## 1 Introduction  
In recent years, the application of various technologies, such as information technology, laser scanners, and modularization, has increased construction productivity. With the introduction of building information model (BIM) technology, problems in the design stage and conflicts in the construction stage are prevented in advance [1]. In addition to introducing BIM, which creates a virtual 3D world and detects faults, the method of collecting information in the real world has also been improved. A laser scanner is used to check whether the structure designed in the pre-construction stage is modified or matched with the drawing, and the structure can be constructed using its 3D model and BIM [2]. In addition, information about the geographic features of a wide area can be obtained rapidly using drones and photogrammetry technology [3]. Despite introducing these technologies, the actual environment in which a structure is constructed can be improved only when the production processes, such as prefabrication, are enhanced in parallel [4]. Prefabrication technology is concerned with the construction production process and serves as a manufacturing platform to improve productivity and safety management [5]. Prefabrication is not a new technology and is used for various purposes. It has been undergoing improvement and optimization for a long time [6]. Applying precast concrete elements to a concrete field can reduce the curing time for constructing structures in the field [7]. Using curtain walls with glasses and frames can reduce the assembly time in the field and avoid the use of scaffolding outside the building. It can eliminate various conflicts related to construction [8]. Using prefabrication technology can shorten the construction period because the components are manufactured in advance in a factory instead of at the construction sites. In particular, it can produce structures repeatedly [9]. Furthermore, the learning effect of the labor units involved in repeated productions increases the efficiency of the work and productivity [10]. The conversion to factory production reduces the processing of materials in the field. The total waste generation rate of construction projects using prefabrication technology is 25.85% lower than that of non-assembled projects. Using prefabrication can effectively reduce most construction wastes (inorganic non-metallic, organic, metallic, and composite wastes) [11; 12]. For this reason, many countries encourage the introduction of prefabrication or modular construction techniques, which is gradually increasing [13]. Recently, owing to the complexity of building systems, the scope of prefabrication has been expanded from construction works such as precast concrete and curtain walls to mechanical, electrical, and plumbing (MEP) elements [14]. In particular, a recent trend in buildings is the use of heavy MEP systems. The prefabrication of MEP components is becoming more sophisticated and advanced, owing to the introduction of BIM technology [15]. Advances in pre-design review technologies such as BIM have improved the environment where MEP coordination can be performed in advance. Therefore, single-trade prefabrication techniques have been replaced with multi-trade prefabrication techniques that combine the elements of complex works in advance [16; 17]. The multi-trade prefabrication technology requires substantially more effort and time in designing and manufacturing than single-trade prefabrication because it solves conflicts between different work types in advance and manufactures by attaching elements of various work types within one integrated frame. However, it minimizes the fieldwork and reduces the construction period; hence, it holds advantages [4]. The benefits of combining various elements in one space can be expected to reduce air and increase productivity more than the existing single-trade prefabrication techniques [18]. A prefabricated MEP rack (PMR) is the most commonly used multi-trade prefabricated member.