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