffle slabs can span larger spaces while using less material. Analogous to a massive cast-in-place space frame, the waffles vertical ribs carry stresses through short beam like planes. Although not common in small domestic architecture the Aldeia da Serra house by MMBB architects stages space beneath a remarkable waffle slab element which covers and expresses limitless modern architectural space.

The waffle elements can be cast on site or waffle panels can be produced in the factory. The resulting two directional structural slabs are ideally composed in symmetrical square patterns varying from small spans to relatively large spans up to 15 metres. Combining waffle slab with beam principles, the Holedeck (holedeck.com) formwork, further reduces weight as openings are cast in each vertical rib. The central openings are positioned in the plates’ neutral axis where compressive stresses cancel tensile stresses to develop a network of voids which can be used as mechanical spaces for ducts, piping or electrical wiring.

Gatti wool factory - Yale Art Gallery - Aldeia da Serra house - Holedeck