Bibliometric Review of Prefabricated and Modular Timber Construction from 1990 to 2023: Evolution, Trends, and Current Challenges

By inergency On Mar 4, 2024

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1. Introduction

Off-site construction, also commonly referred to as prefabrication, involves manufacturing various structural components within a controlled factory environment, subsequently facilitating the transportation of complete assemblies or subassemblies to the designated construction site where the intended structure is to be established [1]. Modular construction is widely recognized as a fundamental approach within offsite construction [2]. This technology can achieve a remarkable level of completion, reaching upwards of 95% before its transportation to the designated location [1]. While some researchers state that modular construction is constituted by a dimensionally unified spatial unit or a 3D volumetric section [3,4,5,6], others describe it in the form of 2D planar components (pre-assembled elements) or fully volumetric units [7,8,9]. From the viewpoint of these authors, modules can cover a diverse range of prefabricated elements or systems, including component groups and complete units of significant size and volume.

These two approaches will be appropriate depending on the demands of each project. The assembly work involved in a 2D panelized system is intricate due to its requirement for additional internal finishing. Nevertheless, it provides superior flexibility. In contrast, volumetric units require adaptation to building specifications, which demands early involvement in the design process to ensure the desired result. Still, it has the potential for the most favorable efficiency and time savings [10,11]. If existing criteria are no longer satisfied, prefabricated elements or systems can be removed individually for expansion and reused to produce new buildings, providing the opportunity for adaptability.

Even though the materials utilized in modular buildings are generally identical to those employed in on-site installations, they exhibit enhanced quality since the components are fabricated within regulated conditions [12]. Timber constructions possess a notable competitive edge in this aspect due to their substantial degree of prefabrication (quick and precise assembly), lightweight nature [13,14], and simplicity of connections [15]. Furthermore, timber structures “can be renewed, manipulated, and laminated” [1]. These are favorable conditions for the effective recycling and repurposing of complete architectural elements, specific sections of buildings, and individual components [16], reducing expenses and shortening installation schedules [17]. In addition, the waste produced can be reused in the manufacturing process for various applications, which ensures a significantly cleaner and more regulated environment. Simultaneously, it enhances health and safety because the operational process is greatly minimized [15]. Given all these benefits, timber may be considered an optimal modularization material. Therefore, the utilization of timber as an off-site material has been extensively researched and promoted in the industry.

Recently, there has been a comprehensive evaluation of prefabricated/modular structures, examining various aspects related to management strategies [18,19,20], structural systems in high-rise buildings [7], building design [8], environmental and economic performance [21], as well as the features of modular construction utilizing exclusively cross-laminated timber [22]. However, the specific advancements and recent innovations in prefabricated/modular timber structures, which include light-frame, mass timber, and prefabricated hybrid elements, systems, or structures have not been reviewed and critically analyzed to identify current trends and challenges in the field.

Therefore, the objective of this research entails the assessment of the current state of the art regarding modular timber construction, analyzing its evolution as a research field and critically assessing the current trends and challenges. This contribution aims to assist researchers in understanding the current state of technology and orient them towards researching the most impactful needs. To achieve this, the applied methodology included the following: (i) to quantitatively evaluate the existing literature; (ii) to develop a scientific mapping analysis utilizing a co-word network approach; (iii) to establish the evolution of the underlying themes within this domain; (iv) to identify future trends and gaps in the current literature; and (v) to provide valuable insights and directions for future research endeavors. The organization of the article encloses a comprehensive methodology section (Section 2), a scientometric analysis and presentation of the results (Section 3), a critical analysis of the results (Section 4), and a conclusion section (Section 5), which encapsulates the essential findings and potential approaches to further investigation.

4. Analytical Description by Subperiod

According to the insights provided by the bibliometric analysis, an analytical description of the leading research topics and current challenges is outlined as follows:

4.1. Superiod 1990–2004

It demonstrates a notable inclination toward studying timber-framed panel walls’ design and structural behavior. These walls are commonly employed as the primary bearing capacity elements in constructing prefabricated timber structures [78]. Furthermore, there is a noticeable growth in the development and utilization of simulation tools, such as FEM, to analyze the behavior of timber structures. Overall, this period denotes a significant advancement in the research and exploration of prefabricated timber structures, beginning the way for further improvements and innovations.

4.2. Superiod 2005–2019

The utilization of CLT as a prefabricated component in medium- and high-rise buildings became critical, and it worked hand-in-hand with connections. While concrete is still used, it assumes a different role in this context as a material that complements emerging prefabricated timber structures, such as hybrid systems. It is also clear that model approaches remain an essential tool for comprehending the behavior of timber structures. Two fundamental concepts emerge within the construction industry: life cycle and industrialized construction. Unquestionably, one of the most significant advantages associated with employing prefabricated timber systems in construction lies in reducing costs, labor, and time, and the subsequent decrease in carbon emissions and their impact on the structure’s overall life cycle.

4.3. Superiod 2020–2023

During this subperiod, the researchers kept attracting attention by exploring the structural behavior of prefabricated and modular timber buildings (mass timber and composite elements). Vital aspects involved examining new wood species that exhibited potential for being employed in the manufacturing process of CLT and innovative proposals for developing connections. Furthermore, attention was being given to improving the energy efficiency of timber buildings by enhancing building envelope systems. There were ongoing research studies on prefabricated timber walls (light timber-framed and mass-timber external assemblies) due to wood’s susceptibility to humidity and temperature changes. Apart from these matters, the construction industry was also witnessing the emergence of the circular economy concept as a prominent topic. Prefabricated timber technologies possess various qualities that make them suitable for implementing a circular construction approach. According to the existing literature, these qualities include a faster construction process, reduced pollution, and the potential for material reuse and recycling.

Finally, another significant change observed in the timber construction industry during this period was the application of new digital technologies into industrialized construction practices. There was a clear indication of interest in the extensive application of technology to provide a holistic framework for designing, managing, manufacturing, and producing prefabricated/modular timber buildings.

5. Conclusions

The primary contribution of this study is the identification of relevant topics, emerging trends, and prospective pathways in the research of prefabricated and modular timber structures through the analysis of strategic diagrams and cluster networks with the aid of the SciMAT software.

The thematic evolution revealed an initial interest in understanding the structural behavior of timber buildings with light structures. This interest was achieved using numerical factors, analytical models, and experimental tests. The rationale behind this focus was based on the fact that this system constituted a substantial percentage of the houses constructed in North America, Australia, New Zealand, Japan, and Europe [36]. Subsequently, with the introduction of mass timber products and composite elements, the design and construction methods demanded increased concern in detailing critical elements and new planning methods. Consequently, research efforts were primarily concentrated on the study of two critical elements, namely, connections (which impact the structural functionality, manufacturing, and construction of the entire building system) and the building envelope systems (which safeguard the timber building structure from undesirable weather conditions during the construction phase and enhance the energy efficiency of timber buildings during their operational lifespan). Despite the increasing demands and challenges, the number of CLT modular projects achieved a remarkable milestone in 2017, reaching its peak with the completion of 26 buildings [22].

Furthermore, the analysis results indicated a strong correlation between design and prefabricated and modular timber structures. This correlation was observed from the first subperiod and represents potential opportunities for future research. It was found that prefabrication and modularization offer numerous advantages, such as flexibility in design and the ability to customize and standardize constructive elements. However, Ribeiro et al. [79] identified specific barriers associated with prefabrication. One such barrier is that architectural design often fails to consider the modular construction aspect, primarily due to aesthetic limitations. Similarly, Svatoš-Ražnjević et al. [80] pointed out that only some modular timber projects differ from the traditional rectangular and orthogonal footprints. From the publications analyzed, the modular design approach has predominantly been applied to houses rather than large-scale projects, potentially due to the limited exposure to modular technology in the popular literature and timber databases.

In addition, the analysis of thematic clusters and trends revealed that two factors influence the development of prefabricated and modular timber structures:

Circular economy. Researchers have shifted their focus towards the reusability and efficiency of prefabricated elements. Prefabricated timber construction is characterized by utilizing large-format building elements and the associated logic of joining them together [16]. This characteristic makes it suitable for disassembly, reuse, and recycling. Therefore, adopting strategies and measures that reduce the carbon footprint, minimize waste, and increase the life cycle of structures aligns with the principles of circular construction. To this end, various initiatives have been developed, exemplified by the MAI-Ivalsa Modular House. This environmentally sustainable system of prefabricated housing modules employs reused CLT for the load-bearing structure, allowing for the prefabrication of components at one site and their successful transportation to another [81]. Another example is the Collegium Academicum IBA, the first multi-story modular timber student residence that offers significant spatial flexibility through simple modular pieces that can be easily disassembled and recycled [82,83]. Additionally, a ten-square-meter prototype utilizing a fully modular precast lightweight engineered timber structural system, as developed by [84], has demonstrated the potential of building-scale additive manufacturing with cost-effective materials to enable deconstruction and material recovery. Studies focusing on hybrid structures have recently begun incorporating assessment strategies such as design for disassembly or deconstruction (DfD). For instance, Grüter et al. [85] presented two case studies of modern residential timber hybrid buildings in Switzerland, employing digital tools to evaluate strategies that facilitate a circular design process for timber elements. These strategies specifically address the elements’ start and end of life, thus serving as a solid foundation for future research. Similarly, Derikvand and Fink [86] proposed directions for future developments and advocated for DfD in timber–concrete composite (TCC) floors, emphasizing deconstructable connectors that enable material recovery and reuse as the preferred end-of-life scenario. These projects underscore the importance of integrating sustainable practices into the design and construction processes. Despite the recent release of a standard by the International Organization for Standardization (ISO), which provides general rules for integrating design for disassembly and adaptability (principles of circular economy) into the service life of structures [87], few studies have, thus far, presented innovative solutions and designed buildings that fully address these strategies. From the literature content analysis focusing on connections and building envelope systems (as was established before, they are critical elements in the design and construction of prefabricated and modular timber structures), only one study stands out as it introduced a novel reversible mass timber connector for prefabricated systems [88] while another explored the design and construction of fully prefabricated façade components to reduce materials and production energy [89].
Digital technologies. Digital architecture, computer science, engineering informatics, virtual reality (VR), and building information modeling (BIM) are critical in prefabricated and modular timber construction. Integrating these parameters into industrialized construction practices necessitates the implementation of automation in timber manufacturing. For instance, in Canada, there is a strong focus on integrating BIM information into the construction phase, specifically in the context of automated fabrication. A study by [90] delved into a BIM-based framework that aims to automate the machine capability evaluation of timber frame assemblies. This proposal highlights the system’s ability to accurately determine whether a user-selected machine can effectively manufacture a construction product that has been pre-designed using BIM software. In the United Kingdom, the research developed by [91] adopted BIM models in the game environment (gamification) to facilitate the implementation of small- and medium-sized architectural and construction practices dedicated to visualization creation. The collected evidence showcases that combining a game-like platform and BIM can lead to simplified data delivery to clients, ultimately resulting in customer satisfaction, confidence, and increased sales of timber frame houses. According to [92], “the level of automation is high in the initial stages of prefabrication but relatively low in the assembly stages, which require flexibility and human knowledge.” To address this, some research groups are focused on developing robotic systems. Robotic fabrication, directly connected to a precisely planned virtual model, significantly reduces the risk of construction mistakes and ensures high global precision. This results in a cost-effective and efficient construction system [93]. The literature reveals that robotic systems have enabled the construction of unconventional structures, such as the BUGA Wood Pavilion. Furthermore, companies like Weinmann and Randek have gained recognition for adopting these technologies in their processes [94]. Similarly, researchers in the United States have devised a simulation-based methodology to evaluate the automated assembly of wood frames through BIM, utilizing various robotic systems [95]. By employing simulation techniques, researchers can assess the effectiveness of different robotic systems in automating the assembly of timber frames.

However, among the selected papers, only a few studies delved into the practical implementation of these technologies. This can be attributed to the persistent obstacles that hinder the adoption of digital technology, such as the fragmented and disorganized nature of reverse supply chains, resulting in a lack of practical information sharing among stakeholders. Moreover, these supply chains’ intricate and diverse nature renders the recreation of accurate (BIM) models representing the existing conditions often expensive or time-consuming. Additionally, actors involved may lack the necessary knowledge or skills to embrace certain technologies, further impeding their application [96]. Furthermore, establishing and improving manufacturing environments, which typically necessitate substantial investments, also pose a significant challenge in adopting these technologies.

Drawing from the results obtained in the investigation, it becomes evident that the market ratio of these construction technologies continues to evolve. This development highlights the interrelation of digital technologies in effectively supporting and facilitating advanced circular economy strategies in the design and construction processes. These strategies include supporting forestry, reducing expenses, shortening installation schedules, and ensuring a significantly cleaner and more regulated environment, which enhances workers’ health and safety, as the operational processes are greatly minimized. However, it is essential to acknowledge a significant gap in research and knowledge surrounding these topics, which necessitates further exploration and understanding. Studies conducted in Austria and Australia focused on developing perception surveys and interviews of relevant industry stakeholders to gain insight into the barriers to the widespread application of digital technologies and circular strategies on prefabricated and modular timber construction. For instance, Santana-Sosa et al. [97] highlighted a lack of knowledge and experience, limited implementation of digital tools such as BIM, inadequate capacity of current infrastructure and procedures, and absence of standardization in design, planning, optimized construction management, and manufacturing. Similarly, in Australia, the study conducted by [98] identified a lack of regulations, performance concerns, limited application experience among local manufacturers and suppliers, inadequate regulatory and insurance policies, financing challenges, difficulty in incentivizing clients, and a shortage of a skilled workforce.

Considering the significance of these findings, it becomes crucial for multiple sectors to act:

Reevaluating the regulatory framework is necessary to encourage research collaborations between countries, facilitating the transfer of specialized technology knowledge to the labor market.
Open new research areas in the construction sector, contributing to the growth of existing scientific systems and advancing these fields.
Implementing professional training programs in industries can promote competitiveness and employment opportunities.
Integrating timber expertise, including designers, producers, and construction companies, at different stages of the construction process can further enhance the adoption of prefabricated and modular timber construction.
Raising society’s awareness and knowledge transfer of timber’s potential, such as its safety, cost-effectiveness, adequate structural performance, and environmental aspects, can foster the acceptance of these technologies.

Ultimately, the findings of this research significantly contribute to the existing body of knowledge and serve as an initial comprehensive platform that supports further investigations of these construction technologies. Future research will explore the relationship between circular economy strategies concerning prefabricated and modular timber construction, such as design for disassembly/deconstruction (DfD) and design of adaptability (DfA).

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