EntryPrefabricated Building Systems—Design and ConstructionTharaka Gunawardena *and Priyan MendisFaculty of Engineering and Information Technology, The University of Melbourne, Parkville, VIC 3010, Australia;[email protected]* Correspondence: [email protected]: DefinitionModern Methods of Construction with Offsite Manufacturing is an advancement from prefabricated technologies that existed for decades in the construction industry, and is aplatform to integrate various disciplines into providing a more holistic solution. Due to the rapidspeed of construction, reduced requirement of labour and minimised work on site, offsite manufacturing and prefabricated building systems are becoming more popular, and perhaps a necessity forthe future of the global construction industry. The approach to the design and construction of prefabbuilding systems demands a thorough understanding of their unique characteristics.Keywords: offsite manufacturing; inter-modular connections; Design for Manufacturing and Assembly (DfMA); structural design; modularisation; modular construction; panelised construction;connection design—worked example; design for transportation; lifting and handling Citation: Gunawardena, T.; Mendis,P. Prefabricated BuildingSystems—Design and Construction.Encyclopedia 2022, 2, 6Academic Editors: Raffaele Barretta,Ramesh Agarwal, Krzysztof KamilŻur and Giuseppe RutaReceived: 20 November 2021Accepted: 28 December 2021Published: 6 January 2022Publisher’s Note: MDPI stays neutralwith regard to jurisdictional claims inpublished maps and institutional affiliations.1. IntroductionA prefabricated (prefab) building, by definition, is where an entire building or anassembly of its components is manufactured at an offsite facility and assembled onsite fromself-sustained volumetric modules or separate panels. Prefabrication has existed in construction for many decades in various forms such as dry wall systems, structural insulatedpanels (SIP), prestressed beams, prefabricated roof trusses, prefabricated reinforcementcages, etc. [1–3]. Modern Methods of Construction (MMC) with Offsite Manufacturing(OSM) have arisen to integrate these various technologies into a more holistic and systematic solution, and most modern prefab manufacturers will cater for various architecturaldesigns with prefab units (modules or panels) of creative geometries and innovative connection systems. Such prefab units are mass produced in factories with upskilled specialistworkmanship, and at times with automation and robotics in an offsite manufacturingfacility, transforming the traditional site-based and labour-intensive approach to construction. Prefab building units have been widely used for residential, commercial and publicinfrastructure, post-disaster structures and many other applications around the world.Particularly in Australia, the use of modular construction in public infrastructure is a highlight in applications such as railway stations, schools, hospitals, police stations, childcarefacilities, etc. .Offsite-manufactured prefabricated building systems are built in three main types ofconstruction, as listed below:1.Copyright: 2022 by the authors.Licensee MDPI, Basel, Switzerland.2.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/3.Modular (volumetric) construction: manufacturing of fully self-contained units in anoffsite facility to be transported to site to be assembled to form a complete structure.Panelised construction: manufacturing of flat panel units in an offsite facility to betransported to site to be assembled to form a complete structure.Hybrid prefab construction (semi-volumetric): combining both panelised and modularmethods. Compact modular units (pods) are used for the highly serviced and morerepeatable areas such as kitchens and bathrooms, with the remainder of the buildingconstructed using panels or modules.4.0/).Encyclopedia 2022, 2, 70–95. /www.mdpi.com/journal/encyclopedia
EncyclopediaEncyclopedia 2022,2022, 22, FOR PEER REVIEW271shown inin FigureFigure 1,1, prefabprefab structuresstructures cancan bebe ding(includingAs shownengineered woodwood products),products), concreteconcrete oror aa edtotraditionalprefab construction are speed of construction and reduced labour compared to traditionalmethods ofofofprefabconstructionaremethodsof construction.construction. featuresfeaturesprefabconstructionasfollows:are as follows:The reducedreduced needneed forfor labour,labour, especiallyespecially forfor skilledskilled bles r-orientedoperationthe construction process to move away from being a labour-oriented operation totoaamore process-orientedconstructionmoreprocess-oriented to achievereducedon-sitecongestion,wasteand andpollution.tionprojectsto achievereducedon-sitecongestion,wastepollution.The prefabprefab units,units, especiallyespecially volumetricvolumetric ethemainmain ityofprefabunitscontributesstructure for future reuse or relocation. The reusability of prefab units contributessignificantly toto prefabprefab buildingsbuildings havinghaving aa muchmuch lowerlower lifelife cyclecycle energyenergy ..significantly efabpanelsormodulescan Enables construction to commence earlier than usual, as prefab panels arationandfoundationsworksgetcan be manufactured in the factory while onsite preparation and foundations worksunderwayconcurrently.get underway concurrently.Prefab constructionconstruction uctiontimetimebybyatatleastleast50%50%fromfroma asitesite toftheprojectstartsgeneratingintensive building . This ensures that the client of the project starts generating usual. esultingresulting The construction process is significantlyinanevenfasterconstruction.in an even faster construction.Figure 1. A steel modular unit with softwood wall frames (left) and a steel corner supported modFigure 1. A steel modular unit with softwood wall frames (left) and a steel corner supported modularular unit with a cold formed light-gauge steel wall frame (right), showing various materials used inunit with a cold formed light-gauge steel wall frame (right), showing various materials used inprefabricated construction.prefabricated construction.Panelised units are loadbearing in nature, and most of these are integrated with inPanelised units are loadbearing in nature, and most of these are integrated withsulation, cladding and internal plasterboards. Structural insulated panels (SIP) are a popinsulation, cladding and internal plasterboards. Structural insulated panels (SIP) are aular prefabricated panel type in use, especially in countries like Australia. Structural syspopular prefabricated panel type in use, especially in countries like Australia. Structuraltems of modular buildings are developed using two main types of modules according tosystems of modular buildings are developed using two main types of modules accordingtheir load transfer mechanisms , namely:to their load transfer mechanisms , namely:1Loadbearing modular structural systems—The perimeter wall structure of the mod1Loadbearing modular structural systems—The perimeter wall structure of the module carries adequate stiffness and continuous connectivity to transfer the gravityule carries adequate stiffness and continuous connectivity to transfer the gravity loadsloads to the modules below. The roof beams and floor beams are often assisted byto the modules below. The roof beams and floor beams are often assisted by studstud columns to add stiffness to the walls. This wall stiffness contributes to resistancecolumns to add stiffness to the walls. This wall stiffness contributes to resistanceagainstactions yloadingscenariosagainst rackingracking toandfromas moving and handling inside factories, loading and unloading to and from truckstruckstransportationand duringand handling.systemisduringduringtransportationand duringonsiteonsiteliftingliftingand handling.ThisThissystemis onlyonlyfeasiblefor low-riseapplicationsto naturethe natureof loadbearingsystems.feasiblefor low-riseapplicationsdue dueto theof uralsystems—CornercolumnsintheCorner-supported modular structural systems—Corner columns in hemtothecolumnsbelow.Thissystemisalsocapableup the floor loads and transfer them to the columns below. This systemis alsoofresistingloads suchloadsas earthquakeand wind forces—asas the concapableof horizontalresisting horizontalsuch as earthquakeand ,withadequatestiffness,andcomplyingto designtoas the connections are designed properly, with adequate stiffness, and complying
Encyclopedia 2022, 272design standards—thus ideal for multi-storey applications. In most current modularapplications, a structural system such as this, with columns and structural connections,is made from steel.As discussed above, most multi-storey modular buildings found around the worldare assemblies of corner-supported modules that are laterally connected to a cast in situconcrete core. This in situ core effectively acts as the primary lateral load resisting elementand, in many of these, the floors are poured with concrete after installing the modules.Although these methods are innovative and do save construction time initially, they donot characterise the structures as purely modular nor provide them with the previouslymentioned benefits of modular construction. Gunawardena et al.  introduced a newconcept of an advanced corner-supported structural system, where the shear core of thebuilding could be formed by special modules with infill concrete walls. This would allow amulti-storey building to be constructed using corner-supported modules that could laterbe dismantled to construct other buildings elsewhere. Since then, similar concepts haverealised into prefab structural systems in high-rise modular buildings such as the La Trobetower in Melbourne, Australia .2. Approach to Structural Design in a DfMA (Design for Manufacturing andAssembly) FormatAs shown in Figure 2, the design stage of a traditional construction project wouldfollow a path where initially, a conceptual structural design, also known as a schematicdesign, would be drafted. At this conceptual design phase, the design loads on the structureare determined; locations and arrangement of the structural elements are decided; andpreliminary sizing of all structural elements are carried out. This conceptual design wouldproduce structural GAs (general arrangements) that contain a preliminary arrangementand the dimensions of all structural elements with a construction detail of foundations (asfoundations need to be constructed first). These schematic designs would be adequate tobe produced for approvals from councils (or a similar regulatory body), and even to callfor tenders from potential builders. The take-offs for bills of quantities (BOQs) are usuallyobtained from these schematic GAs using rules of thumb to estimate quantities where nodetailed designs are provided (such as the quantities of steel reinforcement). Thereafter, adetailed structural design would be produced, with all the necessary details needed to fullyconstruct the structure (for example, reinforcement details of concrete beams, columns, etc.and connection details of steel and timber structures).The design process of a prefab building takes a somewhat different and an arguablyimproved approach (Figure 3). This is mainly due to the necessity of completing the projectin a much shorter time and also because the structural design takes place centred aroundthe particular prefab builder, while the specific design task may be subcontracted to anoutside structural engineering firm. It is significant how the collaboration of both thebuilder and the structural engineer is critically important in achieving the design of aprefab building. An efficient structural design of a prefab building will only be achievedwith continuous consultation with the builder, and particularly their in-house teams thatlook after construction logistics and procurement. Mainly, two different types of prefabprojects can be identified as they reach the office of a prefab builder or a designer:Type A—Projects that were conceptualised from the early stages to be built as a prefabbuilding.Type B—Projects that were initially conceptualised to be built with traditional constructionmethods (in situ) but are subsequently required to be built using prefab methods.
2 FOR PEER REVIEWEncyclopedia 2022, 2,734FigureEncyclopedia 2022, 2, FOR PEER REVIEW5Figure 2.2. PhasesPhases signdesign processprocess forfor ananin-situin-situ constructionconstruction alongsidealongside otherotherrelatedphase ofof thethe project.project.related activitiesactivities thatthat occuroccur concurrentlyconcurrently withwith eacheach phaseThe design process of a prefab building takes a somewhat different and an arguablyimproved approach (Figure 3). This is mainly due to the necessity of completing the project in a much shorter time and also because the structural design takes place centredaround the particular prefab builder, while the specific design task may be subcontractedto an outside structural engineering firm. It is significant how the collaboration of boththe builder and the structural engineer is critically important in achieving the design of aprefab building. An efficient structural design of a prefab building will only be achievedwith continuous consultation with the builder, and particularly their in-house teams thatlook after construction logistics and procurement. Mainly, two different types of prefabprojects can be identified as they reach the office of a prefab builder or a designer:Type A—Projects that were conceptualised from the early stages to be built as a prefabbuilding.Type B—Projects that were initially conceptualised to be built with traditional construction methods (in situ) but are subsequently required to be built using prefab methods.Figure 3. A typical DfMA (design for manufacturing and assembly) process for a prefab construcFigure 3. A typical DfMA (Design for Manufacturing and Assembly) process for a prefab constructiontion alongside other related activities that occur concurrently with each phase of the DfMA process.alongside other related activities that occur concurrently with each phase of the DfMA process.Out of these, a Type A project would be designed initially by an architect who is quiteknowledgeable and experienced in prefab construction methods and would result in arather smooth process thereafter for the structural designers and the prefab builders. Onthe contrary, converting a design that was initially conceptualised to be built as an in-situbuilding (Type B) to then be a prefab building is a rather tedious and at times a very complex activity.
Encyclopedia 2022, 274Out of these, a Type A project would be designed initially by an architect who isquite knowledgeable and experienced in prefab construction methods and would resultin a rather smooth process thereafter for the structural designers and the prefab builders.On the contrary, converting a design that was initially conceptualised to be built as anin-situ building (Type B) to then be a prefab building is a rather tedious and at times a verycomplex activity.In a nutshell, the design process of an offsite manufactured building has one singledesign phase (Figure 3), and it takes place centred around the prefab builder (contrary toa traditional design, which is centred around architectural and structural design firms).This design phase, when structured properly, takes the form of a Design for Manufacturing and Assembly (DfMA) format. In this DfMA setup, all relevant faculties of thedesign, i.e., architectural, structural, MEP and interior designs, are integrated with thenecessary detailing and instructions for its in-factory manufacturing, transportation andon-site handling and assembly. A well-organised Building Information Modelling (BIM)framework would ideally integrate of all such specialities. The quantity surveying activityfor cost estimation of a traditional construction project gets enriched into a much broaderprocess involving logistics and procurement. Principles of ‘Lean Manufacturing’ havebeen adopted to incorporate logistics management of both in-factory and on-site work inrecent research [9,10] and are gradually being adopted by prefab builders, especially theones that have a degree of automation within their facilities. Concepts from blockchainand cryptocurrency have also been considered in recent research for developing smartprocurement methods and smart contracts to add further efficiencies to prefab buildingprocesses [11–13].2.1. ModularisationOnce an architectural concept or a developed design reaches a prefab builder’s office,the first design activity that occurs is the modularisation of the given floor plans. For arectangular or a similarly simple floor plan, it could be as straightforward as dividingup a given plan into segments, of which the sizes represent the viable modular or paneldimensions. However, every modularisation activity requires the builder to consider anumber of key parameters prior to arriving at his viable modular or panel dimensions andtheir arrangement. Some of these key parameters are listed below:12Architectural constraints—The architectural space arrangements will dictate wherepossible modular splits can be drawn for a modular or flatpack design. Similarly,the locations and dimensions of architectural features such as windows, openings,facades, etc. will dictate where the panel boundaries can be located for a paneliseddesign. A ‘Type A’ design, as discussed previously, will result in a fairly straightforward modularisation process, since an architect who has experience in prefabdesigns will have prepared architectural general arrangements (GAs) to allow formore efficient modular or panel splits. However, if prefabrication was not envisagedfrom the beginning (Type B), the splits would need to be carefully drawn aroundspaces and architectural features, considering how they might impact the rest of themanufacturing, transport, handling and installation processes.Transportation limitations—The capability of the fleet of trucks that a particularprefab builder has access to will dictate the maximum length of a module or panel.While, for example, in Australia, the maximum allowable length for transportation is30 m (Table 1), most trucks can carry modules and panels up to about 16 m in length.The lane widths of the relevant arterial road network will reflect upon the allowedmaximum width for transportation. This may not be a great concern for panels, sincethey are usually transported vertically. However, for modules, this is one of the maindesign limitations. A 5 m maximum is allowed in Australia (Table 1), and similar limitsapply in other countries as well. Depending on a particular project, other dimensionaland weight limits could very possibly apply to the design. Issues such as the conditionof access roads, nearby obstructions and other applicable regulations and constraints
Encyclopedia 2022, 27534567(for example, heritage structures that cannot be demolished) can impose conditionson the dimensions of prefab units. Similarly, if the prefab units are to be shipped,the volumetric and weight constraints of the vessel will also apply, in addition to thetransportation limitations that apply to the embarking and disembarking countries orregions.Lifting and handling limitations—Prefab units need to be designed to dimensionsand weights that are practically feasible when it comes to lifting and handling bothwithin the manufacturing facility as well as at the onsite installation. If modules areto be shipped, the lifting and handling limitations at ports will also apply. Dependingon the fleet of lifting vehicles such as mobile cranes that are accessible to the prefabbuilder, and the site access conditions of the particular project site (which dictates thelifting angles and distances), the weight of each module and panel will need to belimited. At least a preliminary understanding of the lifting and handling strategy of aparticular project needs to be developed for an efficient modularisation.Onsite installation—The sequence of installation and locations and access to structural connections are important parameters to be considered when dividing up astructure into modules or panels. One of the main intentions of a prefab structure is toreduce the necessity for skilled trades to be on site. Therefore, the onsite installationneeds to reduce skilled activities such as welding to a minimum. The connections alsoneed to be safely and conveniently accessible. The arrangement of prefab units needsto be carried out with these in mind.Manufacturing process—The modularisation activity needs to consider the dimensions of various activity stations of the manufacturing plant and its sequence ofoperations. The prefab units need to move around the facility from one station to theother without the need to restructure any of their geometries and without hinderingthe normal operations of the factory.Preliminary locations of structural columns and connections—A preliminary ideaof the required structural design needs to be considered during the modularisationactivity. The location of columns and their structural connections is a critical aspectof efficient modularisation. In contrast to a traditional structural design, the locationof structural columns will not only relate to the eventual structure, but also to thestructural integrity of a given prefab unit during its transportation, lifting and handling stages, since the lifting connectors should ideally be located on the structuralcolumns.Preliminary locations and dimensions of service spaces (MEP)—A preliminary ideaof the required spaces for mechanical, electrical and plumbing (MEP) services suchas service ducts and ceilings and heavily serviced areas such as toilets and kitchensneed to be known at the modularisation stage. Modular and panel splits will needto be arranged accordingly, and any necessary alterations could be sorted via a wellformed BIM platform or by communicating with the architects and building servicesengineers.Table 1. Dimension and weight limits to be followed in Victoria, Australia by vehicles transportinghouses and prefabricated buildings. Adapted from ref. .1DimensionMaximum LimitWidthHeight 1LengthWeight 25.0 m5.0 m30.0 m43 tonnesThe total allowable height includes 1.2 m of trailer deck height (i.e., the height of modules or panels need tobe less than 3.8 m). 2 The total allowable weight includes the weight of the steer axle concession, which variesaccording to the vehicle.
Encyclopedia 2022, 276The abovementioned parameters in a modularisation activity will affect key decisionvariables that depend on these parameters. Cost and logistical items such as labourand equipment requirements related to the manufacturing process, lifting and handling,transportation and any other part of the project will depend on the outcomes of themodularisation activity.2.2. Structural DesignAs identified in the Australian design standards, and similarly stated in many international codes of practice and guidelines that use a ‘limit state design’ approach, anystructural design should comply with three main design criteria, namely:123Stability;Serviceability; andStrength.Structural design standards found elsewhere in the world follow the same principles but may identify these criteria under various different terms (for example, the term‘Ultimate limit’ is used instead of ‘strength design’ in many standards). None of thesewould and should change in designing a prefabricated structure. While prefabricationand offsite manufacturing are the methods used to replace traditional construction, theend product is still a building used to serve the same traditional functions. Therefore, theauthors, in principle, see no particular need to have separate modular or prefab designstandards. However, the authors are quite supportive of modifying the already existingstructural design standards to incorporate modular and other prefab construction methods.A good example for such an inclusion of prefab concepts into a traditional design standardis the latest version of the Australian concrete design code, AS3600:2018 , where newclauses were added to provide guidelines on minimum levels of strength and safety toprefabricated concrete structures, especially in the design of connections .As previously discussed, the structural design of a prefab project is carried out in onesingle phase, although the specific tasks related to conceptual design and detailed designare still mostly distinct.In terms of identifying the loads acting on a given prefab structure, as in any traditionalstructural design, the usage and importance level need to be determined according to itsarchitectural design and the relevant codes of practice (for example, for structures built inAustralia, AS1170.0:2002  provides guidance on choosing importance levels according totheir intended purpose and usage, with further guidance on load combinations and returnperiods (probability of exceedance); AS1170.1:2002  provides guidance on minimumlive loads; AS1170.2:2021  provides guidance on minimum wind loads to consider andAS1170.4:2007  provides guidance on minimum earthquake loads to consider).The selection of most construction materials for facades and other finishes will bedecided according to the architectural design. However, the structural design being centredaround the prefab builder becomes a significant factor affecting the choice of materialfor the main structural elements. Depending on the normal practice of the particularprefab builder, the main structural form could be either concrete, timber, steel or a mix ofthese, and this extends to partition wall frames and the building envelope as well. Thedecision to go for cold-formed steel frames or softwood frames, or another form, such asSIP (structurally insulated panels), mostly depends on the normal practice of the prefabbuilder. Having prior knowledge of the finishing materials and their weights adds an extraadvantage to the overall structural design, since there would be minimum uncertainty onhow heavy some of the finishes and cladding would be (especially since there is minimalopportunity for these choices to be changed later, as the project timeline is very short). Theexact values of their weights with a lower factor of safety can be used in determining someof the superimposed dead loads such as cladding, partitions and finishes.The general arrangement of structural elements will take a different approach to atraditional conceptual structural design. The location of structural columns, especially fora modular building design, would already be decided during the modularisation activity.
Encyclopedia 2022, 277The floor and roof elements would then be designed within the frame of each module.There could still be many structural components that need to be built in a traditionalform, such as foundations, basements, shear walls and podiums, and they would followthe traditional format of design and would usually be built by separate contractors. Apanelised building could have even more structural components that need to be built insitu. However, as far as the structural design of the panelised parts is concerned, the designapproach would be similar to that of a modular building.The abovementioned steps of a conceptual structural design of a prefab building arestill principally similar to those of a traditional in situ building. However, there are a fewfurther unique steps required for a prefab building design. These unique requirements ariseas a result of the structural design being centred around the prefab builder, as discussedpreviously. Firstly, similar to the modularisation activity, the conceptual structural designneeds to consider the strategy for the transportation, lifting and handling of the prefabunits within the manufacturing facility and afterwards, following a DfMA format. DfMAconsists of two components, namely, Design for Manufacturing (DfM) and Design forAssembly (DfA) [21–23]. The arrangement of structural elements and frames need to becompatible with the existing factory setup and manufacturing (fabrication) process of theprefab builder. Some activities, such as framing of steel or timber modular floor and ceilingframes, could be fully or partly automated within an offsite manufacturing facility. Thestructural design should be carried out in a way that a
building, the advantage of knowing the ﬁnishes with more certainty could result in more accurate predictions. 3.2. Design for Transportation, Lifting and Handling The entire design process of a prefabricated building needs to be comprehensively aligned with the transportation, lifting and handling strategy of a given project and the