# A Review on Design Considerations and Connection Techniques in Modular Composite Construction
## 1 Introduction
Although the body of research on hybrid systems remains relatively  
Figure 3: Research trends in precast structural elements and connection techniques over the past two decades. limited compared to well-established methodologies, the increasing emphasis on these techniques indicates a rising interest in superior connection solutions to improve seismic resilience and construction efficiency. The analysis reveals that grouted and bolted connections continue to lead research efforts due to their structural dependability and extensive application within the industry. At the same time, hybrid systems signify a burgeoning trend. Investigations concerning cast-in-place joints primarily function as benchmarks, illustrating the progression from conventional monolithic construction to precast modular solutions. The distribution of research highlights the ongoing evolution of connection technologies, concentrating on enhancing efficiency, resilience, and adaptability in precast construction. Cluster analysis of keyword co-occurrence within the modular construction literature elucidates the interrelation and prevalence of terminology utilised in this domain, as shown in Figure 4. The visualisation of this network, depicting nodes as keywords and edges as their co-occurrence, underscores significant trends spanning from 2014 to 2022. The size of the nodes signifies the frequency of terms, while the colour gradient transitioning from blue to yellow signifies the temporal aspect, thus illustrating the evolution of keyword utilisation. Fundamental keywords like'modular systems', 'cemettitious materials','reinforced concrete techniques', and 'precast elements' appear vital, emphasising their significance in scholarly debates. Following 2020, yellow lines denote connections to newly emerging key terms, including'seismic performance', 'bearing capacity', and '3D printing'. In contrast, the blue and green lines from 2014 to 2020 correspond to foundational concepts such as'structural design', 'building materials', and 'bridge decks'. This transition reflects the evolving landscape of research focus and novel technologies' adaptation to contemporary challenges. This shift in research focus is accentuated in recent years by the rise of significant terminology pertinent to seismic performance within modular construction, as illustrated in Figure 4. Seismic performance is paramount, particularly in areas prone to seismic activity. Figure 4: Co-occurrence of keywords in publications on modular construction. Achieving the seismic resilience of concrete modular systems necessitates tackling the distinctive challenges that prefabricated volumetric units and their interconnections present. Advanced research is focused on refining connection methods to enhance energy dissipation and structural stability during seismic occurrences. For example, novel approaches such as ductile connectors [17, 23, 24] and bolted joints are under investigation to bolster the capacity of these systems to absorb and dissipate seismic forces, thereby minimising the likelihood of structural failure. Integrating seismic isolation mechanisms into modular edifices is another promising strategy to alleviate seismic impacts and enhance overall safety [25]. As the corpus of research expands, it becomes essential to weave these technological innovations into the design and execution of modular construction to guarantee that these systems not only fulfil but surpass the requisite safety criteria. This study focuses on various connection techniques adopted in various precast elements. The connections range from simple bolted connections to more complex composite connections, thus illustrating a spectrum of design intricacies to improve performance demands, such as in scenarios of extreme seismic demands. Emphasis is also placed on Non-Destructive Evaluating (NDE) algorithms and techniques [26] that can assess the performance of composite connections both throughout their service life and after seismic events.