Prefabrication experiments - 524 - Modlines

 

Completed to high levels in a factory, incorporating the most refined details and according to rigorous quality control measures, modular volumetric boxes are shrink wrapped to protect them during transport and carried on flatbeds to construction sites as giant building blocks. The design-to-delivery process is meticulously planned down to the required staging areas where the convoy’s just-in-time sequence is orchestrated for the boxes to be craned and set into position. Spreader beams carried by cranes provide an augmented lifting surface to account for the width of the volumes to be set.  Faces are juxtaposed and stacked to form what are referred to as matelines (where modules meet or mate) or modlines in volumetric construction. Matelines usually refer to joints or segments where panels meet in planar building systems whereas modlines delineate the gridlines associated with volume edges.

 

Modules are typically stacked with one box’s roof constituting the drop ceiling of a flat or room while the next box's floor structure is strapped and bolted to the previously set volume forming a discrete ceiling and floor connection. A vertical sequence of two boxes in width is piled up to the roof, before going on to the next series. This vertical sequence ensures that roofing is quickly installed to protect volumes during construction as manufactured boxes are delivered without permanent weather protection. 

 

Once set in place, mechanical, electrical and other systemic connections proceed. Systems are either capped off or modularized to facilitate on-site coordination. Access hatches, traps or other facilitating devices are incorporated to be finalized on site. When exterior cladding is installed off site, perimeter edge strips are left unfinished to be weatherproofed, insulated and covered. This approach exposes the modline grid on the finished building. Another strategy is to transport incomplete boxes where weatherproofing and cladding is contracted and completed on site dissimulating modlines behind a continuous cladding to resemble a conventional build. The standalone nature of the stacked factory-made boxes comes with a certain degree of material redundancy (double structure for floor and roof compositions), however this redundancy, when detailed properly, can lead to much greater acoustic performance as compared with conventional builds where two stacked flats share a single floor composition.

Setting of factory-made volumes

Prefabrication experiments - 523 - Standards and In-factory Inspections

 

Buildings erected using factory-made subassemblies or large volumetric modular chunks must conform to the same codes and regulations that apply to conventional construction methods. Fire ratings, means of egress, dimensional criteria, and sound transmission are all defined by prescriptive construction assemblies. These provide the basis for minimal construction quality to ensure the health and safety of occupants as well as a building's stability and durability. Architects, other professionals or authorities having jurisdiction (AHJs) supervise or inspect assemblies on site to confirm conformity with building codes. 

 

The logistical nightmare associated with having to adapt factory finished elements on site argues for their upstream inspection and for any corrective measures to be applied in the factory before they are delivered to site. Industrialized building is effective when it is part of a streamlined and cost-efficient design-to-delivery process. In Canada, the CSA A-277 standard provides an outline for qualifying factories and certifying that modular volumetric sub-assemblies, prefabricated buildings or prefabricated panels meet minimum requirements before they leave the factory.

 

A277 provides requirements for  « a) certification of the factory quality program;  b) certification of the prefabricated product; c) auditing of the factory quality program; and  d) in-factory inspection of the prefabricated product.» First developed for the prefabricated housing sector as an evolution of the still applicable CSA Z240MH which states the minimal requirements for manufactured homes fabricated on a steel chassis, CSA-A277 can apply to all building types and is being used to certify factories producing modules for schools, hospitals and collective housing blocks. 

 

Third party independent inspection agents certify factories and their output.  AHJs and professionals can cooperate with manufacturers, ideally during the design stages to understand their processes and outline the required scope of stakeholder engagement to account for in-factory certifications. As is the case with many other modularized factory-made components, unitized curtain walls, mechanical equipment, kitchen cabinets, and specialized devices, conformity is established as part of a harmonized inspection process.  

 

CSA standard series Z250, Z251, Z252 (not yet referenced in Canadian codes) further regulate modular construction considering the current uptake of offsite construction to standardize elements that include transporting, setting, juxtaposing and assembling factory-produced boxes.

CSA A277 conformity sticker

Prefabrication experiments - 522 - Rebel DfMA - The «Over-the-Wall» Problem

Each major advance in manufacturing techniques spawned experiments for bringing the same efficiencies to building construction. DFMA - Design for manufacturing and assembly is a management methodology related to breaking down the barriers between designers’ visions and fabrication parameters. Cited as the «over-the-wall» problem, authors Boothroyd, Dewhurst and Knight (1983) from the industrial design field, described a framework to reduce waste as well as to streamline tasks through an optimized production process synchronizing supply chains and design with production criteria. Reducing the number of pieces and parts in an assembly is the conceptual basis for this lean industrial design strategy. 

 

Recently conceptualized for building construction, DFMA or construction-DFMA rewires conventional construction practices through maximised use of offsite manufactured sub-assemblies to facilitate the onsite erection of any type of edifice harnessing the already well documented advantages of prefabrication. Platform DFMA (another conceptual variant) offers further gains by centering the design of diverse buildings on similar components and their shared details. 

 

Rebel DFMA is an American company with a value proposition articulated to digital modelling as a fundamental part of an integrated DfMA process. Virtual modularization, high-level-of-detail modelling, streamlined shop drawing generation along with the production of onsite user-friendly assembly manuals outline the company’s services. The highly detailed models encapsulate segments, modular and coordinated parts that can be contracted to factory partners and delivered just-in-time to building sites to be put together according to precoordinated processes. DfMA brings the designers to the factory floor before design so that initial design ideas account for their manufacturability. 

 

DFMA requires advanced planning and systematization of tooling, materials, methods and supply chains making this process applicable to any building. BIM has provided an inflection point for digitization to reform construction. It has also provided a wave of interest for new prefabrication theories to emerge. Although DFMA certainly provides a lean framework, it can be argued that it has become just another catchphrase for shedding the archaic connotations associated with prefab, manufactured building or industrialized construction. 

Image from Boothroyd, Dewhurst and Knight (1983) 

Prefabrication experiments - 521 - A Third Emerging Business Model - Project Specific Prefab

 

Industrialized construction is and has always been linked to standardization and repetition. This was undoubtedly the case for post-war, low-cost construction strategies that sought to leverage mass production. Previous blog posts examined vertical integration (post 518) and digital integration (post 520), both harnessing some type of replicable process or product parameters to scale and regulate factory resources. A third emerging business model examines offsite construction not to repeat projects but to mitigate some of the coordination concerns and conflicts in conventional construction; Onsite construction continues to be hindered by lagging productivity linked to systemic entanglement, labour shortages, rising management costs, and overall trade fragmentation. 

 

A growing number of general contractors are exploring the promise of digital processes and tools like BIM (Building Information Modelling) to facilitate factory production for preselected, agreed upon and coordinated chunks of singular buildings. They aim to synchronize design with project management, trades, materials, their delivery and assembly onsite to avoid the costly errors associated with traditional builds. Flying factories, multi-trade coordinated contracts for distributing responsibilities, and near-site prefabrication are all variations of this virtual design and construction business model. Project components and their interrelationships are precisely outlined, modularized or panelized, as large sub-assemblies fully coordinated in an up-to-date, well-equipped and controlled setting. 

 

Project-specific prefabrication is less concerned with replicable standards than with repeatable processes, making it possible to design and construct unique buildings. Digital coordination and file-to-factory-to-site management propose the same overlapping of tasks that speeds up construction while resolving coordination bugs in advance of their discovery on site. 

 

Age-old narratives associated with factory production are being swapped for the high-quality potential of a general contractor / integrator managing on as well as offsite elements along with their systemic synchronization, attributing them to sub-contracted trades and producing them through manufacturing methodologies.   Multinational builders like Skanska and Turner Construction are adopting similar approaches redefining their role of management specialists to include lean manufacturing and efficient onsite assembly facilitators to prefabricate everything from mechanical modular rooms to hospital bed walls and everything in between. 

Temporary production facility set up by Skanska for the Miami Valley Hospital