# Scheduling Optimization of Prefabricated Construction Projects by Genetic Algorithm
## 1 Introduction  
In recent years, the construction industry has paid increasing attention to the use of modular prefabricated parts. In China, prefabricated buildings have recently become a popular form of construction [1, 2, 3]. The prefabricated building transfers part of the traditional on-site construction work to the factory for completion [4]. The modular components are completed in the factory manufacturing center, and then transported to the final project location to form the building [5]. The prefabricated building splits the construction process into two parts, namely, the off-site prefabricated component production process and the on-site assembly construction process; thus, the on-site construction scheduling arrangement of the prefabricated building project must consider the production and supply of prefabricated components [6, 7]. Related studies have found that project scheduling has relatively high value in theoretical research and specific applications. It not only promotes the development of engineering projects but also serves as an important reference for engineering project decision-making, especially under resource-constrained conditions. An effective scheduling plan is a necessary condition for successfully completing the entire project [8, 9]. The unreasonable project scheduling plan formulated due to the failure to fully consider the characteristics of the prefabricated building will make it difficult for on-site construction activities to be implemented as planned, resulting in a series of negative rippleconsequences; hence, prefabricated construction projects cannot be completed on time and the cost increases [10]. In the past few decades, Critical Path Method (CPM) and Program Evaluation and Review Technique (PERT) are the main planning methods to solve project scheduling [11]. The two methods of CPM and PERT usually assume unlimited resources [12; 13]. However, resource supply is generally limited in an actual project implementation, and the project schedule generated by CPM or PERT usually needs to be further optimized after considering resource constraints. Accordingly, scholars began to consider the resource-constrained project scheduling problem (RCPSP). In the past 10 years, many researchers have devoted themselves to solving RCPSP through various meta-heuristic methods. At present, researchers mainly focus on enhancing the performance of meta-heuristic algorithms by using the knowledge of genetic algorithms (GAs) [14]. However, the resource-constrained project scheduling implementation method of traditional buildings cannot be completely adapted to the realization of prefabricated construction project scheduling for the problem of prefabricated construction project scheduling. The project scheduling method of conventional buildings is only adequate for scheduling repetitive tasks in prefabricated building schedules [15; 16]. Assembly construction is constrained by a variety of limited renewable resources, such as management personnel, lifting equipment (crane, flatbed truck) and labor workers (crane driver, installer), etc. Although the project scheduling of prefabricated buildings still belongs to the scope of the resource-constrained project scheduling problem, it is significantly different from the traditional building that the construction site activities of prefabricated buildings can only start after the required prefabricated components arrive on time. Under normal circumstances, the production of prefabricated components is scheduled according to the schedule of on-site construction. However, the business of manufacturing companies is usually operated under the condition of multiproject execution. Resource sharing and competition exist between projects, and the failure to timely supply prefabricated components is often an important reason for the failure to complete prefabricated construction projects [17]. Accordingly, it is difficult to directly apply the traditional scheduling method to the scheduling research of prefabricated buildings [18].