Prefabrication experiments - 367 - Modern structural archetypes - 07 - Harmonized tension and compression

Since industrialization reformed engineering and its education, normalized structural frameworks’ materials, sizing and detailing have orchestrated building construction. In post and beam braced frames, mathematical calculations and catalogued precedents became the way forward to inform component specification. For all load-bearing elements, engineers evaluate and size according to constraints to shape a stable structure with manufactured parts: beams, struts, posts. Before industrialized frames, in masonry construction, geometry, namely arch effect was deployed more intuitively in domes, arches, and vaults to resist vertical loading in a shape inversely proportional to gravitational loads. Catenary arches and funicular figures represent idealized lines of stress that minimize tension, important for masonry structures. Modern engineering principles and longstanding arch effect have also been combined to optimize the link between structural form and a material’s tendencies. Freyssinet's prestressed concrete using tended cables to compress concrete is perhaps the greatest expression of harmonized tension and compression in structures to produce a superlative material. 

 

Another structural genius, Robert le Ricolais, professor at Penn State in the 1950s explored potential lightweight spatial structures, their geometry and the interplay of compression and tension to propose some of the most unique structural spanning elements of the 20th century.  His Polyten Bridge developed in 1968-69 while still at Penn is a notable example of using the prestressing principle that is normally applied to concrete to compose a resistant geometry. The unit leverages, the bowstring truss principle to tie and arch a superior cord. Symmetrical king posts expand the shape at its center to resist the greatest loads.  Together, top cord and bottom cables connected by a web of interrelated struts (short columns suspended within the framework) create a thick wing-type space frame. Akin to tensegrity (compression suspended in tension) the frame could be used for roofs reducing the amount of material that would normally be used by a monolithic element to cross the same distance. The Polyten bridge's geometry and structural effect accord compression and tension to arrange a robust structural framework that literally hangs arched and linear struts within a web of stretched cables in the service of an idealized structural efficiency.

Polyten Bridge - Robert le Ricolais

Prefabrication experiments - 366 - Modern structural archetypes - 06 - Outrigger Frame

 

Structural frameworks are explored and optimized by fine tuning geometry to increase spans while reducing material use. As a structure's dead weight increases, structural capacity is diminished; Achieving great spans with minimal material is the basis of imaginative structural form. Anthony Hunt, a famous structural engineer who worked with Norman Foster and Richard Rogers also taught structures to architects. He defined structural engineering as maximum result (span) with minimal weight (material). This ideal relationship is particularly important in buildings that require large open spaces: stadiums, auditoriums. 

 

Strategies for large spanning roof structures are often based on truss effect or shell / membrane effect to define form actively increasing spanning capacity. Tall buildings also demand an efficient structural ratio as each stacked floor multiplies weight on foundations and imposes greater rotational and moment forces as the edifice rises much like a long vertical cantilevered beam. Reducing these moment forces lessens the lateral forces on a building's framework. The outrigger frame designed for tall buildings uses a solid core and perimeter column organisation with outrigger beams that connect core and columns at certain levels creating a rigid bond between the center and the perimeter of the building. The outrigger beams can be monolithic or trussed but must rigidly connect center and periphery. This equilibrium force is analogous to how outriggers are used on watercraft to increase lateral stability. Floors where outriggers are positioned are usually less flexible than floors where only cores and columns are present freeing up the floor plate from of any structural obstacles. 

 

The Montreal Stock Exchange Tower  (Place Victoria Tower today) designed by architect Luigi Moretti and engineer Pier Luigi Nervi in the 1960s is an elegant example of the outrigger frame principle. Well-known for its four towering external structural pillars, it was once the tallest reinforced concrete tower in the world. Mechanical floors 5, 19 and 32 are crossed by diagonal full floor height reinforced concrete truss beams that stabilise core and perimeter.  The full height trusses rigidly connect the central stabilized core with floor slabs and corner columns strengthening the entire structural tube. The reinforced concrete prism is not only stabilized vertically by the outrigger beams, but the criss-crossing beams also buttress against rotational constraints.

Place Victoria Tower - stabilized outrigger core in red

Prefabrication experiments - 365 - Modern structural archetypes - 05 - J.H. Gray Column

 

The construction of tall buildings implies the efficient and economical use of materials to minimize dead loads for compounded stories. Weight and functional loads from each floor plate are transferred to columns that transmit them down to foundations. Steel works particularly well for tall buildings as structural elements can be profiled to reduce material use and increase span to weight ratio both horizontally and vertically. The iconic H shape of columns and beams depicts this type of material efficacy. The assembly of rolled posts and beams in platform structures became an iconic representation of the early Chicago or New York City style skyscrapers. 

 

The Reliance Building designed by Burnham and Root with Charles Atwood in the early 1890s is often cited as the archetype of the towering brace framed steel skeleton. The fourteen-story structure built in (1894-1895) with a floor plate of 56 by 85 feet exemplified the modern canon of separate structure and skin; glass and glazed terracotta panels were hung or even cantilevered from the steel grid foreshadowing the development of lightweight modular curtain walls. Recognized for its use of projected bays, the building's simple façade demonstrated what would become the commercial urban glass building of the twentieth century. 

 

An expression of industrialization's advances in mechanization (elevators) and pig iron's refinement into steel, the tall building skeletons were devised as large-scale kits-of-parts. The Reliance Building's structure is a basic assembly of a type of balloon frame where continuous pillars carry floor plates composed of primary and secondary beams. The chief innovation in terms of structure was the use of an open web column invented by civil engineer J.H. Gray. Contrary to the Z-bar riveted closed iron columns that had become common in steel assemblies, Gray invented an open trellis framework, a type of rising chase, that would allow for electrical and piping distribution. The posts were made by riveting plates at 30-inch intervals to 12-foot long continuous angles; columns were spliced at every 12 feet. The open web concept reduced column weight, increased accessibility for fireproofing and straight edges made standardized assemblies for beams and columns more efficient. 

Reliance building sketch and J.H. Gray column details

Prefabrication experiments - 364 - Modern structural archetypes - 04 - Castellated Beams

Beams and arches are archetypes of horizontal spanning structures. In particular, beams make it possible to span horizontal distances without the geometric and vertical constraints of arched, domed or vaulted shapes. Beams resist vertical loads by their in-built strength and distortion through physical inertia linked to their material properties, their sections and dimensions. Henry Grey’s wide flange beam revolutionized construction as its iconic «I or H» shapes were rolled and normalised to varying heights, widths and thicknesses. Further, for the first time in the history of construction, the beams employed a material, steel, that was equally resistant in tension and compression making them formidable tools to construct vertical buildings. Even with its great density, steel can be shaped to reduce material use.

 

Understanding beam effect under bending, compressive forces in the top flange, tensile forces in the bottom flange and a neutral axis in the center of the beam, informed the canonical beam shape. Inertia, a function of the vertical distance between both flanges, and a robust axial web resist torsion. This simple principle of all bending structures applies to wood, steel, concrete beams, slabs and space frames. Any horizontal spanning structure can be analysed and optimized through this beam effect.

 

Grey's beam invention remains largely utilized in construction and many have sought to further optimize its beam effect; The Castellated beam invented in 1926 by Fredellia H. Moyer studied a type of optimisation that expands on the reaction forces in bending structures. By eliminating material close to the beams' neutral axis, without affecting inertia and compressive or tensile constraints, the beam can me made lighter and therefore span longer distances as it flexes less under its own dead weight. The production process of a castellated beam illustrates these principles by symmetrically cutting a standard beam longitudinally through its web in order to form a half/hexagonal zigzag pattern. When the beam halves are matched to reveal the hexagons and then soldered they create a much lighter continuous structural element. 

 

The hexagonal openings can create an open network for wiring or ducting making floor thickness far more flexible than with solid beams. Castellated beams have been and are still used in architecture and construction to illustrate the possibilities of augmenting a component’s span to weight ratio simply by reformatting it in relation to beam effect. Openings in beams can also be circular or rectangular and are a function of maintaining the web's shear and torsion resistance.

Castellated beam pattern

Prefabrication experiments - 363 - Modern structural archetypes - 03 - Reticulated and lamella structures

Arches, vaults and domes are not ordinarily associated with industrialized construction or with modern building methods. Braced frames became the way forward for building with materials like steel and reinforced concrete replacing compressive masonry-based systems. With both steel and concrete skeletons, diagonal elements are used to buttress assemblies ensuring lateral stability with lightweight linear, compressive or tensile elements. Using intersecting patterns to configure and strengthen structural systems is not limited to frame construction but has also informed more complex geometries. Geodesic domes use a network of triangles in a similar way to organise reticulated spherical forms; triangles are arranged through lines of stress to construct robust lattices that act as structural membranes. Trusses use these diagonal, bracing principles to alleviate weight by creating a network of oblique segments to achieve structural inertia and optimize beam effect while using a small portion of the material that would be necessary for similar spanning monolithic beams. 

 

Triangulation is not limited to reticulated orthogonal frames. Interweaving elements in lamella-shell roof structures reinforce shapes in a similar way. Invented in 1908 by inventor / architect Frederich Zollinger the elemental ribbing is based on an analogy to mushroom cap structures. The gills or lamallae that grow underneath the cap are very thin and support a covering (the cap); together they form a very strong umbrella structure using minimal matter. The lamella roof uses this basic idea: a series of diagonal arches criss-crossed to reinforce and compose an overall space covering shell. A network of lamellas can be arranged in any vaulting structure, archetypical or free formed. The skin layer covering the ribs provides a diaphragm that bonds with the lamellas to form a thickness that is a fraction of the weight of a full shape. This principle was beautifully deployed by Pier Luigi Nervi in his airplane hangars and was also adopted in timber and steel warehouse structures. Similar concepts are used in reinforced concrete waffle slab construction, ribs and gills designed in varied grids give the slab structural strength and reduce its weight.  

Lamella archetype structure

Prefabrication experiments - 362 - Modern structural archetypes - 02 - Space Frame Urbanism

 

The development of three-dimensional trusses, space and trellis frames, from corresponding, dimensionally coordinated and repeatable linear components patterned to cover and limit architectural space is linked to iconic names in architecture: Fuller, Wachsmann and Le Ricolais explored the links between geometric archetypes and building structures. The three engineers/architects/industrialists/inventors are synonymous with large spanning frames articulating a symbiotic relationship between structural form and architectural space through reducing material use; maximum span with minimal weight. Founded on the principles explored earlier by Alexander Graham Bell, the triangular pyramidal truss is an inherently stable modular unit aggregated, clustered or amassed to shape lightweight frames that distribute structural constraints throughout their composing elements and reticulated network. Fuller's geodesic domes, specifically the double shell domes used a triangular pyramid, the tetrahedron, to fashion form and space. Its compositing equilateral triangles seen as a simple metric for elemental fabrication. These robust, triangulated and filigree structures were used to posit new designs for housing, factories, exhibit edifices, and most surprisingly as enclosures for futuristic cityscapes. 

Inspired by Fuller's great circles and by mentor Yona Friedman (famous for his aerial megastructure city proposals), David Georges Emmerich united reticulated structural form with his singular vision of building new cities over existing ones. Involved with groups like the Congrès Internationaux d’Architecture Moderne (CIAM) and a founding member of the Groupe d’Études d’Architecture Mobile (GEAM) in 1957, Emmerich envisioned space frame structures as an opportunity to develop mobile, flexible, adaptable and everchanging megastructures in response to the rapidly evolving modern city.  His representations of Coupoles Stéréométriques present the links with both Fuller and Friedman. The «coupoles» or domes, imagined as simple to assemble shells, would enclose multi-level neighbourhoods floating within the overarching limits of pure structural form. Patent drawings provide a clue to the simple building methods that would reform cities and buildings. In a similar way to his contemporaries who explored the potential of mass-produced components to solve housing crises and offer a way forward for stagnating city form, the structuralist vision of at once covering, limiting, controlling, relating and measuring «stéréométrie» architectural space remains a unique reverie of the post-industrial city.

Stereometric Domes and patent drawings for domes

Prefabrication experiments - 361 - Modern structural archetypes - 01 - Suspended Frames

 

Tall buildings' development was a direct result of industrialization principles applied to construction. Mechanization (elevators and machines) and mass production (replicated components) simplified the assembly of larger and taller spanning edifices and increased capacity to service these vertical communities. Skeletal frames composed of normalized and optimized steel profiles made it possible to build these structures without substantially increasing their weight to span ratios. Enlarging structural capacity through standardized components is a central theme in modern architecture inspired by manufacturing principles and leveraging them in building construction. Architects envisioned steel and glass cathedrals reaching upward and searched for ways to express weightlessness. 

 

The suspended frame, a major contribution to structural engineering, is a notable example of two specifically modernist canons related to skeletal frameworks: open planning and cantilevered floor plates. Unobstructed floors are carried from above and transmit their loads to a centralized core that braces the entire structure. Outrigger frames are a variation of suspended frames with «outrigger beams or trusses» attached to the core that support one or more floor plates. 

 

A beautiful example of the suspended frame was designed for British Petroleum in Atwerp, Belgium by Léon Stynen architect in 1963. The system is a straightforward expression of the archetype: rooftop cross beams or gallow beams shape a unidirectional hanging system supported by 2 longitudinal mega-beam trusses that transfer floor hanging loads to the central core. The spanning floor plates are stayed by tensile elements suspended from the gallow beams and determine the façade’s grid.  

 

The 11 floors float over the neighboring context and seem at first glance to be supported only by the central element; Once perceived from the rooftop, the megastructure holding up the perimeter is analogous to a masterful hand holding up a rigid marionette. The rigorous structural grid is transmitted throughout the building with the curtain wall expressing the link between structure and skin; a corporate internationalization of architecture generalized during the twentieth century. All elements of the hanging structure are manufactured parts that come together to shape a flexible and adaptable large-scale building kit. The typical floor plate also deploys the basic core and adjacent spaces pattern associated with many tall buildings.

Photo by Paul Hermans licensed Creative Commons-Share Alike 4.0 

Prefabrication experiments - 360 - Fabricating Modern Structural Form

 

Notably during industrialization with the invention of new materials, methods and building types, achieving maximum spans with minimal material informed structural theory and pedagogy. Correlating these two objectives, modern architects and engineers explored varied and variable structural systems that have become synonymous with their era. From ribbed slabs to castellated beams, reducing weight while maintaining and profiling structural integrity requires intelligent constructions, assemblies, and geometric finesse.  The geodesic domes of Buckminster Fuller, the grid shell tessellations of Pier Luigi Nervi, August Komendant's concrete space frames and Robert LeRicolais' lightweight beam experiments all materialized structural conceptualizations by decreasing material use; all shaped geometry, points, lines and their networks to respond to stresses and strains in an optimal manner. 

 

Through examining beam principles, compression, and tension in upper and inferior parts of a beam with forces neutralized closer to the center of the beam, matter is directed to eliminate waste and in response to loads, producing systems that are physical illustrations of their most favorable load transmissions. Today, generative design tools make it possible to optimize the representational and structural relationship even further between modular elements, structural form and geometry; grid shell systems combine resistant forms and shapes with lightweight struts or other modular elements that can be arranged according to underlining physical criteria and material characteristics. These structural optimizations can also lead to construction efficiencies based on similar ideals of modularization, assembling edifices from multiplied dimensionally coordinated components; An idea eloquently defined by Pier luigi Nervi's patent for structural prefabrication.  

 

Stacking boxes, aligning frames, organizing, or mapping shapes from tile-like elements all speak to the relationship between structures and reproductible components - a type of piecework quilting to achieve building form.  Further, modern structural prototypes with increasing spans defined a uniquely modern syntax linked to universal, flexible, and adaptable open spaces free from any structural obstacles.  The next ten blog posts will examine structural archetypes, the modern use of grid, modularity, geometry, and replicability to showcase innovative building systems.  Prefabrication, industrialized building systems, offsite construction and platform principles applied to architecture, all, in some way, employ similar modular schemes to propose the efficient production of edifices. 

Gatti Wool Factory ribbed isostatic slab by Pier Luigi Nervi

Prefabrication experiments - 359 - Containerization of housing

 

Residential housing design employs repetitive organisations and has been the subject of industrialized production using volumetric boxes or panelized building systems in a variety of materials. Spans are relatively small and rectangular. Apartments are often repeated from one floor plate to the next with interchangeable modular built-ins for wet spaces (kitchens, baths).  These patterns guide the housing market making it an ideal sector for normalized fabrication and process replication. A notable production analogy devised to improve efficiencies in housing uses the ISO container storage unit, a stackable block determined by intermodal transport with standardized dimensions and connections as a model for building construction.

 

Normalized containers generalized and communicated a way of shipping things across the planet. The boxes also outlined a common way of storing things, aboard a ship, in a port or in warehouses.  A similar containerization strategy is behind English company Verbus’ way to quickly put together mid-rise housing.  The basic building block, a generic module looks and works much like a shipping container that has been modified and tuned according to multi-unit residential requirements. The volume, 3,6m width x12,2 m length x2.9 m in height includes openings for a central corridor, which defines a double loaded floor plate of small studio flats. Windows and balcony elements are provided on the units’ extremities.  Service shaft holes above and below the container structure standardize duct placement to pre-set vertical mechanical distribution. 

 

More than just a container, the Verbus system has been designed with attachment points to vary massing from straight linear plans to rhythmic alternating and protruding compositions. Cladding and brickwork hangers are also incorporated for any number of wall systems to be attached to the basic steel structure. The generic floor plan presents a 12’ grid with 5 juxtaposed containers, 3 of which have the corridor scheme while the two end containers complete 2-bedroom units, without the hallway space. The steel structured volumes can be stacked up to 16 stories high and have found resonance with hotel design, a definitive outlet for a containerized design.

Verbus massing and floorplate diagrams

 

Prefabrication experiments - 358 - Sterckeman caravans and a house for everyone

For better or for worse, prefabrication often evokes the preconceptions associated with the mobile home. Objectively, the negative press, suspect construction methods and low quality materials have long been forgotten. Today’s manufactured dwellings have nothing to do with pre and post-war prefabs. The obsession with mobility, the possibility of towing one’s home and using industrialized materials and processes to offer accessible dwellings endure as the objectives of the mobile home. The Sterckeman family, based in France in the small town of Seclin near the Belgium border started producing mobile homes or «caravanes», in accordance with the optimistic leisure-based dreams post WW2. As was the case with other mobile home companies, the Sterckemans also attempted to produce inexpensive fixed housing prototypes. 

 

A long-lasting partnership with well-known French architect Paul Chemetov, who had designed commercial and industrial buildings for the family, led to the company's experiment related to one of modern architecture's central themes: A house for everyone. Charles and Ray Eames, Jean Prouvé, Frank Lloyd Wright, Alvar Aalto, Cedrick Price, and the list could go on have all taken a stab at the problem of affordable, manufactured, and well-designed single-family dwellings. The Chemetov-Sterckeman relationship spawned a house made from mass-produced components. Developed on modernist canons, the house floats on pilotis (slender posts) but is grounded by a vertical service core that leads to and serves first story living spaces. A core-house in its most canonical expression, the centralized outward radiating grid-based geometry arranges the plan, its structure and supports external appendages suspended from the steel skeleton. The house showcases its elements as a part of an industrial kit; bay windows are made from repurposed skylights and tubular railings expose the design's link to off-the-shelf zeitgeist and narrative that architects love to argue for to reduce costs. 

 

The architectural prototype is a protected heritage landmark and was extended during its service life. The Sterckeman dwelling was part of an exhibit on Chemetov's work at the Cité de l'architecture & du patrimoine in 2012, showcasing the continued interest in the modernist dream of combining manufacturability with architectural impetuses.

Sterckeman - Chemetov house and excerpt from caravan catalogue