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

Prefabrication experiments - 116 - Structures - 7 - Planar construction from monolithic panels to structural insulated panels

A load bearing organization of vertical and horizontal thick flat surfaces is the simplest form of planar construction. Prefabricated panels can be manufactured in any material from lightweight steel or wood to more heavyweight reinforced concrete. Panels can be dry fastened and bolted to transfer vertical and horizontal stresses through the panels' thickness.  Architectural concrete panels, steel stressed skin panels or the contemporary cross-laminated timber panels are the most common manufactured elements used in this type of moderately spanning structures. Habitually flat packed, panels are easily transported and produce an easy to assemble construction kit, where floor panels span from one wall panel to another.

Whether timber, steel or concrete, exposed joinery connects overlaid panel edges in simple "T" or "L" patterns. Once the bearing walls are anchored to a foundation, the floors and walls are continuously stacked in a platform arrangement to reach varying heights. If simplicity is a major advantage, planning flexibility is one of the drawbacks, as the vertical planes impede free flowing open plans. The 9 m x 6 m prism like spans offer less overall building adaptability as compared to open skeletal frame structures.

Concrete and mass timber panels have the additional drawback of weight and make sense for large multi-story buildings. For smaller scale structures, commercial or residential, structural insulated panels (SIPs) are a variant of this type of planar construction, with a much lighter panel.  A SIP is a type of stressed skin lightweight panel composed of en external sheathing of plywood or other material with a high density polyurethane foam core. This type of structural panel can be manufactured to varying thicknesses and manufactured with windows, doors or wiring networks being milled by numerically controlled cutters in the factory. 

Architects Ian Hsu and Gabriel Rudolphy explored SIPs in their recent project for the Casa SIP m3 prototype. The casa SIP showcases the modular panels as walls, floors and roofs and reproduces a simple type of cardboard model like building system using rectangular flat surfaces. The casa SIP project employs this simple construction method to produce dynamic volumes and spaces as well as a clear tectonic expression planar construction.

Mass timber planar construction (left) Casa SIP by Ian Hsu and Gabriel Rudolphy (right)

Prefabrication experiments - 115 - Structures - 6 - Trusses, Space frames and Tensegrity

Structural strategies share the same basic objective: to span, cover, support, protect or shape architectural space. The principle of spanning the largest area with minimal material use while establishing an elegant shape is the basis of the historically generative relationship between architects, structural engineers and industrialization. Twentieth century world fair exhibit architecture typified this relationship. Trusses and space frames demonstrate efficiency along with open industrialized construction as the repetitive assembly of manufactured components relate to buildings of any shape, size and scope.

Reducing weight is important in any structure but is particularly important in large spanning structures that are free of any vertical obstructions. Triangulated structures or trusses are systems that systematize geometric patterns of consistent components to transfer loads and stresses. Mimicking simple beam behaviour, trusses use triangulation for stability and to position material toward maximum efforts in either compression or tension, all the while eliminating material from low-stressed areas. An interrelated network of stable triangles is the simplest form of a two dimensional truss. Extrude this network axially and the resulting space frame can cover large spaces with a limited amount of material. The space frame applies truss principles in a third axis utilizing triangulation in three directions: length, height and width. The space frame is normally composed of an interdependent matrix of regular repetitive linear components in a thick plate-like structure. 

Triangulated grid lattices can be made to follow any surface, from arches, to domes to textile like organic shapes. Konrad Wachsmann's universal construction kit, or Buckminster Fuller's geodesic dome structures most emblematically portray the space frame’s potential. Tensegrity takes similar principles one-step further for maximum structure with minimal weight. Based on this principle, one of Buckminster Fuller’s students / disciples, engineer, Jeffrey Lindsay patented his structural framework in 1960. An enhancement of the triangulated space frame this structural framework explored a type of structural efficiency using tension cables and compression tubes in a synergetic relationship. The compression elements, tubes or other profiles, keep cables in tension while cable elements stitch compression elements together. The resulting lightweight lattice structure can be employed as a universal, open, flexible, adaptable building system. 

Jeffrey Lindsay's structural framework - tensegrity dome and patent drawings