Cost Case Analysis丨Comparison of cast-in-situ and SPCS structural systems of Yucheng Building 2!

Prefabricated buildings have become the main direction of the development of China's construction industry, and are encouraged and supported by the state and governments at all levels. At present, the development of prefabricated buildings in China is mainly driven by top-down policies, mainly because the cost of prefabricated structural engineering is slightly higher than that of cast-in-place structural engineering, which hinders the market promotion of prefabricated buildings to a certain extent. Most construction units' understanding of project cost is limited to the explicit cost of the whole life cycle, which separates the integrity and systematization of the project as an organism. In order to maximize the overall benefits of the project, it is necessary to further broaden the breadth and time horizon of the cost. The scope of cost should not only include the explicit cost of the project, but also include the hidden cost or hidden benefits such as the construction period cost and environmental cost, and the time span of the cost should include the construction stage, the use stage and the demolition stage.

This paper takes Yucheng Building 2 as an example to analyze the economic benefits of the whole life cycle of prefabricated buildings and cast-in-place structures. The project is a reinforced concrete shear wall structural system, and the above-ground structural parts of Building 2 (cast-in-place) and Building 2 (prefabricated SPCS) are selected for analysis (two practices in one building).

1. Economic benefits in the design stage

The design cost of Building 2 (prefabricated SPCS) is 30 yuan/m2, and the design cost of Building 2 (cast-in-place) is 22 yuan/m2.

The economic benefits in the design stage are: F1 = 22-30 = -8 yuan/m2

2. Economic benefits in the construction phase

The cost per unit floor area in the construction stage is 1149.37 yuan/m2 for Building 2 (prefabricated SPCS) and 895.84 yuan/m2 for Building 2 (cast-in-place).

The economic benefits of the construction stage are: F2 = 895.84-1029.43.03 = -133.59 yuan/m2

3. Economic benefits in the use stage

There is little difference between the prefabricated SPCS and cast-in-situ in Building 2 in terms of property management fees, and the difference in water-saving measures is small, so the economic benefits brought by property management fees and water-saving are not considered.

(1) Daily maintenance cost C1

Daily maintenance cost: 40 yuan/(year/m2) for prefabricated buildings; The cast-in-place type is 65 yuan/(year/m2).

(2) Overhaul cost C2

The life span of prefabricated buildings and cast-in-place buildings is calculated as 50 years. Through the investigation of the literature, it is found that the maintenance cycle of waterproofing engineering and main structure engineering is 8 years and 20 years respectively, the maintenance cost of waterproofing engineering of prefabricated buildings is CA=3.8 yuan/㎡, the maintenance cost of main structure is CB=15.4 yuan/㎡, and the maintenance cost of the main structure after 40 years is CC=30.8 yuan/㎡, while the cast-in-place building is CA=5.7 yuan/㎡, CB=30.8 yuan/㎡, and CC=61.7 yuan/㎡.

(3) Energy consumption cost C3

Prefabricated building is a new type of energy-saving and environmentally friendly building, and the reduction of energy consumption cost is significant. The cost of energy consumption mainly considers the heating, electricity, cooling and gas costs in the use stage. According to the survey data, the energy consumption cost of prefabricated buildings is 13.56 yuan/(year/m2), and cast-in-place buildings are 22.56 yuan/(year/m2).

Without taking into account inflation, the discount rate is 8%.

Net present value of the cost of use of Building 2 (prefabricated SPCS):

NPV1 = (C1 + C3) * (P/A, 8%, 50) + CA * {(P/F, 8%, 8), + (P/F, 8%, 16) + (P/F, 8%, 24) + (P/F, 8%, 32) + (P/F, 8%, 40) + (P/F, 8%, 48) } + CB* (P/F, 8%, 20) + CC * (P/F, 8%, 40) = 655.23 + 4.35 + 3.3 + 1.42 = 664.3 yuan/m2

Net present value of the cost of use of Building 2 (cast-in-place):

NPV2 = (C1 + C3) * (P/A, 8%, 50) + CA * {(P/F, 8%, 8) + (P/F, 8%, 16) + (P/F, 8%, 24) + (P/F, 8%, 32) + (P/F, 8%, 40) + (P/F, 8%, 48)} + CB* (P/F, 8%, 20) + CC * (P/F, 8%, 40) = 1071.16 + 6.53 + 6.61 + 2.84 = 1087.14 yuan/m2

The economic benefits of the use phase are:

F3 = 1087.14-664.3 = 422.84 yuan/m2

4. Economic benefits in the demolition phase

The final remainder of the prefabricated building is 10% of the total construction cost, and the final remainder of the cast-in-place building is 3% of the total construction cost.

Net present value of proceeds from the demolition of Building 2 (prefabricated SPCS):

NPV1 = 1149.37 * 10% (P/F, 8%, 50) = 2.64 yuan/m2

Net present value of proceeds from demolition of Building 2 (cast-in-place):

NPV2 = 895.84 * 3% (P/F, 8%, 50) = 0.619 yuan/m2

The economic benefits of the demolition stage are: F4 = 2.64-0.619 = 2.02 yuan/m2

5. Economic benefits of the construction period

The savings caused by the construction period include the savings in financing costs, the early return of funds brought by early sales, the capital income of housing rental and the savings of direct costs of engineering entities.

The proportion of pre-sale receipts for Building 2 (prefabricated SPCS) and Building 2 (cast-in-place) is 30%, and the sales price is 7,000 yuan/m2. Considering the cost of decoration and construction, the construction cost of Building 2 (prefabricated SPCS) is 1149.37 yuan/m2, and the construction cost of Building 2 (cast-in-place) is 895.84 yuan/m2. During the construction process, the structural part of Building 2 (prefabricated SPCS) was shortened by 27.5 days compared with the construction period of Building 2 (cast-in-place). The construction period of the main structure of Building 2 (prefabricated SPCS) and Building 2 (cast-in-place) is 49.5 days and 77 days respectively.

The parameters for the calculation of the duration benefit assume that the loan interest is 10% and the opportunity cost of the capital is 12%.

(1) Capital income (early collection)

The construction period of the No. 2 building of the prefabricated residential building is short, the opening is early, and the capital benefit brought by the early collection is: R = 7200 * 30% * 12% / 365 * 27.5 = 19.53 yuan / m2

(2) Loan interest

C = 7200 * 0.1 / 365 * 27.5 = 54.26 yuan / m2

(3) Earnings of engineering entities

Compared with the traditional cast-in-place structure, the prefabricated SPCS structure system saves about 20% of the construction period, and the corresponding direct costs mainly include tower crane leasing fee of 5 yuan/㎡, scaffolding leasing fee of 7 yuan/m2, and management fee of 12 yuan/㎡, with a total saving of about 24 yuan/㎡.

The economic benefits brought by the construction period are:

F5 = 19.53 + 55 + 24 = 98.53 yuan/m2

6. Policy economic benefits

All provinces in the country have given certain preferential conditions to the prefabricated structure, among which the preferential policies of Dezhou City, Shandong Province include land use policy, tax policy, financial policy, science and technology policy, and other policies. According to the construction area of the prefabricated part of the exterior wall (no more than 3% of the total planned construction area), it can not be included in the calculation of the plot ratio of the transaction plot, and the prefabricated construction area of the exterior wall of Yucheng Building 2 is about 115 square meters, and the development cost is 2,600 yuan/㎡.

The economic benefits brought by the policy are:

F6 = 115 * (7200-2600) / 3823 = 138 yuan / ㎡

7. Environmental benefits

Because the building will have a certain impact on the environment at all stages of its life cycle, so in the process of promoting the development of prefabricated buildings, people are very concerned about its environmental benefits, that is, whether the prefabricated buildings can be coordinated with the external and internal environment to develop together, and the prefabricated buildings just implement the national policy of "four sections and one environmental protection", which has significant environmental benefits and can save about 25 yuan per square meter.

From the perspective of the whole life cycle of Building 2:

The incremental cost of the prefabricated structure compared with the cast-in-place structure is: F cost = |F1|+|F2|=8+135.59=141.59 yuan/m2

The incremental benefit is: F benefit = F3 + F4 + F5 + F6 + F7 = 422.84 + 2.02 + 98.53 + 138 + 25 = 686.39 yuan / m2

The benefits of the whole life cycle prefabricated structure of Building 2 compared with the cast-in-place structure are: F = F benefit - F cost = 686.39-141.59 = 544.80 yuan/m2

It can be seen that the initial construction process of prefabricated buildings will bring certain incremental costs. However, from the perspective of the whole life cycle of the project, the incremental benefits such as the construction period benefits brought by shortening the construction period through the prefabricated construction method can reduce the incremental costs brought by the prefabricated building to a certain extent, and even produce considerable economic and social benefits.