Prefabrication experiments - 374 - State-of-the-Art - 04 - Collins House kit-of-parts assembly

 

Skyscrapers are contemporary marvels of engineering symbolizing wealth and power, just as gothic cathedrals or tower-houses were in middle-age cities and towns.  Industrialization of steel and reinforced concrete along with advances in structural computations inspired verticality to showcase modernity.  Building tall frames requires imaginative ways of buttressing these thin cantilevered beams sprouting from the ground, reaching for the sky, and anchored by increasingly small floor plates. Stacked small floor areas also require novel building methods in dense urban environments constraining traditional construction methods. 

 

Bates Smart Architects, 4D Workshop structural engineers and Hickory, general contractor, designed and erected the Collins House in Melbourne Australia in 2002 based on an innovative interpretation of an age-old idea – the box beam: a hollowed out structural section braced by its perimeter surfaces. With a height to width ratio of 16:1, the 58-floor building's structural strategy employs 4 shear walls:  2 lateral walls and 2 cross walls that form an overall H-shape braced in both directions. The service core portion of the H shape includes large vertical openings for an egress scissor stair and three elevators. The building's first 14 storeys are outlined by urban property lines, while floors above the 14th gain a 4,5 m cantilever on purchased aerial rights over an adjacent heritage building. This 44-floor cantilevered appendage is post-tensioned to its supporting lateral shear wall. 

 

The building is a manifest of current offsite construction methods applied to unique one-off edifices. Shear walls, exit stairs, cantilevered volumetric sub-assemblies were all produced offsite, delivered, set, and stitched on site.  Precast sections of post tensioned bearing walls were stacked along with complete floor sections; elements were then joined with structural mortar connections. A notable example of multi-trade prefabrication, systems are fully factory coordinated by multiple trades working together as they would onsite, but in a factory setting. Hickory group distributed contractual roles and responsibilities among trades and manufacturers. The unitized curtain walls, and temporary floor jack-posts were also installed in the factory reducing onsite work to a minimum. Modularization is applied to all building subsystems making this tall structure a giant Meccano kit-of-parts designed for precise assembly in a tightly woven urban context.  

Elevation and structural strategy

Prefabrication experiments - 373 - State of the Art - 03 - From the mobile home to the CrossMod®

 

According to the U.S.A.-based Manufactured Housing Institute's Industry Overview for 2021, the average construction cost of a manufactured home is 57$ per square foot compared to 119$ per square foot for an onsite built home - 50% less. This affordability is evidence of the successful application of industrial principles developed throughout the 20th century. From the earliest trailer coaches in the 1920s, to single and double-wide’s evolution in the aftermath of World War II, mobile homes influenced single family home production in the United States; even referred to as American vernacular by modern architect Paul Rudolf. Negative undertones associated with subpar construction forced United States Congress to adopt the National Manufactured Housing Construction and Safety Act in 1974. This increased and imposed national standards for quality to stabilize the industry's credibility. The term manufactured home also conveyed manufacturing potentials and dissociated the industry from the entrenched idea of impermanence.

 

Still representing 9% of all single-family housing starts, the dream of the industrialized house is alive and supported by dynamic forward-thinking producers envisioning greater demand coming from the current lack of housing supply and a dearth of traditional construction trades. Redefining the industry and promoting design and efficiency also led to a new term: CrossMod®. An industry initiative in 2016 highlighted the renewed need to elevate industry standards and more importantly to reach a new generation of consumers still inhibited by enduring connotations of less than par products. Linked to the crossover that has become an immensely popular breeding of cars with SUVs in the automobile industry, the CrossMod® presents some of the features of a site-built home, most notably a permanent foundation. Modular sections can also be combined and stacked to construct homes that are difficult to distinguish from their site-built counterparts and that come with the advantages of constantly improving manufacturing processes. 

 

Short some 5.4 million single family dwellings according to recent studies (USA), the market potential is clear. It remains to be seen if this most recent change in terminology will reform the industry and increase market share: 9% of all new housing starts represents a figure that is strangely unchanged from the 9% of housing starts in 1954.

Evolution from the Mobile home to the CrossMod®

 

Prefabrication experiments - 372 - State of the Art - 02 - Cross Laminated Timber's Disruptive Potential

Once pioneered, some materials revolutionize construction and architectural production. Both Steel and reinforced concrete highlighted by industrialization became the two materials most associated with modernity. Steel was touted as strong, durable and easy to put together, while reinforced concrete provided fire-proofed edifices for densely populated cities. Modernity in architecture relegated timber to small scale building embodied by  stick platform framing. Glue laminated timber, an equally modern material, made it possible to attain large spans and was largely used but has only recently been as disruptive as steel and concrete; Current climate change imperatives and regulating carbon footprint are driving its developments and use in building structures. 

 

Cross laminated timber in particular, a variant of glue laminated timber, a type of «hyper plywood» of compressed, layered, weaved and bonded pieces is offering an alternative to concrete construction. Its weight can be a third that of reinforced concrete. The CLT panels are produced as made to stock sheets, or engineered to order pieces that are cut in the factory to include openings for windows, doors, ducts or any other required openings. Once the pieces are numerically cut (2-3mm precision), they are loaded in sequence and delivered to sites. They are assembled much-like a bolted precast concrete panel system would be. Dry joinery also makes these systems as easy to dismantle as they are to assemble. 

 

A warehouse built in British Columbia, Canada in 2020, designed by Studio 531 Architects displays how straightforward this formidable building material is to work with. Once the slab on grade was cast and cured, vertical 25-foot panels were lifted to form the structure’s perimeter. The bearing wall panels function like tilt-up concrete panel systems and were considered a viable eco-friendly alternative by constructor Citta Group. The 5 ply panels were simply bolted together with cross glulam beams to carry conventional roof trusses in an efficient wall / post and beam / truss building system. Supplied by now defunct Katerra, the building took only two weeks to erect.  Saving time along with sequestering Carbon are the main advantages of this efficient building material that is rapidly gaining ground on conventional building systems.

Interior view of the simple to assemble timber kit (panels, post and beam, trusses)

Prefabrication experiments - 371 - State of the Art - 01 - Construction 4.0 and tall modular

 

A state-of-the-art study on offsite construction, including prefabrication, industrialized building systems or modularity reveals a notable uptake in interest and in use throughout the last decades. Promoted as a productive way forward in construction, novel factory or manufacturing methods are being driven by information technology and reforming a fragmented building industry into a data centric cooperative community of stakeholders. BIM (building information modelling) is at the core of this transition. Data sets are incorporated into a virtual model earlier in the design process potentially streamlining design with manufacturing.

 

Digital design, modelling, fabrication, and monitoring geared to the making of buildings has been referred to as construction 4.0. This fourth industrial revolution is disrupting conventional construction and promises to create more efficient buildings. As Ford and Toyota radically altered the way things were made, DfMA principles along with new manufacturing technologies will become mainstream within the construction industry; robotics, additive manufacturing, generative design, and artificial intelligence are developing a space for new materials, systems and processes to replace conventional means. 

 

Even with all the potential for newness, modular volumetric and panelized construction remain go-to systems. Both have existed within the offsite space for decades and panelized has transformed light wood framing due to the rarity of traditional trades. Modular volumetric is promoted as the most modern of these methods, due to its speed of erection. Stacking factory made boxes certainly epitomizes Offsite construction as in principle only infrastructure and mechanical connections are completed on site. Currently, tall modular is providing a window into how industrialized construction can be deployed to increase productivity. Built from 2018 - 2020 in London by owner, developer, builder, Tide construction, 100 George Street is a notable example of combining the potential speed, precision, and quality of volumetric modular. Two 38 and 40 story towers are made up of 1500 boxes produced by manufacturer Vision Modular Structures. The use of digital modelling tools to organize and harmonize different stakeholders reflects current practices. The owner, developer, builder collaborative model exposes an integrated design-build approach that makes it possible to involve manufacturers in the planning phase. Including offsite criteria in planning is essential to fulfilling the touted advantages of offsite construction.

100 George Street by Tide Construction

Prefabrication experiments - 370 - Modern structural archetypes - 10 - Waffle slabs: monolithic space frames

Industrialization stimulated a generative relationship between structure and architecture. Ideas federated from both disciplines forged rational and robust buildings through the rigorous repetition of components and assemblies. Networks of linear lightweight members, monolithic waffle slabs and ribbed shells symbolize this union’s aim for maximum architectural space with minimal material use.

 

All manner of reproducible elements were knitted into rigid textile-like curved or planar surfaces «packing space» as Buckminster Fuller would articulate using his tetrahedral truss. Triangulation, truss effect and materialized lines of stress symbiotically increased structural inertia and stability. These modular grids and networks argued in favour of prefabrication in skeletal frameworks, in timber or steel, as well as in lamella vaults and filigree domes. Ribbed surfaces allowed similar optimizations in monolithic concrete structures, exposing a lattice of beams, girts and sustaining segments. In medieval ribbed vaults, compressive lines of stress materialized load transmission. These spines can be formed with equivalent or varying depths to further express load transmission. A beautiful play on structural rhythm, the waffle slab used by Louis Kahn in the Yale Art Gallery in New Haven, Connecticut or John Lautner’s modern masterpiece, Sheats–Goldstein Residence, came to characterize modernism with its rhythmic sequence and gothic imagery. Cast in orthogonal, triangular grids or even more dynamically by Pier Luigi Nervi, all were deployed to reduce reinforced concrete’s prohibitive dead load (2400 kg/m3). 

 

The simplest systems place and align dimensionally uniform prefabricated boxes over flat formwork to define a pattern of voids into which rebar is positioned and concrete poured, hardened and cured - the resulting waffle effect and criss-cross pattern buttresses a relatively thinner horizontal slab over the formed ribs. The formwork could be reused or left as part of the structure’s geometry if made of some type of thin shell material. The voids from waffle forms could be used for architectural lighting to amplify the systemic patterns.

 

Today, digital technology allows for this type of structural rationalization to develop ideal patterns and link this data to code machines to create specific formwork components to further optimize ribbing or arching patterns.

Left: Yale Art Gallery; right: Sheats-Goldstein residence

Prefabrication experiments - 369 - Modern Structural Archetypes - 09 - Panelized Frames

The relevance of factory production in architecture and construction has often been expounded by crises. The enormous material and human devastation of World War 2 combined with accumulated housing shortages in Europe underwrote massive housing production and its normalization. In North America, returning soldiers and the baby boom also pressured housing supply; both continents needed to build more and quicker. Standardization became key to two evolving building systems that in a sense represented their contexts’ political and social progression. Balloon and platform light timber frame construction complemented by onsite rigorous sequencing became the go to system for single family dwellings whereas standardization elevated the use of concrete panel systems for collective flats in Europe. The large panel building was democratized using a very simple structural system: the wall and slab monolithic hive archetype combines surface elements to create inhabitable cells in plan, section, and elevation. Bearing walls and spanning floor surface elements arrange simple one floor unidirectional tube-like apartments on either side of a central corridor.

 

The planar bearing elements and spanning slabs are connected through monolithic joinery. Extending reinforcing steel bars are tied together and specially formulated high strength low-shrink mortar fills the gaps bonding vertical and horizontal elements.  The panels were sometimes cast in place or in a factory but usually on some type of horizontal casting surface or vibrating table. Flat-packed and transportable, the reinforced concrete panelized structures are monolithic, inherently fire-proof and robustly stable with vertical and horizontal panels contributing to lateral resistance. Panels for exterior walls could be faced with ceramic, brick, or any other material, but it's the structural simplicity that made the system successful in so many different contexts. The panel and slab system including standardized planning principles could be deployed and improved from building to building, deploying replicable details and processes. The wall and floor frames have recently been examined once again in relation to affordable housing stock only this time highlighting mass timber potentials. CLT panel and slab buildings use similar configurations to concrete large panel typologies with the added advantage of carbon sequestration.

Large precast reinforced concrete panels and prototype building in USSR

Prefabrication experiments - 368 - Modern structural archetypes - 08 - Moment frames and connections

 

In skeletal construction, post and beam, box, balloon, platform or regular braced frames, joints and connections between vertical bearing elements (columns/posts) and horizontal spanning elements (beams / girders) are either nailed, screwed, bolted or riveted. These pin connections resist vertical gravitational loads and transfer constraints from one element to another taking the path of least resistance. Generally flexible, these joints are located where moment and shear forces are at their summum. Flexible frame connections require some form of bracing, diaphragm planes, or diagonals to lock connections in position to resist lateral loads. Moment Frames are skeletal constructions where conventional flexible fixations are replaced with more complex rigid or moment connections to keep the frames stable. 

 

Portal moment frames like the infamous Butler steel building frames, invented in 1940 by Wilbur and Kenneth Larkin of the Butler Manufacturing Co., express compression constraints by tapering elements to mid span where bending moment is at its minimum and enlarging elements at points of maximum stress. The connections are rigid; instead of standard bolted beam to column joinery, the constituent parts are either welded together or bolted with larger stiffening plates. The connection is further reinforced by welding plates in webs to ensure a complete transfer of bending moment and shear stresses from the horizontal to the vertical. Deformation or displacement of the moment connection implies bending or distortion of the entire frame which makes the whole system behave with monolithic geometry. Tapering composing parts reduces material use but connection complexity makes this frame's use appropriate only where completely open spans are required, arenas, factories, etc. 

 

A beautiful example of a moment frame designed by Camenzind Evolution in 1998, the Sports centre in the city of Uster in Switzerland employs hinged rigid frames to reduce effective spans and present an elegant structural solution derived from structural analysis; wide flange beam are shaped according to stresses. In these types of portal frame structures, rigid joints resist lateral constraints in their traverse direction while roof surfaces play a structural role bracing frames in their longitudinal direction.

Butler Frame (left), Sports centre frame by Camenzind Evolution (right)

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