JOURNAL OF ASIAN ARCHITECTURE AND BUILDING ENGINEERING 2021, VOL. 20, NO. 3, 285–297 https://doi.org/10.1080/13467581.2020.1782917 CONSTRUCTION MANAGEMENT Multiple work zone strategy for repetitive on-site work of modular construction using parallel station method a b c d e Hosang Hyun , Minhyuk Jung , Inseok Yoon , Hyun-Soo Lee and Jeonghoon Lee a b Institute of Engineering Research, Seoul National University, Seoul, Korea; Institute of Civil and Environmental Engineering, Seoul National University, Seoul, Korea; Department of Architecture and Architectural Engineering and Institute of Construction and Environmental Engineering, Seoul National University, Seoul, Korea; Department of Architecture and Architectural Engineering, Seoul National University, Seoul, Korea; Department of Architectural Engineering, Gyeongnam National University of Science and Technology, Gyeongnam, Korea ABSTRACT ARTICLE HISTORY Received 14 October 2019 Modular construction offers benefits such as high quality, low cost, and short durations owing Accepted 14 May 2020 to the high productivity of repetitive production. To maximize productivity, modular construc- tion involves repetitive schedules; however, the scheduling methods exhibit limitations when KEYWORDS applied to on-site work. These methods are optimized by adjusting the production rate of Modular construction; on- activities; however, the bounds of the production rates of modular construction on-site work site work; labor allocation; are limited because of workspace limitations in the units and varying amounts of work repetitive work; parallel between activities. This results in idling time in the scheduling methods. Thus, in this research, station method the parallel station method (PSM) was employed to ensure a flexible production rate. A discrete event simulation model was developed and employed to estimate the number of workers and work duration. The results demonstrated the following: 1) The developed scheduling method exhibits better results than the method for stick-built construction. 2) When applying the PSM, the line-of-balance method is cost-effective, while the TACT method is time-effective, implying that scheduling methods should be selected based on the primary objectives of modular projects. The findings of this research will contribute toward improving the accuracy and applicability of repetitive scheduling methods and reduce the labor cost and duration of on- site work. 1. Introduction objective, repetitive scheduling methods are used (El- Rayes and Moselhi 2001; Ioannou and Yang 2016). In developed areas such as the United States, Australia, With repetitive scheduling methods, activity duration and Singapore, the construction industry suffers from is set by adjusting the production rates. Production rate labor shortages, resulting in project schedule and cost refers to the productivity of a particular crew and can be overruns (Arif, Espinal, and Broadway 2002; Lu 2009). To adjusted through crew size and crew allocation strate- overcome these problems, modular construction has gies (Moselhi and El-Rayes 1993; Fan, Sun, and Wang attracted attention. This method comprises manufactur- 2012). This adjustment helps optimize the work sche- ing and on-site work and offers benefits such as dule (Altuwaim and El-Rayes 2018; Carr and Meyer 1974; improved productivity, short construction periods, better Chrzanowski and Johnston 1986; Ioannou and Yang quality, and safety due to controlled factory work envir- 2016; Ipsilandis 2006; Lee et al. 2015; Nassar 2011; onments of repetitive production tasks (Eastman and Reda 1990; Salama and Moselhi 2019; Zou et al. 2016). Sacks 2008; Mullens 2011; Shaked and Warszawski These studies used a single work zone is used for repe- 1992). After production, units are transported to con- titive work, also called a work unit. The space and struction sites and on-site work is done. The proportion number of work zones are equally applied to repetitive of on-site work is estimated to range from 30% in fully activities. However, to improve the efficiency of mod- modular buildings to more than 50%; thus, work should ular on-site work scheduling, a multiple work zone be planned based on the required on-site work (Lawson, strategy is required. The need for such a strategy Ogden, and Bergin 2011). On-site work comprises repeti- stems from the nature of modular on-site work such as tive work activities such as assembling equivalent units workspace limitations in the units and different having similar specifications. Thus, in modular construc- amounts of work for different activities, which causes tion on-site work, repetitiveness is important as in other limitations in assigning more workers or reducing the construction projects (e.g., highways, high-rise buildings, number of workers allocated to activities. These con- and railroad) to improve productivity. Toward this strain the bounds of the production rate and, in turn, CONTACT Jeonghoon Lee email@example.com Department of Architectural Engineering, Gyeongnam National University of Science and Technology, Gyeongnam 52725, Korea © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the Architectural Institute of Japan, Architectural Institute of Korea and Architectural Society of China. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 286 H. HYUN ET AL. affect the efficiency of work schedule optimization. For repetitive schedules applying the PSM were modified example, when interference between activities is pre- according to project characteristics. The scope of this dicted in repetitive scheduling using a single zone strat- research is as follows: 1) It focused on on-site work pro- egy, the production rate of the preceding activity is cesses in modular construction. 2) It only considered labor accelerated to alleviate this interference. However, in resources because modular construction is labor intensive modular on-site work, workspace in the modular unit and suffers from labor shortages. Moreover, one of the is limited and more workers cannot be assigned to the benefits for project managers is the reduced labor unit. Moreover, it is difficult to allocate workers to other resource requirement (Blismas et al. 2005; Blismas and units to increase the production rate without interfering Wakefield 2009). Thus, this research specifically focused with unit assembly and on-site work because during on labor resources. finishing work, units are being assembled simulta- neously, making it difficult to use a single work zone 2. Literature review strategy for multiple units. These constraints limit the production rate. Moreover, the required man-hours for To apply the repetitive scheduling method to modular each activity are significantly different because some construction, previous studies were reviewed. First, stu- building components require little on-site work, which dies to understand the principle of scheduling methods affects the production rate. The limited bounds were studied, identifying constraints of repetitive sche- decrease scheduling efficiency. Therefore, to manage duling for modular construction. Then, previous studies different man-hour requirements without workspace related to PSM were reviewed. Based on this, a method interference, the size of or the number of units in the for applying PSM to repetitive scheduling methods for work zone should be individually allocated for each modular construction on-site work was derived. Figure 1 activity. Using different work zone strategies, interfer- shows the organization of the literature review. ence and total activity duration in scheduling can be reduced. To overcome these limitations, the parallel 2.1. Repetitive scheduling for modular station method (PSM) has been used to improve assem- construction on-site work bly line flexibility by allocating different numbers of work zones to each activity (Askin and Zhou 1997; Modular construction on-site work comprises repeti- Becker and Scholl 2006; Pinto, Dannenbring, and tive work in that equivalent units having analogous Khumawala 1981). By applying PSM to modular on-site specifications are assembled; thus, a single unit is work, the production rate can be adjusted and on-site used as a work zone for this repetitive work. Such work schedule efficiency can be improved. work is conducted in a tightly planned schedule and Therefore, the objective of this study was to develop work activities are concurrently conducted with unit a repetitive scheduling method for modular construction assembly, increasing the complexity of on-site work on-site work by applying PSM. Initially, constraints on the schedules. To prevent schedule overruns and interrup- production rate were analyzed, and then repetitive sche- tions, on-site work should be scheduled based on this duling methods were analyzed through a literature repetitiveness (Mohsen et al. 2008). review. To validate the application of PSM, a simulation To increase productivity, repetitive scheduling meth- model was developed using the discrete event simulation ods are employed for repetitive projects. In repetitive (DES) method, and simulation results were analyzed. scheduling methods, the project schedule is adjusted Finally, to improve the efficiency of the schedule, using the production rate of the activities. According to Figure 1. Organization of literature review. JOURNAL OF ASIAN ARCHITECTURE AND BUILDING ENGINEERING 287 previous studies, the production rate refers to the pro- idle time of each crew; however, increasing interrup- ductivity of a particular crew and can be adjusted using tions up to a specific limit can reduce the project dura- the crew size and accelerated through crew allocation tion. Fan, Sun, and Wang (2012) presented an strategies. In these studies, different terms are used to optimization model for repetitive projects and the soft represent the production rate, such as production rate, logic of activity sequences was applied to this model. output rate, and production output (Moselhi and El- They suggested a time–cost optimal result by changing Rayes 1993; Kaming et al. 1997; Fan, Sun, and Wang the work activity sequence. Zou et al. (2016) developed 2012). To optimize the project schedule by adjusting formal mathematical models and a model to generate the production rate, a number of studies have been approximate solutions. These models are used to mini- conducted to optimize project schedules by adjusting mize project cost without exceeding a given duration; production rates (Carr and Meyer 1974; Chrzanowski this is a time–cost trade-off problem. In these models, and Johnston 1986; Ioannou and Yang 2016; Ipsilandis to increase production rate, if two activities are not 2006; Lee et al. 2015; Reda 1990). In terms of line-based related they are conducted concurrently in a work scheduling methods, repetitive activities are repre- unit and multiple crew allocation is considered. sented as lines with constant and varying slopes, Altuwaim and El-Rayes (2018) presented an optimiza- where the slope indicates the production rate. Line-of- tion model to minimize project duration, crew work balance (LOB) is a line-based scheduling method interruption, and interruption costs. In this optimization derived from the manufacturing industry. LOB graphi- model, the work continuity constraint is relaxed by cally presents any imbalance that suggests a deviation enabling work interruption because the project dura- from the plan and enables the project manager to tion and indirect cost can be reduced. Salama and quantitatively assess the deviation (Suhail and Neale Moselhi (2019) presented a multi-objective optimiza- 1994). When scheduling construction projects use tion model to minimize project costs, duration, and LOB, to prevent interference between activities, idling work interruptions where work interruption is also time is allocated for work continuity, which affects the enabled to reduce the project duration. These studies total duration of the project (Reda 1990). Arditi, focused on reducing the idle time between activities Tokdemir, and Suh (2002) suggested basic principles and optimizing interruption days by adjusting produc- that can be used for planning repetitive schedules and tion rates. However, line-based scheduling methods noted that optimal repetitive schedules can be allocate idle time to prevent or optimize work interrup- achieved by adjusting production rate and crew size tion in the schedule, and this increases the project using strategies to accelerate the production rate using duration. To reduce the idle time represented in LOB to adjust crew sizes. To reduce idle time while Figure 2(a), the TACT scheduling method based on line- maintaining work continuity in a line-based scheduling based scheduling methods is used. In the TACT method, several studies were conducted (Altuwaim and method, a building is divided into equivalent work El-Rayes 2018; Ipsilandis 2007; Nassar 2011; Salama and zones in which the same work activities are repeated. Moselhi 2019; Zou et al. 2016). Ipsilandis (2007) devel- The activities for one work zone are completed in the oped a multi-objective linear programming model for cycle time. The cycle time is equivalently divided and repetitive projects considering trade-offs in project equivalent durations are assigned to each activity. To schedules, where project duration, idle times, cost ele- complete the activities in the cycle time, different ments, and work unit completion duration are optimi- amounts of resources are allocated depending on the zation objectives. Nassar (2011) presented a model characteristics of each activity (Lee et al. 2015). In the using genetic algorithms to minimize project duration TACT method, idle time between activities can be and the number of interruption days for repetitive reduced; thus, the project duration can be reduced. projects. This is a trade-off problem. In other words, However, the production rate of each activity cannot maintaining work continuity can result in a reduced be exactly fitted to the assigned duration; thus, the Figure 2. Idling time in repetitive scheduling methods: (a) LOB method and (b) TACT method. 288 H. HYUN ET AL. work continuity of each activity in the TACT method production line, equivalent task times are assigned to may be lower than that in other line-based scheduling each station, and the products are moved after the methods. Therefore, when optimizing a project sche- assigned time elapses. The production rate of the dule using the TACT method, the cycle time and pro- production line is related to the output because duction rate of each activity are adjusted to improve the assigned duration can be reduced by increasing work continuity by reducing the idle time [Figure 2(b)]. the production rate. However, due to limitations in In previous studies, the production rate of activities resource allocation, there is also an upper bound of was adjusted to reduce idle time, project duration, cost, the production rate of each activity, which also limits and interruptions in project schedules. Production rate is the output of a production line. In the manufacturing increased by allocating more resources to a work unit or industry, the upper bound must be extended (Ege, by concurrently conducting multiple activities in the Azizoglu, and Ozdemirel 2009). To this end, PSM has unit. In terms of the work zone strategy, these studies been used (Alghazi and Kurz 2018; Askin and Zhou used a single work zone and adjusted the size of the 1997; Becker and Scholl 2006; Ege, Azizoglu, and work zone by integrating multiple work units into Ozdemirel 2009; Özcan 2019; Pinto, Dannenbring, a single work zone. However, there are constraints in and Khumawala 1981). PSM can increase the produc- scheduling on-site work when using repetitive schedul- tion rate by enabling activities that extend the cycle ing methods adopting a single work zone. Constraints time to be conducted in a two-sided (parallel) line such as workspace limitations and different amounts of such that workers simultaneously conduct the activ- on-site work limit the production rates. The constraint ity. For example, twice the number of workers can be for regulating the upper bound of the production rate is allocated to the same station for producing big pro- the workspace limitation in modular units because a unit ducts such as cars and household appliances because is used as a work zone. For example, when the approved of the relatively large parts and space. Hence, the upper limit of the number of workers is already allocated activity can be completed within the allocated for an activity, the production rate cannot be increased cycling time as the activity time is reduced by half. because more workers cannot be added to the unit When paralleling work stations, resources such as because of space interference. The workers cannot be workers and equipment should be equally assigned added to several units because the units are assembled to the paralleled stations, thus the cost is increased. simultaneously, which regulates the production rate. Ege, Azizoglu, and Ozdemirel (2009) proposed pro- The other constraint regulating the lower production duction line optimization algorithms to minimize sta- rate limit is the difference in work amounts, which tions on the assembly line and equipment cost. Work means different man-hours are required for different stations were paralleled to complete production in activities. In on-site work, there are activities that require the assigned cycle time and work stations to be little on-site work because some components are basi- paralleled were selected using an algorithm. Alghazi cally assembled in the factory and just connected on site. and Kurz (2018) developed a model for scheduling When reducing the production rate of the activities mixed model production lines to produce custo- where one is allocated, the number of workers cannot mized products that are not identical. For the pro- be reduced, resulting in idling time and loss of produc- duction of customized products, work stations tivity. To summarize, the upper or lower bound of the require different task times. Therefore, an average production rate decreases scheduling efficiency; thus, task time is assigned to each work station. However, the benefits of modular construction, such as short dura- the major drawback of this is that some tasks exceed tions and reduced labor resource requirements, cannot the assigned time. To overcome this, stations requir- be fully achieved. Considering these factors, to increase ing additional time are paralleled and the developed the production rate without workspace interference, model was used to minimize allocated workers. a multiple work zone strategy is required. Using this Özcan (2019) proposed a mathematical model and strategy, the constraints on the production rate can be algorithm to schedule paralleled production lines alleviated and idle time can be reduced; thus, the project that share resources and are called an interline work- duration can be reduced. Therefore, when applying this station. This workstation is different in that the pro- strategy, the limitations of existing scheduling methods duction line is paralleled to share resources. These using single work zones can be overcome. previous studies focused on different objectives such as minimization of workers, cost, and number of sta- tions; however, improvements were observed when 2.2. Parallel station method PSM was employed to extend the bound of the pro- duction rate. PSM can be a solution to make the The production lines of manufacturing industries production rate more flexible and extend the bound comprise a sequence of stations through which of the production rate of activities in modular on-site a set of activities is processed. The stations are linked work such that resources can be selectively allocated and products are generally moved by transportation to increase the production rate of activities. mechanisms such as conveyor belts. To balance the JOURNAL OF ASIAN ARCHITECTURE AND BUILDING ENGINEERING 289 The on-site work schedule, established using conven- 3. On-site work simulation model tional repetitive scheduling methods, is represented as 3.1. Model development a production line in manufacturing industries. During on-site work, workers cannot be allocated to two sides, To quantitatively validate scheduling methods apply- as in production lines, because on-site work is generally ing the PSM and to estimate idle time, on-site work conducted in units. Thus, the work amount of activities duration, and the number of workers needed, to be paralleled is doubled and double resources are a simulation model was developed. The simulation assigned to each unit. For example, if the time taken for method can deal with various scenarios and many one or more activities is longer than the assigned cycle studies have been conducted using simulation meth- time in the TACT method, parallel stations can resolve ods (Jung et al. 2017; Moghadam et al. 2012; this conflict, and the production rate in the LOB method Taghaddos et al. 2012). In the simulation environment, can be adjusted without workspace interference, as each scenario can be tested without impacting exter- shown in Figure 3. When the assigned time is 1 h in nal factors and results depending on the parameters the TACT method, the productivity of electrical work can be observed. Thus, the simulation method is sui- decreases because of workspace interference. To solve table to test scheduling methods for on-site work. As this problem, the activity cycle time of electrical work is a simulation methodology, DES was used. Modular modified to 2 h while not increasing the number of construction on-site work can be represented as workers required to complete the activity in 1 h. The a sequence of activities. DES focuses on the process workers then conduct the activity for two modular units level based on queuing theory and has been used to and the following plumbing connecting work for one model construction processes (AbouRizk, Halpin, and unit is completed in 1 h. Lutz 1992; Alvanchi et al. 2011; Lee, Han, and Figure 4 describes the application of PSM to on- Peña-Mora 2007; Mohsen et al. 2008). DES can provide site work. The figure shows that the electrical work information regarding activities and resources for two modular units is finished with the same required to conduct work activities (Alvanchi et al. amount of resources and half the original number 2011). Thus, DES can be used to develop a simulation of workers assigned to each unit, reducing work- model for modular on-site work to monitor the results space interference. Moreover, the allocated time for according to variations in parameters such as the pro- plumbing connecting work can be maintained at duction rate and cycling time. Further, it can help 1 h such that idling time can be reduced. estimate the number of workers to be allocated (Jung Therefore, the PSM allows a flexible production et al. 2017). As a simulation modeling tool, AnyLogic rate in modular on-site work to reduce idle time. 7.0 PLE, which provides a unified modeling methodol- In other words, repetitive scheduling methods in ogy for DES, was used, making it possible to develop combination with the PSM can be used for modular special-purpose models for on-site work process. The on-site work without constraints on production rate DES model represents the starting point of on-site adjustment. In this research, the PSM was applied to work activities with a start event and various condi- repetitive scheduling methods such as LOB and tions such as resource availability and dependency TACT to adjust on-site work production rates. relationship of activities. The completion of on-site Figure 3. Crew allocation in parallel station method. Figure 4. Parallel station method for modular construction on-site work. 290 H. HYUN ET AL. work is represented as a finished event, and the fin - which is the finish event. This model is based on the ished event is triggered depending on the time of each following assumptions: 1) The activities are not activity (Jung 2016). For model development, on-site affected by other external conditions. 2) Only the work daily report of a military facility modular con- labor resource is considered and other resources are struction project was used as the case project. To con- not considered. 3) The units are supplied at a constant sider the repetitiveness, repetitive work activities that rate regardless of the production environment or sche- are conducted in units are included as the project data dule. The production rate is used as the input and and the one-time work activities such as earthwork adjusted based on the number of workers. The produc- which is not directly related to the completion of tion rate is calculated by multiplying the productivity units were excluded. The facility comprises the same of the workers by the number of workers. The produc- types of units; however, repetitiveness was not consid- tivity of the workers is different depending on different ered when planning the project schedule. Thus, this activities; thus, the number of workers required to project was selected to compare the results derived achieve the same production rate is also different. from 1) schedules that consider repetitiveness 2) sche- This is considered as the characteristic of each activity dules that use PSM, and 3) real project data. Project in the model. In the model, the unit has man-hour data information is presented in Table 1. for each activity. When applying PSM, the work The developed model describes on-site work pro- amount is different for each activity. Using the produc- cesses, excluding transportation and unit assembly, as tion rate and work amount, the duration for each shown in Figure 5. In this model, seven types of on-site activity is estimated and each activity is considered work activities are described and workers are allocated completed after the estimated duration elapses. to each activity as resources. A unit is considered com- Buffers are placed between activities and the time pleted when all activities in the process are completed. spent in the buffers is considered idle time, which is The on-site work in this model starts when the first unit the nonproductive part of the model. When an activity is assembled, which is the starting event, and finishes is completed, the units are stored in the buffer until the when the last activity for the last unit is completed, idling time has elapsed, which prevents interference between activities in the LOB method or cycle time in the TACT method. As output data, the total number of Table 1. Information on Modular Construction Project. required workers, the duration of each activity, and the Modular Construction Project Information on-site work duration are presented. Location Jangdan-myeon, Paju-si, Gyeonggi-do, Republic of Korea Building Type Military Facility (Barrack) 3.2. Model verification Height 1 Story Project Duration 05.09.12 ~ 05.11.01 (51 days) (Off-site : 28 days; On- To validate the reliability of the developed simulation site : 23 days) Number of Units 30 units model, model verification was conducted. Mohsen Figure 5. Modular construction on-site work process in simulation model. JOURNAL OF ASIAN ARCHITECTURE AND BUILDING ENGINEERING 291 et al. (2008) verified the simulation model for on-site workers. By comparing the project data and verifica - work of modular construction projects by comparing tion results, this model is deemed to be reliable as it the results of the simulation model and real project represents on-site modular construction work and can data. If the derived results (e.g., the total number of be employed to test other scheduling conditions. workers, activity duration, and total on-site work dura- tion) agree with the project case data under similar conditions, the model is considered reliable and appro- 4. Simulation experiments priate for testing under other conditions such as repe- 4.1. Simulation information titive scheduling methods applying PSMs. Accordingly, conditions similar to the project data are applied to To estimate and compare the number of inputted work- verify the model. The numbers of workers required for ers and on-site work duration of repetitive scheduling activities listed in Table 2 are the input values. These methods for stick-build construction and repetitive are rounded up based on the number of workers scheduling applying PSM, simulation experiments per day in the project data and the total on-site work were conducted. Individual experiments were con- amount is set to 30 units. ducted and the established production rates of each The activities in the project were conducted at cer- scheduling method were inputted by adjusting the tain time intervals to increase the utilization rate of number of workers in each experiment. Estimated idle workers by maintaining work continuity. In the devel- times considering the production rate of each activity oped model, the intervals between activities are esti- were allocated to prevent interference between activ- mated based on the production rate of the previous ities. The maximum number of workers in each unit was activity. For example, the plumbing work starts after limited to four workers because of limited workspace, the fourth day of the electrical work. During these 4 and although the amount of work was small, which days, electrical work for 13.2 units can be conducted. causes idling time, one worker was allocated for con- Thus, the plumbing work starts when electrical work ducting an activity. Table 4 summarizes the simulation for 14 units is completed and the idling time between experiment information and results. the other activities is allocated in the same manner. In the first experiment, the LOB method was simu- The project data and verification results are compared lated and the production rate of activities was set to 2 h/ in Table 3. unit to reduce idling time between activities. The produc- The verification results demonstrate that the on-site tion rate was set to 2 h/unit because finishing work duration estimation is 22 days and the duration is the requires the longest duration when inputting one same as the project data. However, there is a difference worker, and by inputting four workers, the duration in the total number of workers because the allocated could be reduced by approximately 1.87 h, which is the number of workers for each activity is rounded up and maximum production rate in the LOB method. The pro- the difference between the actual and estimated dura- duction rate of each activity was set to 2 h/unit approxi- tions of each activity is represented as a difference in mately and idle times between activities were allocated input workers. This difference is attributable to the to prevent interference between activities. During the estimation of total number of workers multiplying the idle time, units are stored in the buffer of the model activity duration times by the input number of workers. and the next activity begins when the idle time has Furthermore, some activities started in the afternoon elapsed. The TACT method was simulated and the cycle as work starting time is not regulated in the simulation, time was employed. The activity cycle time in the TACT thus, resulting in an additional difference in input method was set to 2 h/unit to compare the results of the Table 2. Number of Workers Inputted to Model. On-site Work Activities Electrical Work Plumbing Work Tile Work Panel Work Finishing Work Equip. Installation Panel Joints Completion time by one worker 5.76 2.12 6.5 2.12 7.47 4.27 1.6 Activity Duration (Days) 9 4 5 4 5 5 3 Workers/day 2.4 2 5 2 5.6 3.2 2 Input Value 3 2 5 2 6 4 2 Table 3. Model Verification Results. On-site Work Activities Electrical Plumbing Tile Panel Finishing Equip. Panel Total Work Work Work Work Work Installation Joints Workers Total Number of Workers Project Data 22 8 25 8 28 16 6 113 Simulation Results 24 12 30 10 30 20 8 132 On-site Work Duration Project Data 2005.10.10 ~ 2005.10.31 (22 days) Simulation Results 2017.03.14 ~ 2017.04.04 (22 days) 292 H. HYUN ET AL. Table 4. Simulation Information and Results of Scheduling Methods. LOB (2 h/unit) TACT (2 h/unit) TACT using Parallel Stations (1 h/unit) Allocated Activity Required Allocated Activity Required Allocated Activity Required Workers Duration Workers Workers Duration Workers Workers Duration Workers Electrical Work 3 1.92 24 3 1.92 24 3 x 2 (6) 1.92 24 Plumbing Work 1 2.12 9 2 1.06 16 3 0.71 15 Tile Work 3 2.17 30 4 1.63 36 4 x2 (8) 1.63 40 Panel Work 1 2.12 9 2 1.06 16 3 0.71 12 Finishing Work 4 1.87 36 4 1.87 32 4 x2 (8) 1.87 32 Equip. Installation 2 2.14 20 3 1.42 27 3 x2 (6) 1.42 30 Panel 2 Work 1 1.6 9 1 1.6 9 2 0.8 8 Sum 15 13.94 137 19 10.56 160 36 9.06 161 Total Duration 15 days 13 days 9 days LOB and TACT methods. After 2 h, the units proceeded to conduct electrical work for two units, three workers the next activity and the required number of workers were allocated to each unit who worked for 2 h. After was allocated to finish the activity in 2 h. Although the 2 h, workers allocated to plumbing work conducted their activities were completed within 2 h, the units did not work for one unit in 1 h. Thus, the production rate could proceed to the next activity, indicating idling time. be increased without workspace interference. Finally, the TACT method applying PSM was simulated. With this combination (Figure 6), the cycle time was set 4.2. Simulation results to 1 h/unit, and PSM was applied to activities requiring more than four workers. To increase the production rate The simulation results of the scheduling methods were of electrical, tile, finishing, and equipment installation obtained using the developed model and results are work, the number of workers was doubled and two compared in Table 4. The simulation results demon- units were inputted for the activities. For example, to strate that to complete the modular project, LOB Figure 6. On-site work process for TACT applying parallel station method. JOURNAL OF ASIAN ARCHITECTURE AND BUILDING ENGINEERING 293 requires 15 days with 137 workers, TACT requires Table 5. Simulation Results of LOB+P and P+ LOB. Input Workers 13 days with 160 workers, and TACT using PSM TACT requires 9 days with 161 workers. These results demon- using LOB+P P+ LOB strate an inverse relationship between the on-site work LOB TACT PSM (2 h/unit) (1 h/unit) duration and the total number of workers needed. The Electrical Work 216 222 222 216 216 Plumbing Work 79 150 111 78 108 LOB scheduling method required the longest on-site Tile Work 246 300 304 240 288 work duration because the idling time between activ- Panel Work 81 148 111 81 108 ities was allocated to avoid interference between activ- Finishing Work 304 300 296 280 280 Equip. Installation 160 225 222 162 216 ities. Evidently, this idling time increased the total on- Panel 2 Works 68 75 74 60 72 site work duration; however, work continuity between Sum 1154 1420 1340 1117 1288 Total Duration 91 days 80 days 43 days 71 days 41 days activities was maintained such that the total number of workers was reduced. The total number of workers and duration of the TACT method using PSM yielded more conduct the activity for the unit in the buffer. To test effective results than the TACT method because the the effectiveness of the modified scheduling methods, PSM managed the production rate such that idle times namely, the LOB+P and P+ LOB methods, the number of activities were reduced. The shorter an assigned of units in the simulation model was increased from 30 time, the shorter is on-site work duration, thus redu- to 300 units, while other experimental conditions were cing the idle time caused by different work amounts in the same. The results are summarized in Table 5. The different activities (e.g., when the cycle time is 1 h, the results of the experiment are difficult to directly com- idling time is 1.126 h, and when the cycle time is 3 h, pare but can be indirectly compared through relative the idling time is 4.31 h when completing one unit). comparisons. Figure 8 represents the results of the Therefore, the application of the PSM can reduce the experiments. LOB+P outperforms TACT and LOB in on-site work duration by reducing the idle time in work terms of required labor resources and duration. P schedules. + LOB shows better results than TACT using PSM. Therefore, comparative results show that only LOB+P and P+ LOB remain and show relatively better results 5. Modification of scheduling methods than other scheduling methods. However, the results To improve the LOB and TACT methods using PSM, the also show an inverse relationship between on-site work duration and the total number of workers. Thus, scheduling methods were modified. First, the benefits of the PSM were applied to the LOB method (LOB+P). it can be said that the LOB+P method is cost-oriented For example, when twice as many workers conduct while the P+ LOB method is time-oriented. Although the PSM can improve the efficiency of on-site work activities for two units, the production rate can be doubled. By adjusting the production rate in the LOB scheduling, the project manager should choose an method, the activity rates become relatively uniform appropriate on-site scheduling method in accordance and this could reduce idle times between activities and with the purpose of the modular construction project. on-site work duration (Figure 7). Moreover, by applying the benefit of work continu- 6. Discussions ity in the LOB method to the TACT method using PSM, the buffer can be used to reduce the idling time (P In this model, it is assumed that 8 units are transported + LOB). With a buffer, work on a unit need not be and assembled on-site per day. The reduced 4 days of postponed until the cycle time has elapsed when the TACT using PSM compared to TACT means the total previous activity is completed. Instead, workers can project duration can be reduced by 4 days because proceed to the next unit. The completed unit goes to unit production and on-site work can be conducted the buffer until workers complete the activity. When concurrently and on-site work can start 4 days earlier workers have completed the activity for a unit, they using PSM and inputting one more worker. This means Figure 7. Production rate modification in LOB+P. 294 H. HYUN ET AL. Figure 8. Relative comparison of experiment results. that the reduced cost with reduced duration may be of each activity. Therefore, the suggested method greater than the cost of one more worker. Therefore, it indicates that the efficiency of existing TACT and is expected that by using the suggested method, pro- LOB methods can be improved using PSM. However, ject cost also can be reduced. It is also expected that the availability of labor resources at construction sites from the perspective of modular projects, the storage should be considered before using the suggested cost required to meet the production rate of both unit scheduling method, although modular construction production and on-site work can be reduced because on-site work requires fewer resources than those the on-site work production rate can be adjusted if the required for stick-built techniques. Moreover, the resource can be supplied on-site. shorter duration does not guarantee the same project In terms of the scheduling objectives, when the quality and the quality may even be lower than that production rates of activities in P+ LOB are set to fit in projects using existing scheduling methods. the cycle time, there is no idling time in the schedule Therefore, quality management also should be con- and the P+ LOB is similar to LOB+P. However, in sidered during project scheduling. To improve this practical situations, there is idling, and this implies research and estimate the project duration, the rela- that the project manager should choose an optimal tionship between manufacturing schedules and those scheduling method for ensuring project efficiency. of other on-site work activities such as foundation The project manager can use the modified scheduling work should be included. To improve the reliability methods to plan, estimate, and predict the on-site of the simulation model and use its results for mod- work schedule. It is important to consider the on-site ular projects, model verification using other project work schedule before conducting projects because data is required to obtain quantitative verification during on-site work, activities are simultaneously con- results. However, the model developed in this ducted in a tight schedule and in the later phase of research and the simulation results can be used to the project. Therefore, the schedule overrun of on-site compare scheduling methods and test the effective - work is directly related to the project schedule over- ness of the suggested scheduling methods. run; thus, this research focuses on on-site work. The scheduling method and model suggested in this 7. Conclusions research are based on the assumption that labor resources are used to adjust production rate and On-site work in modular construction comprises repeti- other resources are not considered. The experimental tive work activities. To improve the efficiency of this results showed that the scheduling methods requir- work, repetitive scheduling methods have been used. ing more labor resources, such as the TACT method However, when applying these methods intended for and TACT method using the PSM, had shorter dura- stick-built construction to modular on-site work, idling tions than those in the LOB method. However, TACT time occurs because of the limited bounds of produc- or LOB methods using PSM showed better results tion rates caused by workspace limitations and differ - than scheduling methods without PSM. This is ences in the work amounts of each activity. To address achieved by uniformly adjusting the production rate these limitations, the constraints of on-site modular JOURNAL OF ASIAN ARCHITECTURE AND BUILDING ENGINEERING 295 construction work were investigated and the reasons for Funding idling time were analyzed. Using PSM, the idling time in This work was supported by the Housing Environment repetitive scheduling methods could be reduced. Then, Research Program funded by the Ministry of Land, the scheduling methods were modified to improve the Infrastructure and Transport of Korean government efficiency of the LOB and TACT methods using the PSM. [20RERP-B082884-07]. The suggested scheduling methods, namely the LOB+P and P+ LOB methods, yielded better results than the Notes on contributors modified LOB and TACT using PSM. The results also revealed that the LOB+P method is cost-effective, Hosang Hyun received a doctorate degree at Seoul National while the P+ LOB method is time effective, implying University in 2019, and he has been dedicated to research at Seoul National University as a postdoctoral researcher. His that the primary objective of a modular project should main research area is construction engineering and be identified when choosing the scheduling method. management. The practical contributions of this research are as fol- Minhyuk Jung received a doctorate degree at Seoul National lows: 1) The idling time in repetitive scheduling meth- University, and he has taught and researched at Seoul ods can be reduced by using the suggested scheduling National University as a research professor. His main methods. 2) On-site work duration can be predicted research area is smart construction. more accurately compared to existing methods. 3) On- Inseok Yoon is a Ph. D Course at the Department of site work can be tightly scheduled. 4) The improved Architecture and a researcher at the Institute of schedule efficiency can alleviate the labor shortage pro- Construction and Environmental Engineering, Seoul blem faced by the construction industry. 5) In addition National University, Seoul, South Korea. His main research to reducing on-site work duration, project cost also can area is construction schedule optimization in construction management. be reduced by reduced project duration and unit sto- rage cost. The academic contributions are as follows: 1) Hyun-Soo Lee received a bachelor’s degree in 1983 and a master’s degree in 1985 at the Department of Architecture of Applying the concept of multiple work zones in the PSM Seoul National University. He has studied Construction to existing repetitive scheduling methods can overcome Engineering & Management at the University of Michigan the limited bounds of production rates. 2) Alternatives since 1988 and finished doctor’s degree in 1992. And he for work scheduling can be tested in controlled simula- worked for the Dept. of Architecture Engineering at Inha tion environments. 3) The effectiveness of the PSM can University as a professor. Since 1997, he has been working be tested without other external influences. However, as a professor at the Department of Architecture and Architectural Engineering of Seoul National University. His this research has limitations because although work- main research area is Construction Engineering and space limitations differ depending on the types of activ- Management. ities and projects, this research uniformly applies Jeonghoon Lee is an assistant professor of architectural engi- workspace limitations. 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Journal of Asian Architecture and Building Engineering
– Taylor & Francis
Published: May 4, 2021
Keywords: Modular construction; on-site work; labor allocation; repetitive work; parallel station method