The term ‘modern methods of construction’ (MMC) encompasses a wide range of building products and components that are predominantly prefabricated off-site and transported to building sites. This approach facilitates more rapid construction than with ‘traditional’ construction methods (Figure 1). MMC can include panelized structural and non-structural wall and floor systems, panelized facade systems, volumetric modular systems (VMS), etc. (e.g. WRAP 2007; RICS 2018; MBI 2019; AIA & NIBS 2019). Such components can be constructed using a variety of materials, including concrete, timber, plastic, and steel, and in combination as composite systems (Meacham & McNamee 2020).
Use of MMC is widely touted as providing significant economic and sustainability benefits. MMC afford the ability to capture the efficiencies gained by integrating the processes and technologies of design, manufacturing, and construction. In addition, MMC can result in higher quality buildings, delivered in shorter time frames, with more predictable costs, and fewer environmental impacts. Industry assessments reflect an increased use of MMC in recent years and project even higher use in the near future (e.g. Bertram et al. 2019; Buckley et al. 2020). The benefits may be reflected as pre-manufactured value (PMV), defined as value that is created as a result of completing work away from the site (CLC 2018). It is calculated by taking the gross capital cost of the project and deducting the site overhead costs and site labor costs. The result of this is then divided by the capital cost and is reflected as a percentage.
However, MMC come with some concerns, particularly with respect to fire safety performance and its assurance through the regulatory process. In a recent information review on fire safety challenges of ‘green’ building attributes (Meacham & McNamee 2020), a range of potential fire safety concerns was identified with MMC. For some types of panelized systems and all types of volumetric construction, there are concerns with void spaces necessitated by the construction process. A major fear here is the spread of smoke and hot gases, which could lead to life safety impacts in parts of the structure remote from the area of fire origin, in some cases also contributing to fire spread. In situations where connections between MMC components, or MMC components and the structural system, are unprotected in such void spaces, significant fire penetration into the void could lead to failure.
In the case of panelized construction, a wide range of concerns has been identified across material types. These include aluminum composite material (ACM) panels and other metal composite systems with combustible cores (e.g. McLaggan et al. 2021), structural integrated panels (SIPs) composed of oriented strand board (OSB) and an expanded polystyrene (EPS) core (Meacham et al. 2017), mass timber systems (Hoehler et al. 2018; Meacham & McNamee 2020), and high-strength lightweight concrete systems (Phan & Carino 2000). While some of the challenges can be mitigated by fire protection measures when the buildings are complete, many are at risk during construction.
The Grenfell Tower fire in London is a well-known example of the poor fire performance of a form of panelized construction, in this case exterior cladding and insulation systems. A significant amount of information about this fire is available in evidence compiled as part of the Grenfell Tower Inquiry (2022). It was suggested by one of the expert witnesses, Luke Bisby, that the primary cause of rapid and extensive vertical and horizontal external fire spread was the presence of polyethylene-filled ACM rainscreen cassettes in both the building’s refurbishment cladding system and the architectural crown detail. Secondary factors that may have contributed to the fire’s vertical upward, vertical downward, and horizontal spread include the use of combustible insulation products within the cladding system, and the presence of extensive cavities and vertical channels within the cladding system itself, among other items (Bisby 2018). Concerns such as these are present in many cladding systems with combustible components and materials (e.g. McLaggan et al. 2021).
In addition to fire safety performance concerns, MMC can present challenges to current building and fire regulatory paradigms. These largely assume traditional construction practices, and often include inspections during construction to assure proper materials are used, fire protection measures are in place, etc. With some forms of MMC, many building services installations, such as the installation of wiring, plumbing, and other services, are undertaken at the point of fabrication. In such cases, the components are sealed from view with respect to on-site inspection. Some countries have already produced guidance to try to address such concerns (e.g. BCA 2020; ICC/MBI 1200 2021), while others are still developing appropriate guidance. The challenges associated with obtaining building permits and finding qualified contractors for certain types of MMC are just being explored in some countries (e.g. Dorrah & El-Diraby 2019).
The fire safety and regulatory challenges are arguably a function of two factors: (1) many forms of MMC are complex systems (or systems of systems); and (2) the introduction of MMC has occurred more rapidly than some regulatory systems can accommodate. It has been suggested that adequate design, regulation, and safety management of complex systems requires socio-technical and safety systems thinking and approaches (e.g. Meacham & van Straalen 2018; van Hees et al. 2020; Meacham et al. 2020; Meacham 2022). Without such considerations, the fire performance of complex systems may not be well understood, and the building regulatory system may have some ‘blind spots’ to the potential failures that can result from the lack of understanding.
The current paper reviews the literature around current fire performance and regulatory considerations in respect of MMC. It illustrates the types of fire concerns that exist, what mitigation approaches might be used, and where additional research is needed. It also explores the regulatory challenges with assuring regulatory compliance with those systems and components fabricated off-site, reflecting current practice in a sampling of countries, and drawing insight from QA and/or QC approaches used in other industries.
A focused literature review and limited survey of building regulatory entities were conducted. The aim is to frame the situation, and explore regulatory approaches, to address potential challenges with assuring fire safety and regulatory compliance in buildings that use MMC. For the literature review, keywords used for the purpose of searching out relevant published sources reflected a range of MMC construction typologies, construction material types, fire performance indicators, and building regulatory system terms.
Keywords for MMC construction typologies included: panelized structural and non-structural wall and floor systems, panelized facade systems/construction, prefabrication/prefabricated systems/modules, structurally insulated panels/SIPS, permanent modular construction, and volumetric modular construction. Material types included concrete, steel, timber, and plastic (fiber-reinforced polymers—FRPs, resins, etc.). Fire performance indicators included cavity/void spaces, connections, surface flame spread, smoke production and spread, resistance to fire, containment, compartmentation, cables, penetrations, seals/sealants, detection, suppression, and sprinklers. Building regulatory system terms included building codes/regulations, fire codes/regulations, fire risk assessment, review, approval, inspection, on-site, off-site.
A survey was conducted of members of the Inter-jurisdictional Regulatory Collaboration Committee (IRCC). The IRCC is an unaffiliated committee comprised of representatives from primary national lead regulatory and/or research entities that develop or support the development of the country’s building regulatory system. The focus of the survey was to obtain information and experience regarding how MMC are treated by building and fire regulations in the review, inspection, and approval processes, and whether it is anticipated that new or different approaches may be considered in the future.
Each of the above topics could warrant an in-depth review, and in many cases such studies have been conducted, e.g. MMC typologies (e.g. NHBC 2018; Ferdousa et al. 2019; MBI 2019; Bertram et al. 2019), and fire spread in cavities/void spaces (e.g. Choi & Taylor 1984; Rogowski 1985; Meckler 1987; Kurzawski & Ezekoye 2014; Shipp et al. 2015; Livkiss et al. 2018). However, the intersection of MMC, fire performance concerns, and regulatory controls has not been explored in significant depth, and it is this confluence of issues, the potential for unintended consequences, and strategies for mitigating the concerns that is explored herein.
Fire performance challenges of MMC typologies can vary based on such aspects such as where and how MMC components are fabricated and connected, the fire performance of materials, how connections are made at the construction site, and how the constructed assembly is fire protected. To provide a common basis for discussion, a first step is to define a taxonomy.
The literature review identified numerous resources that present typologies of MMC. In some cases there is overlap, while in other cases a different approach is taken. In their state-of-the-art review, Ferdousa et al. (2019) present a table of ‘classification of modular buildings’ that has a particularly material-focused approach (e.g. steel-framed modules, light steel-framed modules, and timber-framed modules). Others, such as the NHBC (2018), focus more on the configuration of the assembly of materials (e.g. panelized systems, VMS), with some examples of materials. This approach was also taken by the Waste & Resources Action Programme (WRAP 2007), the Modular Building Institute (MBI) (2019) in their definitions section, the American Institute of Architects and the National Institute of Building Sciences (AIA & NIBS 2019), McKinsey (Bertram et al. 2019), and Dodge Analytics (Buckley et al. 2020). The UK government has recently published an MMC definition framework that combines aspects of the two approaches (Cast Consultancy 2019).
For present purposes, the configuration-focused taxonomy is adopted, with specific reference to materials in certain cases. Table 1 reflects this taxonomy, which draws significantly from NHBC (2018), with modifications and additions based on other resources (Figure 2).
|Cassette||Prefabricated wall, floor, or roof sub-assembly||NHBC (2018)|
|Closed panel||Panels based on a framing system which are closed on both sides and may include factory-fitted insulation, windows, doors, services, and internal wall finishes. Panels can be structural or non-structural||NHBC (2018)|
|Composite system (element, assembly)||System, element, or assembly composed of two or more materials, often to provide stiffness or other structural property (such as steel and concrete floor systems; e.g. Liew et al. 2019), fiber-reinforced polymer (FRP)-steel composite floor systems (e.g. Satasivam & Bai 2016), cross-laminated timber (CLT) and concrete floor systems (Siddika et al. 2021)||Author/as noted|
|Hybrid systems||Construction that combines a volumetric pod system with a panelized system||NHBC (2018)|
|Modular component||Sub-assembly, subsystem, or combination of elements, including panelized systems, building shells, or bathroom pods, for use as a part of a modular building that is not structurally independent, but is a part of structural, plumbing, mechanical, electrical, fire protection, or other systems affecting life safety||ICC/MBI 1200 (2021)|
|Module||Three-dimensional, volumetric section of a modular building designed and approved to be transported as a single section, independent of other sections, to a site for on-site construction (see also prefabricated prefinished volumetric construction (PPVC) and volumetric modular systems (VMS) below)||ICC/MBI 1200 (2021)|
|On-site MMC||Construction processes carried out on the building site, but which use different processes, technologies, and systems from those used for ‘traditional’ construction. On-site MMC includes thin-joint blockwork, large format blockwork, insulated formwork, and on-site factories||NHBC (2018)|
|Open panel||Panels based on a framing system where all framing members are visible until the wall linings have been fitted on-site. Panels can be structural or non-structural||NHBC (2018)|
|Panelized systems||Building systems comprised of two-dimensional units that are typically manufactured off-site and assembled on-site. Panelized systems include timber frame, light-gauge steel frame, cross-laminated timber and structural insulated panels:
||Adapted from NHBC (2018)|
|Prefabricated prefinished volumetric construction (PPVC)||Specific type of modular construction where the internal elements of the modules (walls, floors and ceilings, mechanical, plumping and electrical, etc.) are prefinished before the modules are installed (see also VMS)||BCA (2020)|
|Volumetric modular systems (VMS)||Building systems composed of three-dimensional units (volumetric modules) produced in a factory and fully fitted out before being transported on-site (see also PPVC)||NHBC (2018)|
|Volumetric pod/pod||Volumetric module forming a single room, most commonly a kitchen, bathroom, or utility space. They can be used either with traditional construction or as part of a hybrid system||NHBC (2018)|
A common aspect of MMC is the prefabrication of modular components of the building, be they closed panels, prefabricated prefinished volumetric construction (PPVC)/VMS, or pods, and the need to connect the prefabricated components together for structural, building services, safety, and aesthetic performance needs. The fit of components will vary by construction, but in several typologies, void spaces (cavities) may be required to facilitate connections.
Concerns about the spread of fire, smoke, and hot gases through void spaces (concealed spaces, cavity spaces) is not new (e.g. Choi & Taylor 1984; Rogowski 1985; Meckler 1987; Kurzawski & Ezekoye 2014; Shipp et al. 2015; Livkiss et al. 2018). Early work focused on smoke spread to locations remote from the fire origin, as well as the potential for ignition and fire spread with combustible insulation materials. As reported by Shipp et al. (2015), fire and smoke spread in concealed spaces within buildings can present a life risk to both occupants and firefighters, and cause widespread damage with extensive, difficult, and expensive clean-up and reinstatement. The particular focus of the Building Research Establishment (BRE) study (Shipp et al. 2015) was void spaces under roof assemblies: a horizontal fire and smoke spread potential. More recently, fire in vertical flow configurations has again become a focus of study, in this case largely around void spaces in facade systems (e.g. Livkiss et al. 2018).
From a research perspective, fire concerns with void spaces associated with the construction of VMS buildings and other forms of MMC systems do not appear to be well studied. In part, this could be a function of the fact that fire and smoke spread in void spaces is a known concern. However, it might also be related to MMC being in many ways, in some countries, still a new construction approach, and not as widely used as traditional approaches.
Nonetheless, the potential fire concerns exist, and anecdotal examples of fire spread in void spaces of VMS buildings has been presented (e.g. Gallagher 2009; CROSS 2021). One example of fire and smoke spread in VMS is a fire that occurred in a single-family, modular construction house in the US state of Massachusetts (Gallagher 2009; Fire Engineering 2011). In this event, it was alleged that the hot gases were transported within the void spaces that connected the modules of the lightweight timber frame system. Glue used to connect components within the modules, including fire-resistive gypsum covering, was ignited, fire resistance was lost, and ultimately the structure was engulfed in fire.
The US fire reflects similar concerns as reported by the UK Collaborative Reporting for Safer Structures (CROSS), which is a confidential reporting system for fire safety and structural safety issues related to buildings and other structures in the built environment. In 2021, CROSS published a safety report on volumetric modular buildings and fire (CROSS 2021). The stated concern is summarised as:
the existence of extensive cavities within the compartments, combined with the lack of appropriate care when connecting the modules could lead to the concealed spread of fire and smoke.
Some fire events within VMS construction were provided as examples.
One of the examples cited in the CROSS report was the Moorfield Hotel in Brae, Shetland, UK, which burned in July 2020 and was previously reported on by the RISC Authority in the UK (Abley 2021). This 100-room hotel had been manufactured in Northern Ireland using SIPs. The SIPs in this case were timber boards facing a core of combustible foam insulation. The SIPs were fabricated into PPVC components, thus resulting in MMC modules (PPVC) composed of MMC subcomponents (SIPs). The RISC Authority article states that the Fire and Rescue Service were unable to control the fire when it got into the cavities between modules and within the SIPs.
Fire sealants and fire stop materials can be used in void spaces to help rectify this concern. However, they must be tested and approved for the specific intended application. As noted by Gerard et al. (2013), some test standards for firestop/sealant material may need to be altered to allow for the tests to be carried out with combustible bases or substrates. Current test standards are based on the fire rating of the wall or floor system in which they are to be installed (e.g. ASTM E814-13a 2017). Such fire-rated wall and floor systems are typically noncombustible. This may pose a particular challenge for mass timber construction, especially if unencapsulated timber is desired. Since mass timber is combustible, fire sealant and firestop manufacturers would need to test a range of products that can then be used in such combustible floor and wall systems. Ultimately, fire-stopping/sealing for penetrations and openings in combustible structures has to achieve the performance requirements for the assembly in which they are used. This requires that the entire assembly, including the product and combustible material it penetrates, achieves the appropriate performance when exposed to the fire testing protocols.
In summary, void spaces are often required for connecting different types of MMC systems and components. If unprotected or inadequately protected, these void spaces, which can be rather larger, can facilitate the spread of hot gases, smoke, and flame. In cases where combustible materials are within or adjacent to void spaces, combustion of those materials could occur.
Fire performance concerns with materials is largely related to three areas of MMC: combustible structural elements, combustible surface lining, and combustible insulation or void filling materials. In many regards these are not particularly different than in traditional methods of construction. However, because of the prefabricated and/or encapsulated nature of MMC components and systems, there can be enhanced concerns. In particular, control of materials used in void spaces (in terms of inspection and verification of material), volume of void spaces (to facilitate flame and smoke spread), and penetrations into void spaces in closed panel and SIP type systems.
As noted above, cladding systems with combustible components and materials can exhibit any or all of these attributes, which can contribute both to concealed and surface fire spread (e.g. McLaggan et al. 2021), as was evident in the Grenfell Tower fire (Bisby 2018). The challenge is that while individual components may meet fire performance requirements under defined test conditions, the fire performance of an assembly (or system) of components may be different due the combination, arrangement, and amount of materials that comprise the assembly. This has been recognized for some systems, such as cladding systems, and efforts to transition to full assembly (system) testing has been recognized (Bostrom et al. 2018; Anderson et al. 2018; van Hees et al. 2020; McLaggan et al. 2021).
It seems likely that the Moorfield Hotel fire reported on by the RISC Authority, as noted above (Abley 2021), experienced fire and smoke spread facilitated by combustible insulation in the SIPs. However, this detail is not reported and cannot be verified. However, this type of ignition and fire and smoke spread in SIP construction was verified in real-scale fire testing at Worcester Polytechnic Institute in the US (Meacham et al. 2017). In this case, SIPs constructed of OSB with EPS insulation were tested in a laboratory to assess fire vulnerability. Tests were conducted in a specially designed test rig that allowed for gas burners to be used in developing a repeatable fire up to 5 MW in size, located within compartments constructed of traditional ‘stick frame’ timber and OSB and EPS SIP panels. In one test that involved SIP wall and roof system components, the inner OSB skin on the SIP roof ignited following the loss of the ceiling gypsum board. The fire then penetrated and burned the EPS foam core. Melting of EPS foam exposed the inner face of the outer OSB skin and resulted in a loss of composite action of the SIPs, thus reducing flexural stiffness of the structural system. Exemplar photographs from the tests are presented in Figures 3a and 3b. This is an indicator of the potential for structural collapse (testing was stopped before collapse to maintain the integrity of the test rig).
Mass timber is another area that warrants attention. Additional emphasis is placed on this construction type within this paper due to the growing use of mass timber in construction. Mass timber is a term that encompasses several mechanisms in which smaller pieces of timber are connected to form large panels of timber sections. These include glue-laminated timber (glulam), cross-laminated timber (CLT), nail-laminated timber (NLT), dowel-laminated timber (DLT), laminated veneer lumber (LVL), and laminated strand lumber (LSL) (e.g. Meacham & McNamee 2020). Mass timber construction falls generally into the panelized and composite categories, but can be post and beam and hybrid construction as well. For tall mass timber buildings, cores may be concrete, floor systems concrete and mass timber, and walls mass timber panels.
Fire performance of mass timber systems has received significant attention (e.g. Hakkarainen 2002; Frangi et al. 2008; Klippel et al. 2014; Hagiwara et al. 2014a, 2014b; Su & Lougheed 2014; Suzuki et al. 2016; Hoehler et al. 2018; Su et al. 2018; Brandon 2018, 2020; Brandon et al. 2021). A significant number of reports on fire safety performance of tall wood buildings can be found on the websites of industry groups and research institutions. In North America, for example, this includes the American Wood Council (AWC) (2022) and the Canadian Wood Council (CWC) (2022), and the US National Institute of Standards and Technology (NIST) publications web portal (NIST 2020), the National Research Council (NRC) Canada publications web portal (NRC 2022a, 2022b, 2022c), and the Fire Protection Research Foundation (FPRF) (2022). Similar is true for various research entities around the world, such as the Research Institutes of Sweden (RISE) (e.g. Brandon 2018, 2020; Brandon et al. 2021), ETH Zurich in Switzerland (e.g. Frangi et al. 2008; Klippel et al. 2014; Hozjan et al. 2019; Voulpiotis et al. 2021), and the University of Queensland in Australia (e.g. Emberley et al. 2017; Cuevas et al. 2020; Nothard et al. 2022). A recent comparison of fire tests with CLT in particular provides many insights to performance and testing challenges (Liu & Fischer 2022) (Figure 4).
While research and test programs have demonstrated good fire performance under defined scenarios, with little challenge to structural resilience or fire spread if fully encapsulated (e.g. protected by noncombustible gypsum wallboard), some concerns still remain (e.g. RISC Authority 2022). This is particularly the case with respect to connection performance (Peng et al. 2012; Petrycki & Salem 2019), cavities where pyrolysis leading to smoldering combustion may occur if sufficient thermal energy penetrates into the space (Hadden et al. 2017), and surfaces where exposed timber is still desired (and delamination and continued burning could occur) (e.g. Su et al. 2018; Ni & Gernay 2020). These aspects are particularly concerning in countries where there is a regulatory requirement for a structure to survive burn out of the fire (combustion of all available fuel, including any contributed by the construction materials, without subsequent collapse or fire spread). A recent overview of these challenges can be found in Ni & Gernay (2020).
In summary, several types of MMC may include combustible materials, which if ignited can potentially spread flame, decompose, and/or lose structural strength. Fire protection measures are available to offset these concerns. However, it is not always easy to determine if adequate fire protection has been provided, and in some systems, such as mass timber, unprotected systems are desired. The performance of unprotected mass timber continues to be researched.
A major reported benefit of MMC over traditional construction is the ability to prefabricate modular components (closed-panel systems, PPVC/VMS, pods) off-site, often in a factory, where many aspects of fabrication can be quality controlled. Another benefit is that MMC can often be assembled into a building in a much shorter time than with traditional/conventional construction methods. As with traditional construction, however, the prefabricated components need to be connected on-site to provide structural stability, and for many modular components to connect the building services that are built into the modular components (e.g. electrical cabling, plumbing). Unsurprisingly, the overall structural stability and robustness of the assembly of modules are significantly influenced by the behavior of module-to-module connections (Ferdousa et al. 2019).
Fire-induced failure of structural connections is not a new issue. Investigations into the fire-induced collapses in structures, including the collapse of World Trade Center 5 building in 2001 (e.g. LaMalva et al. 2009), indicate that failures can originate at connections and trigger the collapse of floors, which can in turn lead to the collapse of the entire structure (Pakala et al. 2012; Lange et al. 2012; Sun et al. 2015). Fire-induced failure of connections can occur with any construction material, and adequate fire protection is always an important factor. Materials such as concrete (Mohd Radzi et al. 2020) and steel have been well studied, and fire protection methods are generally straightforward. However, timber can create challenges. If the connection contains a heat-conducting material, such as steel, there may be ignition concerns (or at least loss of connection due to loss of mass of the timber). There may also be challenges with the adhesion of fire protection materials to the timber.
In fire conditions, the time to failure of a timber connection is mainly reliant on the wood charring rate, the strength of the residual wood section, and the limiting temperature of the steel connectors involved in the connection (Petrycki & Salem 2019). In some fire-resistance tests on bolted wood–steel–wood (WSW) connections and bolted steel–wood–steel (SWS) connections, the fire-resistance ratings of all the tested WSW and SWS connections without protection were less than the target rating for code compliance, with the fire performance of the tested SWS connections being about half that of the WSW connection (Peng et al. 2012).
In summary, fire performance of structural connections is a known issue. While a good understanding of fire protection of noncombustible materials, such as concrete and steel, exists, the understanding of mass timber connections is still developing. With mass timber one concern is that low energy (smoldering) combustion can occur within the timber, in void spaces and behind encapsulating cover (e.g. gypsum), which can be difficult to extinguish. Long-term combustion can weaken the connections, which could lead to a potential failure. Since timber combustion occurs at a lower temperature than other materials (e.g. concrete), such ‘hidden’ combustion can continue after visible fire in a compartment has been extinguished. As such, controlling for this, and combustion of timber in general, is important (Law & Haddon 2020).
There are many different forms of building regulatory systems, the composition of which are dependent upon several factors. These include the legal system, the regulatory system structures (development and implementation), and the regulation type (e.g. prescriptive, performance, objective, or functional based). It also includes the ‘regulatory system infrastructure’, which includes material and product testing, approval and certification (listing), building control system (e.g. permitting, plans review, inspections), licensing schemes, insurance, etc. There are benefits and detriments, and opportunities and challenges, with each component, attribute, and combination of system components and attributes. These are well-described in the literature (e.g. Meijer 2002; Sheridan et al. 2003; Duncan 2005; Lundin 2005; CEBC 2006; May 2003, 2007; Meacham et al. 2005; Meacham 2009; Mumford 2010; Imrie & Street 2011; van der Heijden & De Jong 2013; Burgess & Thomson 2014; World Bank 2015; Meacham & van Straalen 2018; Meijer & Visscher 2017; DHCLG 2017, 2018; Shergold & Weir 2018; Meacham et al. 2020).
For current purposes, two primary building regulatory typologies are considered: prescriptive- and function-based (which for this discussion includes objective- and performance-based typologies). Material/product/system testing, and approval and certification, are explored at the individual material/component level, and at the ‘final’ assembly/system level. At the individual materials and components level, considerations range from assembled panel sections (e.g. CLT wall unit) to structural connection components, to electrical cable, pipes, and plumbing fixtures. At the final ‘system’ level, examples include closed panels, composite floor systems, PPVC, volumetric pods (pod), and VMS. With respect to building control, the primary typologies considered are government control, private sector control (e.g. private certifiers), and self-certification, recognizing that some jurisdictions include all forms.
Discussion around these regulatory system components is focused on three areas. First, how the different MMC components are considered by different regulatory typologies. Second, how regulatory compliance/performance verification for ‘subsystem’ performance is addressed (e.g. materials and components assembled into closed panels, pod, VMS). Third, how control for regulatory compliance/performance verification for the installed modular components is addressed (e.g. when closed panel systems are connected, pods are connected, and VMS are connected to form a building or part of a building).
Most building regulatory systems have a somewhat complex set of interconnected regulatory and private-sector checks and balances for evaluating and accepting for use materials and systems that, when connected together, result in a completed building. These checks and balances begin with testing and approval (certification, listing) of materials, equipment, and systems that go into buildings, from structural materials and systems to building services, and interior and exterior finishes. Testing ranges across a broad spectrum of performance measures, including strength, fire, energy, and acoustics. Testing may be required at the component and ‘subsystem’ or assembly levels (e.g. the fire performance of a door, and of a door system in a wall assembly). With appropriate certifications (approvals, listings), the materials and assemblies are specified for use in building design.
Depending on the prevailing regulatory system, combinations of certified materials and systems are integrated into the overall building design, which may then be subject to approval as the ‘building’ design. Materials and systems may further be subject to inspection during construction for compliance with appropriate approvals/fitness for purpose. At the site inspection stage, inspectors can check for the appropriate indicators of product and system certification (e.g. CE mark, UL label, etc.). Such marks are indicators that that the performance of the material is certified to have passed appropriate testing. Site inspection may also check that installation is according to regulations and design documents (e.g. structural connections, electrical connections, plumbing connections, thermal penetration seals (for energy), fire stop/fire sealant materials and location (for limiting smoke and fire spread), etc.). In some jurisdictions, some level of systems testing or commissioning is required before the certificate of occupancy is issued to demonstrate that the building systems work as expected to deliver the design performance (e.g. a door fan test for leakage, a smoke test for the smoke exhaust system).
MMC can present challenges with the on-site inspection and approval of some systems and components, in the traditional sense, as many of the MMC components are closed from view of the inspection entity. Closed-panel wall systems, pods, and VMS/PPVC systems may include electrical wiring and conduit sections, plumbing system sections or components, fire system sections or components, thermal insulation, and the like, all sealed from view, except at points of connection to adjacent modules. In addition, these points of connection (structural and building services) may not be fully visible and/or accessible, especially after connections are made. Without the ability to control for appropriate materials and systems within the MMC components on site, alternative means of compliance/performance verification may be warranted. This places more of an emphasis on having a robust system for control at the point of fabrication (Figure 5).
Since a key attribute of MMC is the prefabrication of panels, systems, and modules off-site, often in a factory setting, one alternative approach is to allow for inspection and approvals to occur within the factory by the building regulatory official. This can be either for each individual component or system, or on an audit basis in concert with an approved quality management (QM) approach. As per ISO 31000 (2018), QM incorporates QA, which is focused on providing confidence that quality requirements will be fulfilled, and QC, which is focused on fulfilling quality requirements. Stated another way, QA relates to how a process is performed or how a product is made, and QC is more about the inspection aspect of QM.
Two important aspects of a QA/QC program are inspection and audit. Inspection refers to the measuring, examining, and testing that is necessary to assess one or more characteristics of a product or service, and the comparison of these characteristics with specified requirements to determine conformity. The audit process serves to compare actual conditions of products or services be produced with the stated requirements. It is typical that in an approved QA/QC approach, the manufacturer has the responsibility for compliance, some number of inspections is made to check that the specified product is being produced, and only a defined number of samples of the component or systems is assessed as part of the audit process.
In principle, a QA/QC approach for MMC would require the manufacturer of the component or system to put in place a suitable and auditable system to control for the appropriate use of properly certified/approved materials in component/system fabrication. The QA/QC system would also have to assure that the fabrication is in accordance with building regulatory (and other regulatory) requirements. Finally, the QA/QC plan should be approved/accepted by the authority(ies) having jurisdiction (AHJs).
While not particularly different in concept than QA/QC of components or systems that would be used in traditional means of construction, the difference here is that multiple materials and systems, which might be considered separately in traditional construction (e.g. structural, energy, fire, plumbing), are all included in the module QA/QC process. The process can become complex when certain MMC components or assemblies are fabricated in jurisdictions other than where the final building will be located, in particular if the regulatory system and oversight in those jurisdictions are less robust than where the building will be erected. In such cases, arrangements might be needed to have the fabrication comply with the ‘end jurisdiction’ requirements to fulfill the QA/QC requirements.
Another approach is to operate the regulatory system ‘as normal’ but raise the expectation of actors in the system to control for the peculiarities associated with MMC. This might include implementation of governmental certification schemes, inspections or audits during construction, or solely through private sector responsibilities (i.e. where the client is responsible for regulatory compliance without specific government verification/responsibility). The following reflects a range of approaches used in different jurisdictions with different regulatory system structures.
A prescriptive building regulatory system is typified by regulations that are very detailed and specific about what must be done for compliance, and often have strong governmental oversight (i.e. review and approval processes). An example is the building regulatory system in the US. In particular, the US system is prescriptive based, with model codes and standards developed in the private sector and adopted into law (regulation) at the state or local levels. Consensus standards, which are cited by the codes, become adopted by reference. The standards, if not cited in the building code, can be applied voluntarily and/or required by particular jurisdictions. Building code compliance (building control) and often fire code compliance are largely undertaken by building departments at the state or local levels (and sometimes both) depending upon the state. Building departments are generally involved in the review of building designs, issuing building (construction) permits, site inspections, and issuance of certificates of occupancy (e.g. Meacham 2009, 2014).
In the US system, building (and fire) code officials rely significantly on compliance with prescriptive requirements in the codes and standards. As such, having requirements embodied into codes and standards is the preferred approach. For the MMC challenge, the International Code Council (ICC) took the approach of developing a standard that requires the use of a QM system with audit approach. The requirements are embodied in the ICC/MBI 1200, Standard for Off-site Construction: Planning, Design, Fabrication and Assembly, published US (ICC/MBI 1200 2021).
The purpose of the ICC/MBI 1200 Standard is “to provide minimum requirements to safeguard public health, safety, and general welfare, and to address societal and industry challenges for the inspection and regulatory compliance of off-site construction.” It is intended for adoption by government agencies and organizations for use in conjunction with model codes (i.e. the International Building Code (IBC) and the International Fire Code (IFC) to achieve uniformity in the inspection and regulatory compliance of off-site construction. Where adopted into law, this standard requires, among other factors, the following:
In this system, the manufacturer of the module is responsible for building regulatory compliance (e.g. to the IBC, IFC, and other relevant codes and standards), is required to demonstrate a knowledge of codes and standards appropriate to performing this function, must establish a QA/QC program to provide for adherence to the regulatory requirements, and is subject to periodic audits.
Since in-factory compliance issues are only one aspect of the process, the standard also includes several specific requirements associated with transportation and on-site assembly of modules. In particular, for on-site construction, the manufacturer is required to provide instructions that provide details for the following:
Overall, the aim is to have the QA/QC and documentation systems in place to assure regulatory compliance through fabrication and on-site construction.
Associated with the ICC/MBI 1200 standard is the ICC/MBI 1205, Standard for Off-site Construction: Inspection and Regulatory Compliance (ICC/MBI 1205 2021). The focus of this standard is to outline expectations for building officials and other AHJs in the inspection of modular buildings for regulatory compliance. A key component of ICC/MBI 1205 is outlining the duties of building officials as associated with the installation or erection of modular buildings or components. These include the following:
With respect to on-site inspections of off-site-fabricated components, the building official should verify that installation is compliant with the approved manufacturer’s installation instructions, and connections performed on-site are compliant with approved construction documents.
In summary, with a defined QA/QC approach, a process for assuring and assessing quality in the fabrication process, which complies with the building regulatory requirements in which the MMC will be assembled into a building, is possible. An example where the building regulatory system is highly prescriptive indicates significant guidance in the regulations feed into the QA/QC process, and the building and fire officials are familiar with those requirements. In this situation, the factory and on-site inspections, supported by audits, help deliver confidence that the MCC comply with building regulatory requirements.
A functional- (objective- or performance-) based regulatory system typically has regulations that focus on identifying functional or performance requirements for a building, without specifying how exactly those requirements are to be met. Such systems often have compliance documents (deemed-to-satisfy documents, approved documents) that are more prescriptive (specification based) in nature, which may be mandatory or non-mandatory. Government involvement in oversight (review, approval, etc.) may be characterized as strong (highly involved) or more market oriented (i.e. more responsibilities on the owner/developer).
An example of a performance-based building regulatory system that reflects a higher degree of government involvement is Singapore, which has a national, performance-based system, where the building (planning, fire) regulations are developed, promulgated, and controlled at the national level. The Building and Construction Authority (BCA) has responsibility for the Building Control Regulations (Building Code), and Singapore Civil Defense Force (SCDF) has responsibility for the Fire Code. In Singapore there is a significant focus on review within the regulatory system, with requirements for accredited checkers for review of structural designs, and peer reviewers for the review of fire designs (especially for performance-based designs), in addition to government regulatory review by the BCA and SCDF, respectively. To assist actors in the design and construction market, the BCA, SCDF, and other publish several guidance documents, in addition to regulations. Various accreditation schemes are in place as well (for more information, see, e.g. Meacham 2009).
With respect to MMC, a mature system for the design and approval of PPVC is in place. In the first instance, design, construction, and installation of the proposed PPVC system for building construction must comply with the requirements of relevant agencies, from planning to transport, to construction. This includes the BCA, Land Transport Authority (LTA), Ministry of Manpower (MOM), National Environment Agency (NEA), PUB, The National Water Agency, SCDF, Urban Redevelopment Authority (URA), Housing and Development Board (HDB), and JTC Corporation.
To assist stakeholders, specific guidance is provided regarding PPVC design and approval requirements in the Design for Manufacturing and Assembly guide for PPVC (BCA 2020). This comprehensive document addresses a wide range of issues, including:
In addition, to help ensure that the different PPVC systems being used are reliable and durable, the BCA has set up a PPVC Acceptance Framework (BCA 2022), which consists of building regulatory agencies and industry experts, to evaluate the design and materials used. This includes the requirement to obtain In Principle Acceptance (IPA) from the Building Innovation Panel (BIP), which consists of the agencies noted above.
The first step is a sort of preliminary check that the proposed PPVC meets the performance requirements by the BIP secretariat. Once that check is complete, the secretariat sends the PPVC application to the full BIP for additional comment. Once all comments are resolved, an IPA can be issued. In addition, the proposer of the PPVC system is required to comply with the PPVC Manufacturer Accreditation Scheme (MAS) requirements. Accreditation is based on the following criteria (SCI 2022):
Overall, while Singapore has a performance- (functional-) based regulatory system, there are numerous levels of government review and approval, and many support documents, much like the prescriptive approach in the US.
Like Singapore, Japan has a performance-based regulatory system, which could be considered as being characterized by having strong government oversight. In Japan, the primary regulation for buildings is the Building Standard Law (BSL), with the Fire Service Law (FSL) having applicability to fire protection systems. The review and approval are at the local or prefecture level, and may involve private-sector-approved certification bodies (for more information, see, e.g. Meacham 2009).
The BSL, promulgated by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), allows for certification in advance that a standardized building (or part of a building) conforms to a series of regulations such as structural capacity, fire prevention, and evacuation, and requires certification of manufacturers who manufacture or build the said certified buildings (or parts of buildings). In the case of MMC systems, the technical production conditions at the factory, such as manufacturing equipment, inspection equipment, inspection methods, and QC methods, must be deemed appropriate. The certification must be conducted by the MLIT or a designated certification body. In the case of a designated certification body, it must have an appropriate plan for the implementation of the certification work, including its staff, facilities, and methods for implementing the work.
In summary, examples of a functional/performance-based building regulatory system with strong government oversight include Singapore and Japan. In both examples there are requirements for multiple levels of oversight and approval, with certifications by approved bodies required. For MMC, the approach in Japan has some commonalities with both the US and Singapore approaches. In particular, these are the requirements for quality and inspection plans for oversight of MMC produced off-site, in addition to inspections and checks as part of the construction/assembly process.
In some countries that have functional-based building regulatory systems, there is arguably less government control/oversight and more responsibility placed on the owners/developers of specific projects. Development may be by national government or quasi-governmental organizations, with promulgation/administration at the national, state, or local level, with a strong focus on the owner/developer being responsible for regulatory compliance, with government playing less of an ‘approval’ role. Examples include England and the Netherlands (for more information, see, e.g. Meacham 2009). In some cases, there may not be any particular requirements for MMC: just an expectation that the final building complies with the regulations. This means that there may be a wide range of approaches, with quite varying levels of responsibility and avenues for checks and balances.
This is the case in England, for example, where there are no particular requirements for MMC in the Building Regulations or in the Approved Documents, and no particular required special (different) building control or fire (risk) control requirements (MHCLG 2020; CROSS 2021). The expectation is that the owner/developer has the responsibility for complying with the Building Regulations 2010, Regulatory Reform (Fire Safety) Order 2005, and other pertinent regulations and requirements (e.g. there are various national assessment guidelines and European Assessment Documents (EADs), but all of them consider the same performances as required by the building regulations, and they also contain the same verification methods as indicated by the building regulations). As such, a typical approach would be to follow the Approved Documents.
However, as noted in the CROSS (2021) report, it can be argued that volumetric modular is not a common building situation, as it incorporates modern construction methods, and that the technical guidance found in the Approved Documents might not be adequate on its own. In such cases, it is suggested that one could refer to the MHCLG’s Manual to the Building Regulations (2020), which provides clarifications that can assist designers on how to approach the issue of technical guidance for ‘non-standard’ conditions, which arguably exist in buildings that incorporate MMC. In such cases, it is up to the owner/designer to ensure that the Functional Requirements as set out in Schedule 1 of the Building Regulations 2010 are satisfied. With respect to MMC, this includes, among other things, adequate fire protection of cavity (void) spaces, control of interior surface fire spread potential, assurance of structural stability in case of fire, and limiting of fire spread to adjacent properties.
In a government review of MMC in the UK (HOC 2019), the regulatory challenges, including around fire, were noted, and a recommendation to address the situation was stated:
The current suite of Approved Documents is confusing and difficult to comply with. It is particularly difficult for homebuilders that use MMC to apply the regulations to their developments. This could result in compromised safety standards in MMC buildings. The consultation on the building and fire safety system is welcome but does not consider specific guidance for MMC builders to help them comply with current regulations. The Government should urgently set out a clear plan for the review of the whole suite of Approved Documents, including a timeline for implementation. This review should consider how the Approved Documents relate to MMC buildings and where relevant, provide additional guidance on how MMC homebuilders might reach the required standards.
To help address some of the fire concerns, CROSS (2021) suggested that a competent fire engineer be consulted during the early stages of the project to ensure that all aspects of fire safety are addressed, that a site engineer or inspector be present to ensure that all the necessary cavities are adequately constructed and protected, that if structural fire protection is provided through fittings in the module construction, then issues of durability and maintenance should be addressed in the chosen technical solution, and that a structural engineering assessment should be able to help inform influences on fire performance of cavity barriers and fire stopping due to differential vertical movements and possible deflections.
While MMC is not specifically addressed in building regulation, and therefore no specific government required QA/QC programs are in place, as in Singapore, there are programs such as the NHBC (2021) Accepts program. The NHBC Accepts is a review service for innovative products and systems that offers a fast-track route for the acceptance of products and systems for use in homes covered by all NHBC warranty and insurance policies. This includes MMC systems. However, this program is limited to homes and is not applicable to commercial buildings.
At present, the situation is similar in Germany, where there is no special definition and/or provisions for MMC included in the Building Codes. As with England, the German Building Codes set out requirements for individual elements of the building or works (e.g. a load-bearing exterior wall, or a separating, fire-resistant intermediate floor), and the components of MMC, which as a system or individually form these elements, would need to be verified for their compliance with the Building Codes requirements.
A difference in Germany, however, is that the Deutsches Institut für Bautechnik (DIBt), which is a national provider of approvals and assessments, including for innovative construction products and techniques, plays a key role in MMC. For VMS/PPVC modules, approval for use would typically require a module-specific construction technique permit granted by DIBt based on testing. Likewise, fire (and other) performance of innovative prefabricated units or components would also need to be verified by the DIBt.
To summarize, in countries that place more responsibility on the market, there is an expectation that the required expertise will be incorporated into the project, e.g. the use of fire safety engineers on MMC projects in England. From a legal perspective, the expectation is that the final product will meet the regulation. The client, and their experts, will be expected to assure this is the case.
Modern methods of construction (MMC) can offer numerous benefits in terms of sustainability and economy. However, they can also present challenges associated with controlling for certain performance aspects associated with fabrication and means of connection, such as fire safety. In addition, they can also create challenges for regulatory systems that do not specifically consider MMC.
A fundamental issue is that MMC are complex ‘systems of systems’ for which assurance of design, regulated, and installed performance is difficult. On the one hand, this is driven by the complexities of designs and the resulting fundamental performance knowledge (e.g. the fire dynamics of facade systems or of void spaces in other forms of MMC). On the other hand, the regulatory systems in which MMC are approved do not necessarily have mechanisms to address such technical complexities, or they ‘expect’ that they are handled in an extra regulatory manner (e.g. the responsibility of the building owner to employ ‘competent’ persons).
While many buildings can be considered complex, and indeed are, the ways in which ‘traditional’ buildings are designed, constructed, and regulated have developed over time, and the regulatory system has adapted along the way. This includes everything from test methods to design approaches, to construction technologies. However, by definition MMC are not traditional, and the regulatory system in some jurisdictions has not yet caught up. Standard fire tests, for example, are often focused on specific materials, and not on assemblies or systems of materials (e.g. a facade system). Regulatory systems may assume that materials that pass a fire test will perform adequately, however they are used in a building (i.e. independent of how the materials are arranged). Some regulatory systems may assume that on-site inspections of all aspects of the building will be possible, which is not the case with some MMC.
With MMC, fire performance challenges can exist in cavity/void spaces, which may be required to connect closed panels, panelized systems, volumetric modular systems (VMS)/prefabricated prefinished volumetric construction (PPVC), and pods, especially if not properly fire blocked and sealed. There may also be fire concerns with the insulation or structural materials used within panelized systems, VMS/PPVC, and pods, especially if fabricated off-site and not subject to building control or inspection. Use of combustible materials could facilitate fire spread within void spaces depending on the geometry of the void. In addition, some panelized systems are combustible, such as mass timber systems (cross-laminated timber—CLT), which can present concerns for adding fuel load, increasing fire size, and impacting structural connection performance.
If the regulatory system does not require ‘systems’ testing—of assembled panels, VMS/PPVC, etc. in the installed configuration (including stacked VMS/PPVC)—then the actual fire performance of the ‘system’ may not be fully known. If void spaces exist, without proper fire blocking and sealing, smoke and fire may be transported through the void space, and if temperatures rise, some structural connections may be at risk. This does not necessarily mean that the test methods are inadequate, but that the void spaces need to be fire sealed. If there is difficulty in assuring that the void spaces are properly sealed for any specific types of MMC, then perhaps new fire test methods are needed. This is not without precedent: the push for facade system testing is an example of where traditional tests fail to adequately reflect the performance of the system. It may be that systems fire tests would be helpful for MMC in which considerable amounts of combustible material is used (including structural integrated panels (SIPs) comprised of combustible material and unencapsulated mass timber systems).
Aside from the systems testing aspect, the assurance of appropriate fabrication and construction of components and systems warrants attention. In highly prescriptive systems, such as in the US, regulations and standards have been promulgated that anticipate some of issues outlined above, in particular for VMS/PPVC. In the US, the International Code Council (ICC)/Modular Building Institute (MBI) standards provide requirements for the inspection of many forms of MMC components during fabrication, as well as when connected to form a building on-site. Requirements for quality assurance/quality control (QA/QC) and proper certification of components are also specified. Some functional (performance) regulatory systems mirror many of these components, including requirements for QA/QC systems, certification of products and producers, and on-site inspections.
However, some building regulatory systems rely heavily on the owner/developer and designer to assure that the performance of the MMC ‘system’ meets all regulatory requirements, and in some cases, with few requirements for on-site inspection of the building during construction. This approach may warrant reconsideration. The problem with on-site inspection does not change with the regulatory system approach—if the on-site inspection cannot be undertaken by a government official, how can it be expected that the owner/developer can do so? It would seem that such regulatory systems might consider requiring that the fabricator/supplier of MMC components (panels, VMS/PPVC, etc.) be the party responsible for assuring the performance of the building (in this case, the assemblage of components), or at least requiring this responsibility be shared with the owner/developer.
In the end, while every jurisdiction needs to work within their regulatory framework and implement systems that work for them, it seems that with the growing use of MMC, additional regulatory guidance and oversight may be warranted to help increase confidence in the performance of modular buildings, in particular from a fire performance perspective. As a starting point, this might include development of design and installation guidance, as well as regulatory review, inspection, and approval guidance. It may also mean the development of new ‘system’-level test methods or standards, especially if materials or connection conditions (e.g. void spaces) warrant. As many countries have already mandated, it seems that requirement for QA/QC procedures to help assure manufactured/fabricated components comply with regulatory requirements is an essential component.
In the longer term, it is suggested that a more fundamental set of changes should be considered. First, as buildings and building systems become more complex, a sociotechnical systems approach would be helpful to help assure alignment between the technologies, actors, and institutions in the regulatory system (e.g. Meacham & van Straalen 2018; Meacham et al. 2020). Likewise, the design profession needs to adopt sociotechnical and safety systems thinking, which serve not only to achieve alignment, but also aim to deliver ‘fail-safe’ buildings, in which avoidance of catastrophic loss is a requirement (e.g. Meacham 2022). Each of these approaches will ultimately require a more competent and ethical workforce—one that has an engrained safety culture in which the safety of occupants is more important than financial gain. The combination of a systems approach to regulation and design, and a safety culture approach for all actors in the system, will go a long way in helping to minimize the occurrence of failures associated with current MMC as well as whatever new technologies the future may bring.
The author thanks the members of the Interjurisdictional Regulatory Collaboration Committee (IRCC) for providing input related to how their respective organizations and countries address modern methods of construction. Interpretation of that input, and translation into this paper, was the sole responsibility of the author, and the IRCC members bear no responsibility for any incorrect or inconsistent interpretation by the author.
The author has no competing interests to declare.
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