Prefabricated Volumetric Modular Construction Prospects & Challenges

Ajay Chourasia, Sr. Principal Scientist, CSIR-Central Building Research Institute (CBRI), Roorkee, reviews the general concepts, PVMC connections, and recommendations for technological interventions and alternate connection systems.

Prefabricated Volumetric Modular Construction (PVMC) is an efficacious construction technique which has a sustainable behaviour that reduces construction time, waste of resources, and onsite workload, resulting in improved quality, faster, safer, and more environmentally sustainable construction.

PVMC’s adaptation to modern urbanisation is an acceptable alternative to traditional construction in multi-storey structures. Although, its use in low-rise and low seismic region is common, it poses challenges in contrast conditions due to inadequate understanding and knowledge as a result of extreme multi-direction forces.

The seismic behaviour of prefabricated volumetric modular buildings depends on joint connections between modular components. Thus, it is of utmost importance to improve the joint connection details and deploy seismic resisting features in PVMC. Researchers across the world have performed experimental and numerical studies to evaluate the response of such buildings under lateral loading.

PVMC is an industrialized construction process which involves manufacturing of prefabricated volumetric modular components in a factory, followed by transportation to the construction site, where they are assembled and joined, using specially designed connections. PVMC buildings are distinguished by their assembly process, design system, and detailing requirements, as well as a reduction in deadweight, and improved efficiency over traditional structures. Fig. 1 shows the volumetric building construction process from manufacturing stage to installation.


There are three classes of prefabricated construction: 1D single element, 2D panelised system, and 3D volumetric system, based on the degree of prefabrication. Panelised and volumetric construction, also known as modular construction, is the most efficient class of prefabricated construction as it allows for 70% to 95% of a building to be prefabricated in a factory before transporting it for on-site assembly.

The connections between prefabricated modules can be achieved by on-site simple inter-modular connections. Although PVMC has been extensively applied for low-rise buildings over the last three decades, its application for high-rise buildings is still limited at less than 1%.

Structural Systems for Improving Multi-Directional Stability
Joint Connections: The performance of PVMC under lateral loading is the function of its seismic parameters such as lateral strength, deformation characteristics and energy dissipation capacity.

Inter-modular joint connections in PVMC result in movement and volumetric changes due to the consequences of loading, stresses, shrinkage, thermal effect, and other factors. Thus, to reduce the movement and deformation, internal friction between the elements is induced by providing connectors.

Prefabricated modules should be designed to sustain gravity as well as lateral loads in order to deliver highly efficient seismic resistant system, which can be ensured by evaluating and studying mechanism of inter-modular joint connections. The mechanical behaviour of a joint connection can be designated by load-deformation curve (Fig. 2).


Apart from structural requirements, joints between precast panels should also fulfil the functional requirements with regards to sound insulation, thermal insulation and water proofing. Hardware materials used for connections between precast walls include reinforcing bars, bolts, threaded connectors, welds, post-tensioning steel, steel plates, etc. In modular high-rise buildings, the connections between modular units play an important role to ensure the overall structural integrity, stability, and robustness of the entire building. Although welded connections can provide rigidity between adjacent modules, it is not preferred onsite as it requires highly skilled labour, a large working space, and time-consuming inspections after welding.

Inter-modular Joint Connections: An increased number of joining techniques have been developed for the inter-modular connection of modular steel buildings as illustrated in Fig. 3. These connections can be classified into three different types: inter-module connection using tie rod; inter-module connection using connector; and inter-module connection using bolt.


Inter-Module Connection using Tie Rod: This type of connection has been developed to vertically connect the columns of the lower modules to those of the upper modules (i.e. column-to-column connection). In this connection, the vertical tying between lower and upper modules is provided by using a vertical rod, whilst the shear force between the lower and upper columns is resisted by shear keys. The advantage of this type of connection is that it can be installed outside the modules, which thereby prevents any potential damages to internal finishes. In addition, this joining technique can be applied for columns with both hollow bare steel sections and CFST sections, which are necessary for high-rise buildings to retain the same column size.

Inter-Module Connection using Connector: Unlike the tie-rod system, the installation of the connector system is quite simple and flexible because the connector can be easily welded to the beam and column of the module offsite, which have different shapes of cross-sections. Various connector systems have been developed such as Vectorbloc, self-lock, rotary, and bracket connectors.

Inter-Module Connection using Bolt: Bolted connection is considered as an alternative solution to the onside welding approach due to its fast and easy installation and better quality control. Various bolting techniques have been developed for inter-modular connections of modular steel buildings, such as bolted connection with plugin device, wherein a high-strength bolting system is used to establish the vertical connection between the lower and upper modules, whilst a cast plugin device is adopted to establish the horizontal connection between adjacent modules. The seismic behaviour of the connection was experimentally and numerically investigated by Chen et al. (2017) for the corner joint and perimeter joint of modular steel buildings. The results showed that all connection specimens have reasonable energy dissipation capacity, post yielding deformation capacity and connection ductility, although fracture is their governing failure mode. In addition, the connection stiffness is also pronounced and needs to be considered in the design.

Impediments in Current Connection Systems
There have been an increasing number of joining techniques developed recently for modular buildings. However, the current techniques result in limited strength and stiffness, which may not be suitable for high-rise building applications. They also require a certain level of on-site labour. Therefore, there is a need for future research to develop smart joining techniques, which are not only stronger for high-rise applications, but also easy to install.

In addition, the new joining technique should be robust enough to be applicable to a wide range of joint configurations and novel modules with composite members. With these techniques, modular buildings can be built taller, faster and cheaper. For concrete modules, the current inter-module connections require significant on-site labour for laying the rebars and site grouting.

Tie-rod connection has limited moment resistance and its performance is similar to that of a weak semi-rigid connection. This leads to a weak framing action and lateral resistance of the whole building. Therefore, this type of connection is not suitable for high-rise buildings.

Bolted connection using welded plate can provide satisfactory connection ductility, but its strength and stiffness are limited and consequently, it can be classified as semi-rigid connection. This connection is not suitable for the inter-module connections of modular steel buildings as it requires highly skilled labour and working space.

Alternate Connection System: Headed Bars
Headed bars are formed by attaching a steel anchor at the end of beam reinforcement bar through welding or threading. Headed bars are highly advocated by the researchers due to their cost-effectiveness, easy installation, time saving fabrication and construction efficiency, without affecting the structural performance.


Behaviour of Headed Bars: Experimental program

At first intance, headed bars are tested for prefab RC beam-column joints. Ribbed headed bars were tested under displacement controlled cyclic loading and its strength was compared with conventional system of ductile detailed system and partial development length. Fig 5 shows the prefab RC beam-column joint with headed bar and development lengths and Fig. 6 shows the experimental set-up of tested precast RC beam-column joint specimens.

Headed bar specimens demonstrated satisfactory structural performance by surpassing nominal moment strength and design joint shear capacity. The load carrying capacity of headed bar specimens was well comparable with the conventionally detailed specimens, even marginally higher in most of the cases. Thus, headed bars are efficacious in imparting adequate bond strength inspite of absence of development length. Headed bar specimens satisfied the performance criteria of ACI 374.1-05 with regards to minimum drift, strength degradation, relative energy ratio and stiffness, which is considered as an important milestone for their acceptance. Fig. 7 shows the typical hysteretic curves for the three connection systems tested.



Cost Analysis
Cost of implementing headed bars in beam-column joints was estimated and compared to the cost of conventional development length for different rebar diameters. Cost of anchor is considered as per manufacturer’s rate. Rate of steel work is considered as ₹ 60/kg, including labor charges as prevalent in construction. Rebar embedment depth into the joint is considered as 300 mm in case of headed bars, while embedment depth for development length is determined as per IS 456-2000 and IS 13920-2016. Table 1 illustrates the cost analysis and comparison of headed bars and development length for different rebar diameters. Cost analysis indicated that the headed bars are economic substitute to development length, with percentage saving increasing with the increasing rebar diameter (Fig. 8).


Conclusion
Prefabricated volumetric modular construction reduces construction time, resource waste and onsite workload using a volumetric modular construction method. It provides more cost-effective, faster, safer, and eco-sustainable building solutions than traditional onsite construction. In addition to many advantages, the success shown by PVMC in multidirectional forces is increasingly encouraging its adaptation as an acceptable alternative to conventional construction.

PVMC offer a wide range of benefits over cast-in-situ structures, yet their acceptance is uncommon in construction industry, owing to scarce knowledge on the design and force transfer mechanism of joint connections. Previous studies on PVMC revealed that behaviour of modular buildings can be enhanced by providing effective connection systems which impart adequate lateral strength and ductility to the building. Existing research, although competent to understand the response of connections at component level, cannot completely envisage the global seismic performance of modular buildings.

Structural behaviour of existing connection types is still ambiguous when practically implemented to the buildings, for which full-scale prefabricated modular buildings with inter-modular connections should be tested experimentally under lateral loading. Alternate inter-modular connections for PVMC include headed bars which have shown superior structural performance and cost-effectiveness as compared to conventional development length using reinforcing bars.

Current design practices for modular buildings are based on conventional design guidelines for traditional buildings, which are not suitable and joint connections require special design. The structural design guidelines for modular connections remain unavailable. Therefore, there is a need for future research to develop provisions for the structural design of inter-modular connections. With the new design codes, modular buildings can be built safer, which will enable the construction industry to implement advanced modular technologies for high-rise buildings. These endeavors may encourage widespread adoption of PVMC in seismic prone regions of the world.

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