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