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

Prefabrication experiments - 114 - Structures - 5 - Folded Plate Structures

Normally related to but not exclusive to reinforced concrete thin shell construction, the folded or pleated plate structure is essentially a three dimensional geometric assembly of planar elements through rigid edges. The creased surface's stress lines induce a complex transfer of loads across the surface portions and linear folds of the pattern. Folds increase the material distance from the configuration’s rotational centre, which increases the systems’ capacity to resist angular stresses. While an area can be pulled in one direction buy a load other interrelated surfaces push or pull in opposing directions. Originating in paper folding principles, multi-directional folds act as stiffeners.

Divided into surface or truss systems, both strategies create thin surface polygonal envelopes that behave similarly to arches or vaults. Much like the voussoir segments of an arch, loads are transferred through stress line vectors. As the folds are increased in height or depth overall spanning capacity is increased as each surface mimics a vertical beam. Tighter and greater geometric subdivision also increases capacity as the stress and bending moment of each ridge is reduced. The resulting architectural space can be planar or follow curved patterns.  

Skidmore, Owings & Merrill (SOM)'s folded surface and truss triangular arch at the US Airforce Academy Chapel at Elpas Colorado or Herbert Yates’ Plydome folded cardboard agriculture workers’ shelters typified architectural and engineering fascination with the pleated structures throughout the 20th century. The folded plate although not exclusive to modernity became synonymous with innovative structures and materials.

Building on the explorations in engineered timber and the folded plate as a structural and geometrical archetype, the iBOIS laboratory at EPFL (École polytechnique fédérale de Lausanne) is examining and experimenting folded plates’ contemporary potentials. Banking on timbers’ relatively low embodied energy and weight, plywood surfaces reproduce geometrical patterns of paper folding while being inflexibly joined together.   The Chapel St-Loup a collaborative effort between Localarchitecture and the iBois laboratory demonstrates the simple manufacturing principle of fixing numerically cut pieces determined from fold patterns. The joinery is a mix of traditional lap and dovetail to achieve a rigid edge joint. The exciting research proposed by this university lab showcases structural capacity, dynamic space creating qualities and the formal geometric elements of folded architecture.

Chapel St-Loup (construction) and iBois laboratory experiment

Prefabrication experiments - 113 - Structures - 4 - Volumetric or box type construction

Also described as volumetric construction, a box structure is basically a rectangular prism component for building. Volumes/modules are either self-supporting or designed to sit in or to be supported by a separate collective infrastructure. The rectangular prisms are usually defined according to standard regulated shipping dimensions and can be completed to varying levels in a factory. The combination of manufactured boxes informs building erection with a potential «toy building block» variability.

The particular advantage of volumetric systems is that much of the construction process takes place within a quality-controlled environment. The factory setting is accommodating and a streamlined organization of procurement, design and production is possible. Transportation costs and important investment in factory production comprise some of the drawbacks of off-site volumetric construction.

Boxes can be cast in concrete or assembled in wood or steel frames. Iconic 20th century box systems employed reinforced concrete or prestressed concrete. Concrete volumetric construction matured during Operation Breakthrough in the U.S. as the postwar building booms in industrialized nations underwrote the development of various concrete box systems designed and engineered according to their vertical and horizontal aggregation. The Habitat 67 prototype designed by Moshe Safdie in Montréal is one of the flagship examples of concrete box building. Reinforced concrete boxes were amassed in a dynamic form, which produced a canonical 20th century architectural marvel. The boxes were fabricated on site as an assembly line was established adjacent to the building site. Manufactured fibreglass kitchens and bathroom were fitted in the boxes.

Once assembled, volumetric construction is permanent, which renders future changes and adaptability difficult. This challenge has constantly argued for some type of hybrid system. For instance, the assembly of bearing service volumes over open frame structures would allow for some flexibility. Grouping has also been explored as a potential to achieve greater flexibility of volumetric building systems. The Shelley System patented in 1970 explored a checkerboard pattern of boxes in order to avoid double walls of adjacent boxes. Furthermore the pattern structured a series of voids accommodating adaptable spaces. The brick like pattern created a permeable structure. The staggered units were attached and post-tensioned by cables running through superimposed faces and ridges.

Shelley system patent drawing