Component-level construction schedule optimization for hybrid concrete structures
Introduction
Due to the increasing labor cost and the requirement of green construction, prefabricated construction developed rapidly in recent years, especially in countries with large demands of new buildings, such as China. Compared with traditional cast-in-situ (CiS) construction, prefabricated construction has better quality, higher efficiency, wastes less and is more environmentally friendly [[1], [2], [3], [4]]. According to the 13th five-year plan of China, by the end of 2020, more than 15% of newly built buildings should be prefabricated. Categorizing in terms of material, there are three major types of prefabricated construction: wood structure, steel structure and reinforced concrete structure [5]. Because of the market supply and demand, the most commonly used prefabricated structure type in China for buildings is reinforced concrete structure, which is also known as precast concrete (PC) structures. In addition, due to the constraint of design codes in China, most PC structures adopt hybrid concrete (HC) structures, which integrate PC components and CiS components to make the best advantage of their different inherent qualities [6]. HC structures are also being applied in other countries, such as Japan. In HC structures, PC components are connected by CiS components, thus a neighboring PC component becomes a part of the framework of the CiS components, so that both kinds of components can have influence on each other during construction.
Schedule is essential to the success of a construction project, since it is used to guide the whole construction process, and has influence on the makespan, cost and resource utilization of the project. For traditional CiS structures, construction schedule is commonly on construction zone-level, in which an activity in the schedule is a construction process of a floor or a construction zone. For example, reinforcement work of the 1st floor, mold work of the 2nd floor, concrete work of the construction zone A, and so on. Such kind of schedule is suitable for traditional CiS structures, since the construction unit is a floor or a construction zone. However, the construction unit for HC structures may attain to a single PC component or CiS component, instead of a floor or a construction zone, so the construction schedule for HC structures should be component-level. Namely, in such cases, each single activity may correspond to a single component. As a result, the construction sequence and accurate construction time of each component need to be specified in such component-level construction schedule, and the time unit of the schedule should be refined to hour or even minute. Since PC components and CiS components have influence on each other, the component-level construction schedule can contribute to eliminating the construction conflicts by specifying the accurate construction time of each component. Besides, similar to traditional construction schedule, such a schedule should be optimized in order to shorten the construction makespan, decrease the construction cost, and enhance the resource utilization efficiency. Thus, the optimized component-level construction schedule is crucial to promote refined construction management for HC structures.
Construction schedule optimization is commonly modeled and solved as a resource-constrained project scheduling problem (RCPSP), which is a widely studied problem in construction field. The basic RCPSP model minimizes the construction makespan under the constraint of certain quantities of resources [7]. There are also a number of extended models for RCPSP, such as the model that considers multiple modes or multi-skill workers, the model that considers minimum or maximum time lags, and so on [7]. In a recent study by Wang et al., a general RCPSP model which can cover most information requirements and extract required data from information model automatically was proposed and solved by constrained programming method [8]. However, few studies about RCPSP take construction sequence of components into consideration, thus cannot be used to optimize the component-level construction schedule for HC structures.
Several studies have been dedicated to component-level consideration and optimized the construction sequence of prefabricated construction. Shewchuk et al. proposed a lean approach to optimize the locations of stacks, stacking locations and assembly sequence of prefabricated wall panels to minimize the quantity of stacks, panel material handling distance and the work required to position and brace panels, while the interference of the panels was ignored [9]. Rausch et al. presented a framework for optimally planning the assembly sequence of prefabricated components to minimize the on-site rework caused by geometric variability in production [10]. Wang et al. proposed a BIM and genetic algorithm-based method to generate an optimal assembly sequence of PC interior walls considering the construction difficulty and interference [11]. These component-level studies optimized the construction sequence of prefabricated components by considering the aspect of the minimization of workload, rework, or construction difficulty. However, they did not take time, cost, resources into account, thus are inadequate for component-level construction scheduling. In addition, when optimizing the construction sequence, they only considered PC or other prefabricated components without considering CiS components, thus can hardly be used for the construction of HC structures.
Liu et al. developed a BIM-based approach for detailed construction schedule optimization for light gauge steel construction, in which the construction schedule was component-level [12]. However, the outcomes of this study cannot be applied to HC structures, because PC and CiS components have different nature compared with light gauge steel components as far as construction planning is concerned.
In summary, to the best of the authors' knowledge, no existing study has dealt with the optimization of the construction schedule for HC structures from the aspects of construction makespan and cost considering both PC components and CiS components on component-level under resource constraints. Therefore, the first research question of this study is how to properly model the component-level resource-constrained project scheduling problem for HC structures (C-RCPSP-HC).
To solve the C-RCPSP-HC model, an efficient multi-objective optimization algorithm needs to be formulated to solve the model. In construction scheduling field, a number of meta-heuristic algorithms, including genetic algorithm-based algorithms [13], particle swarm optimization-based algorithm [12], and ant colony optimization-based algorithm [14], has been used by researchers. However, most of them need a trial and error process to determine the most proper parameters, i.e. crossover probability and mutation probability for genetic algorithm-based algorithms.
Symbiotic organisms search (SOS) is a novel meta-heuristic algorithm, which simulates the symbiotic interactions, i.e., mutualism, commensalism, and parasitism, adopted by organisms to survive and propagate in the natural ecosystem [15]. Similar to the crossover operation and the mutation operation in genetic algorithm, SOS adopts three operations, i.e., mutualism operation, commensalism operation, and parasitism operation, to create new feasible solutions from existing feasible solutions [15]. The original SOS was designed to solve single objective and continuous problem, such as structure design optimization [15]. Then, it was developed to discrete versions (DSOS) to solve traveling salesman problem [16], and multi-objective versions (MOSOS) for continuous problem to solve multi-objective structure design optimization [17].
In this study, SOS is selected as the basic optimization algorithm to develop a new optimization algorithm to solve the C-RCPSP-HC model for the following reasons. Firstly, compared with other meta-heuristic algorithms, SOS involves no control parameters, thus has no trouble of parameter tuning. Secondly, in construction scheduling field, DSOS or MOSOS has been proved to have better performance than other meta-heuristic algorithms in solving time-cost-labor utilization trade-off problem [18], large-scale time-cost trade-off problem [19], time-cost trade-off problem in repetitive project [20], and resource leveling problem [21].
Considering the C-RCPSP-HC model is multi-objective and discrete, existing DSO or MOSOS cannot be directly used to solve it. Therefore, the second research question of this study is how to develop a multi-objective discrete SOS (MODSOS) to solve the C-RCPSP-HC model.
To answer the research questions, this paper is organized as follows. Firstly, taking HC shearing wall structure as an example, Section 2 analyzes the construction process of HC structures, to determine the scope of the C-RCPSP-HC. Based on the scope, Section 3 formulates the C-RCPSP-HC model by specifying its decision variables, constraints and objectives. Then, Section 4 develops a multi-objective discrete symbiotic organisms search (MODSOS) to solve the model. Section 5 presents a case study from a real HC structure project to verify the approach. Finally, Section 6 draws the conclusions and suggests for future work.
Section snippets
Analysis on construction process of HC shearing wall structures
In China, HC structures are mainly used for residential buildings, specifically in the form of HC shearing wall structures, i.e., the shearing wall structures based on HC structures. Thus, this study takes HC shearing wall structure as an example for the modeling and solving of the C-RCPSP-HC. The approach can also be applied to other structure types, such as HC frame structures, with a number of minor modifications.
In a PC residential building, normally the foundation, basement and the first
Proposed C-RCPSP-HC model
Based on the analysis in Section 2, the C-RCPSP-HC model is formulated by extending the basic RCPSP model.
Firstly, in the basic RCPSP model, the quantities of renewable resources are usually modeled as known input parameters, which are determined by project planners directly, thus they may not be optimal. Considering that the quantities of renewable resources have influence on the construction makespan and the construction cost, the C-RCPSP-HC model considers the quantities of renewable
MODSOS for solving the C-RCPSP-HC model
In this study, a multi-objective discrete SOS (MODSOS) is developed to solve the C-RCPSP-HC model. Fig. 3 presents the flowchart of the MODSOS, in which the process of the algorithm is the same as in [18] and an organisms coding method integrated with a modified serial schedule generation scheme is developed specifically for this study. Details about the MODSOS are illustrated as follows.
Case study
To verify the proposed approach for component-level construction schedule optimization for HC structures, a case study derived from a real project is conducted in this section.
Conclusion
This study proposes a novel approach to optimize the component-level construction schedule for hybrid concrete (HC) structures. Firstly, the construction process of HC shearing wall structures is analyzed to determine the scope of the component-level resource-constrained project scheduling problem for HC structures (C-RCPSP-HC). Then, the C-RCPSP-HC model is formulated, which aims to optimize the construction makespan and cost under the resource constraints and precedence relationships on
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study has been supported by the Tsinghua-Glodon Joint Research Center for Building Information Modeling.
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