Keywords

1 Introduction

Prefabricated building is a kind of building that is fabricated by prefabricated components on the construction site [1]. Because of its fast construction speed, environmental protection and energy saving, Prefabricated building have obviously become an important driver of the development of building industrialization.

With the continuous improvement of urban construction, in addition to building a large number of residential buildings, it is also necessary to construct supporting public service buildings. The supporting public service buildings, such as shopping malls, hospitals, schools, are mostly frame structures.

Compared with traditional cast-in-place mode, prefabricated frame structure has the problem of higher cost. The prefabricated frame structure system is a standardized design that separates components such as columns, beams, slabs, stairs, balconies, and exterior walls according to the characteristics of the building and structure. It is standardized prefabricated and produced in the factory, and mechanically installed and reliably connected on site to form a frame structure building [2].

Against the backdrop of the country vigorously promoting the transformation and upgrading of the construction industry, it is a challenge for most projects to meet the requirements of national or local construction administrative authorities for building assembly rates while effectively controlling project construction and installation costs.

This article takes the comprehensive building project of a certain hospital in Chongqing as a research example, quantitatively compares and analyzes the changes in the construction cost of each component of the prefabricated frame structure under different assembly rate schemes, and proposes corresponding cost control measures.

2 Project Background

2.1 Project Overview Description

The construction scale of a hospital project in Chongqing is approximately 42000 square meters. Among them, the comprehensive building has 9 floors above ground (including 3 podiums and corridors), totaling approximately 18400 square meters; The second floor of the underground garage is approximately 13000 square meters, with a planned height of 40.9 m.

2.2 Basis for Determining Assembly Rate

Assembly rate refers to the ratio of prefabricated component concrete volume to all concrete volume used within the scope of a building unit [3]. So the size of the assembly rate will inevitably affect the construction cost.

Therefore, this paper takes a hospital complex building project in Chongqing as the object, and according to the Detailed Rules for Calculation of Assembly Rate of Prefabricated building in Chongqing (Version 2021) [4], three sets of assembly schemes under different prefabricated assembly rates are formulated.

3 Assembly Plans with Different Assembly Rates

3.1 Cast-In-Place Frame Structure Scheme

The comprehensive building project adopts a traditional cast-in-place construction scheme. The specific methods are as follows: Beams, columns, and slabs are fully cast-in-place; 50% of the exterior walls are made of self insulating blocks, and 50% of the exterior walls are curtain walls; 50% exterior wall decoration adopts plastering + real stone paint; The interior wall adopts traditional masonry and plastering; The kitchen and bathroom are constructed using traditional techniques.

3.2 Plan of Assembly Rate 50%

According to the Evaluation Standard for Prefabricated building [5], those with an assembly rate of less than 50% cannot be called Prefabricated building. In order to meet the requirements of building assembly, an assembly scheme with a minimum standard assembly rate of 50% has been formulated.

The specific assembly scheme adopted is as follows: Using vertical components such as columns, supports, load-bearing walls, and ductile wall panels with a 15% ratio, score 10 points; Using 75% proportion of horizontal components such as floor slabs, stairs, balcony panels, and air conditioning panels, scoring 7 points; The proportion of standardized components used for prefabricated components reaches 70%, with a score of 2 points; 80% of the formed steel bar processing and distribution are integrated, with a score of 1 point; The non load-bearing enclosure wall adopts a thin masonry process wall with self insulation function, and scores 3 points; The internal partition adopts an 80% proportion of prefabricated internal partition integrated with pipelines, with a score of 7 points; 6 points for fully decorated public buildings; Using a 77% proportion of integrated toilets, scored 4 points; Using a 90% pipeline separation ratio, score 6 points; Effective transmission of BIM [6] data in design, production, and construction, with a score of 1 point; Digitize the identity of on-site management personnel by using electronic signatures and seals, with a score of 1 point; Digitize construction operation behavior and management behavior, score 2 points.

3.3 Plan of Assembly Rate 65%

On the basis of the minimum required assembly rate of 50%, increase the assembly type indicators of some projects.

The specific plan is as follows: Increase the proportion of prefabricated components used for horizontal components such as floor slabs, stairs, balcony panels, and air conditioning panels from 75% of the original plan by 76%, and increase the score by 1 point; The plan for non load-bearing retaining walls has been changed from 100% using thin masonry walls with self insulation function in the original plan to 50% using prefabricated retaining walls integrated with insulation, insulation, and decoration, with an increase of 7 points; Increase the integrated kitchen project by 90% and increase the score by 6 points; Add real-time generation of digital archive projects, increase score by 1 point.

Through the adjustment of the above four parts of the project, the assembly rate of the project can reach 65%.

3.4 Plan of Assembly Rate 90%

In order to further investigate the impact of increased assembly rate on project cost, the assembly index of the project was further improved on the basis of the 65% assembly rate plan.

The specific plan is as follows: Increase the proportion of vertical component prefabrication from the original 15% prefabricated columns to 75% prefabricated columns, with a score increase of 15 points; Increase the use of 50% prefabricated beams and increase the score by 5 points; Increase the standardized proportion of prefabricated components from 70% to 80%, and increase the score by 1 point; Increase the use of 70% overhead, dry laying, or thin pasting processes, with a score increase of 2 points; Increase the proportion of integrated bathrooms to 80% and increase the score by 2 points;

Through the adjustment of the above five parts of the project, the assembly rate of the project can reach 90%.

4 Comparative Analysis of Frame Structure Costs Under Different Assembly Rates

4.1 Scope of Cost Analysis

To reflect the cost differences under different assembly rate schemes, floors above ± 0.00 were selected as the object, including the main body, rough decoration, external wall insulation and facade decoration engineering fees, measure fees for individual buildings, temporary construction construction fees, and project management fees; But it does not include the cost of roof, fine decoration, electromechanical, door and window, and railing engineering.

4.2 Principles of Cost Analysis

This project adopts the bill of quantities pricing model [7] to calculate the cost of each sub project of the project. Firstly, use BIM modeling software to calculate the quantities of each sub project. Secondly, the comprehensive unit price [8] of each sub item shall be determined in combination with the documents such as Chongqing Housing Construction and Decoration Engineering Pricing Quota [9] and Chongqing Prefabricated building Engineering Pricing Quota [10]. Finally, determine the cost of each sub project according to formula (1), and obtain the unit cost index of each sub project according to formula (2).

$$ {\text{Cost}}\;{\text{of}}\;{\text{sub}}\;{\text{projects}} = \Sigma \left( {{\text{Bill}}\;{\text{of}}\;{\text{quantities}}\;{\text{for}}\;{\text{sub}}\;{\text{items}} \times {\text{comprehensive}}\;{\text{unit}}\;{\text{price}}} \right) $$
(1)
$$ {\text{Divisional}}\;{\text{and}}\;{\text{sub}}\;{\text{item}}\;{\text{unit}}\;{\text{cost}} = {\text{Divisional}}\;{\text{and}}\;{\text{sub}}\;{\text{item}}\;{\text{project}}\;{\text{cost}}/{\text{building}}\;{\text{area}} $$
(2)

4.3 Cost Comparison Analysis

Comparative Analysis of Main Structure Costs

Based on the pricing principle mentioned above, under different assembly schemes, the unit cost data of the main structure of the comprehensive building project is shown in Table 1:

Table 1. Comparison Table of Cost Differences in Main Structure
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Cost Comparison and Analysis of Outer Envelope Structures

The unit cost data of the external maintenance structure of the comprehensive building project is shown in Table 2:

Table 2. Comparison Table of Cost Differences in Peripheral Structures
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Comparative Analysis of Single Unit Cost of Comprehensive Building Project

The trend of project cost per square meter cost changes for different assembly rate schemes based on traditional schemes is shown in Fig. 1:

Fig. 1.
figure 1

Run chart of change of single square cost of complex building under different assembly rates

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4.4 Analysis of the Relationship Between Assembly Rate and Cost

After integrating project data, it was found that there is a certain relationship between the increase in construction cost and assembly rate. Therefore, Origin software was used for analysis, hoping to find a regression formula to describe the changes in construction cost under different assembly rates.

According to the Origin fitting results, the relationship between the construction cost increase (y) and the assembly rate (x) of the Prefabricated building system project can be expressed as formula (3):

$$ {\text{y}} = - {\text{Ax}}^4 + {\text{Bx}}^3 - {\text{Cx}}^2 + {\text{Dx}} - {\text{E}} $$
(3)

According to relevant research [11], the assembly rate of Prefabricated building is between 10% and 20%, and the curve shows an upward trend with a large slope, that is, the construction cost increases significantly with the increase of the assembly rate; The assembly rate is in the range of 20% to 40%, and the curve is almost horizontal, indicating a stable increase in construction costs, maintaining around 35%; The assembly rate is in the range of 40% to 65%, and the curve shows an upward trend again, reaching the peak of construction cost increase when the assembly rate reaches 65%. The slope is relatively large but smaller than the slope value of the assembly rate in the range of 10% to 20%; When the assembly rate exceeds 65%, the curve shows a downward trend, meaning that the increase in construction costs slows down with the increase in assembly rate.

Therefore, based on the trend of the curve and the use of regression formulas, the possible increase in construction costs can be estimated simply based on the application scale or assembly rate of prefabricated components. There are differences in the cost increases corresponding to different projects at the same assembly rate. There are differences in the construction techniques used in different projects. Previous studies have shown that achieving more efficient utilization of tower cranes on construction sites can achieve the effect of saving construction costs. Under the same assembly rate, the more efficient utilization of mechanical equipment and the increase in project schedule will have a certain impact on construction costs.

5 Conclusion and Suggestions

5.1 Conclusion

Essentially, there are significant differences in construction processes between prefabricated and cast-in-place construction methods. The different construction processes and organizational management methods result in different economic, social, and environmental benefits.

The construction costs corresponding to different prefabricated construction schemes vary greatly, and the incremental and reduced costs must be based on the technical system, which cannot be simply measured by assembly rate.

There is no strong linear positive correlation between the level of assembly rate and cost increment, and selecting an appropriate assembly plan is crucial for cost control of assembly construction; After selecting the appropriate assembly plan, the higher the assembly rate, the lower the construction cost. Similarly, when implementing fully assembled dry construction in low-rise and multi-story buildings, the cost is more advantageous.

On the basis of reducing costs through large-scale production, the maturity of the technical system can also significantly reduce construction costs. At the same time, the quality improvement, construction period reduction and policy dividend brought by the cost increment of Prefabricated building should be comprehensively considered.

5.2 Suggestions

The scheme comparison and selection of Prefabricated building should be considered at the scheme stage. In the scheme design stage, only by adjusting the prefabricated scheme and solving the problems of standardization, modularization and cost optimization from the source can the problem of high cost of Prefabricated building be fundamentally solved.

Summarize the cost reduction path of the design and coordination system by comprehensively balancing the safety, quality, cost, and progress of the project. For example, standardization of household products, PC components, molds, production layout optimization, and final assembly plan optimization can further reduce costs throughout the entire process.