2.1.1. Component Analysis of Unit Modular Using DSM
To componentize unit modules and derive the optimal process, a close examination of the unit module production process is required. This study monitored factory production to analyze the unit module production process. The most popular type of residential modular construction of a company with extensive experience was selected as the subject of monitoring. Monitoring included route, input personnel and equipment, materials, and working time.
Unit module production processes are formed by one production line, so the work is carried out sequentially. The factory is largely divided into a workspace inside the factory, a yard space, a floorplate production, and a concrete placement space. In particular, the deck is made by concrete pouring, which is arranged outside the factory and requires several transportation operations. Except for the external workshop, the internal module production flow is U-shaped and consists of a linear production line [
16,
26]. Lee, Park, Lee, Kim, Kim and Hyun [
16] have 12 stations for modular factory production, and KICT [
26] have 7~12 stations for unit module production.
The main purpose of analyzing the modular units is to identify the module production process. Therefore, this study selected a simple but representative residential modular construction in Korea. Specifically, this study selected a dormitory with four walls (3300 mm × 6600 mm × 3000 mm) as the unit model to generalize the production process. Although the unit module area is not ample, the unit module incorporated most activities required for the residential building, including a floor heating system. Despite satisfying the minimum requirement of area per person for social background, the dormitory offers an additional separated area on the first floor of each building. Furthermore, multiple unit modules can be combined to provide a larger space for one cell [
26,
27]. The unit module is a corner supported type with a steel column beam and a cast-in-place concrete floor. The gypsum board was used as the basis of the wall and ceiling. Depending on the area, different types of gypsum board (fireproof, general) were used in two layers. Wallpaper was used to finish the gypsum board. The floor was finished with vinyl tiles after the insulation. Lightweight foamed concrete, cement mortar, and hot water pipes were installed. The insulation performance of each part of the modular unit is designed to meet the standards outlined in Article 14 of the Green Buildings Construction Support Act (Submission of Energy Saving Plans) notified by the Minister of Land, Infrastructure, and Transport of the Republic of Korea. Additionally, this study monitored factory production to analyze the unit module production process. The most popular type of residential modular construction of a company with extensive experience was selected as the subject of monitoring. Monitoring included route, input personnel and equipment, materials, and working time.
Unit modules can be broken down into components based on the DSM to understand what components can be prefabricated for assembly because many components can be assembled both internally and externally in a station [
28]. The study used the dependency structure matrix (DSM) for analyzing the connectivity among components and preassembly for clustering [
29]. DSM is a network modeling tool to express the interaction between elements of a system in a N × N matrix [
30,
31]. For the application of DSM, interaction type and interaction strength should be defined.
Interaction type means a type of relationship among components that construct a single unit. The connection between components can be categorized based on how the components meet and how they physically merge. First, the methods by which materials meet can be classified using surface, line, and dot. Based on such categories, they can be divided into six types: surface versus surface, surface versus line, surface versus dot, line versus line, line versus dot, and dot versus dot. As an example, if the gypsum board is installed on the lightweight steel that constitutes a wall, the lightweight steel would be a line, and the gypsum board would be the surface, which means the gypsum board (surface) is on top of the lightweight steel (line). Therefore, joining occurs when a surface comes in contact with a line. The type of joints refers to how the components attach. There are six types of joints: welding, bolts, screws, pin, regular joints, and simple joints. A simple joint here means there is no joint between the two (A, B) components, but a third component is joined to each, which causes A and B to be joined. The joint method in the above example (lightweight steel and gypsum board) is a screw joint. The physical junction relationship can be further divided into 36 combinations, depending on the bonding type and method.
where IS is the interaction strength, IT
CA is the contact area, and IT
CM is the bonding method. In accordance with the DSM analysis order, the strength of the impact and relationship of the interaction type must be derived among singular module components. Interaction strength can be defined as binary or by assigning a weighted value [
32]. In this study, two types of factors (contact area, bonding method) were multiplied to calculate the relative interaction strength [
30,
32] as shown in Equation (1).
Therefore, the contact area would have the biggest impact on the surface versus surface and the smallest impact on the dot versus dot. In other words, starting with the smallest value of 1 (for dot versus dot), each unit increased by 1, with the surface versus surface at the highest value of 6. On the other hand, a simple joint would be 1, a bond or tape bond would be 2, a pin connection would be 3, and a screw connection would be 4 points. However, welding and bolting would have relatively higher values (bolting 9, welding 10) because welding and bolting are much stronger than other bonding methods. Based on such assumptions, a total of 36 types of the interaction strength between components were calculated. A simple junction is the smallest when it is the dot versus dot and biggest for welding on the surface versus surface with 60 points. A matrix was formed using such an assumption, which was then subjected to a quantitative analysis of the relationship between components. As a result, there are 7 clusters defined: Cluster 1 (short-side wall) has 13 components, Cluster 2 (long-side wall) has 9 components, Cluster 3 (outer wall) has 2 components, Cluster 4 (floor) has 9 components, Cluster 5 (ceiling) has 10 components, Cluster 6 (pipe shaft) has 2 components, and Cluster 7 (bathroom) has 10 components.
Figure 1 is a visual representation of the DSM matrix of short-side wall. Depending on the interaction strength, green represents a bigger correlation, and red represents smaller interactions [
16,
33]. For example, the strongest interaction for the short-side wall (i.e., Cluster 1) are the materials that constitute the structure; studs and tracks, the backbones of the finishing materials for the wall, have the strongest interaction with the structure. Studs and tracks also have strong interactions with the finish, as shown in
Figure 1.
2.1.3. Rule-Based Modular Assembly Production Process
The linear production method can benefit the most from the factory layout and prefabricated components. Often referred to as continuous production, linear production refers to a process where each production step proceeds in order. The workers work as all the modules, workers, equipment, and materials are located in a designated place with a rail or conveyor belt installed [
4]. In other words, the number of production lines and cycle time determine the process of continuous production, which means prefabricated components may have a bigger impact on this method than any other production method. The modular planning process in linear production can be categorized using the concept of the station. The station refers to the point in linear production where the work takes place. It is typically divided into the Pre-Station, Station, and Post-Station. The Pre-Station refers to steps until unit modules form a box frame, the Station refers to the process where unit modules move along the rail for finishing, and the Post-Station refers to remaining work and packaging as well as transport (shown in
Table 2). Here, the manufacturing time for each process was derived by monitoring the production process in modular factories. In addition, in the study, each task was allocated in an identical task area with the consideration of the detailed process, cluster, and work continuity (as shown in
Figure 2).
Once the activities in
Table 2 are integrated as prefabricated components, depending on the component combination, the corresponding activities disappear, and the deleted activity is replaced with the prefabricated components’ installation activity. Once the activity is replaced, the process may face a partial change, and therefore, the task time will change. Here, the time required to perform the altered process is not the sum of the times of activities to manufacture the corresponding components; rather, it is replaced by the time of the earliest work that constitutes the prefabricated component. Additionally, if there is any change in the process due to prefabrication, 10 min are added to reflect the quality inspection process [
32]. In addition, the installation difficulty level will increase during prefabricated component manufacturing. According to the analysis result of manufacturing time and installation time for the prefabricated lightweight steel and gypsum board, the production time lengthened.
Table 3 presents the results of the analysis of the production time and installation time for the prefabrication of lightweight steel and prefabrication of both lightweight steel and gypsum board. The result shows that the total time increased when both the lightweight steel and gypsum board were prefabricated. In other words, even when using prefabrication, converging lightweight steel and gypsum board will cause both walls to be blocked, which means directly screwing the lightweight steel to the structure frame is no longer possible. Instead, connecting hardware must be additionally installed. The study reflected the additional time for the installation of the connecting hardware (L-shaped angle). In other words, when both lightweight steel and gypsum board are prefabricated, an additional 20 min are added [
34], which includes the short and long sidewalls.