1 Introduction

With the rapid development of the world economy, the high consumption of energy resources and the impact of environmental pollution continue to plague the world's ecological civilization. In order to comply with the global climate change trend and China's national conditions, China has put forward a clear goal of carbon neutrality. In 2022, China's total carbon emissions were about 12.1 Gt CO2, accounting for 33% of the global total [1]. However, the total carbon emissions of buildings in China in 2020 were 5.08 billion t CO22, accounting for 51% of the national carbon emissions [2].

Currently, there are extensive research studies on carbon emissions across various types of constructions. This encompasses studies on commercial and residential buildings [3], low-carbon communities [4], rural buildings [5], prefabricated buildings [6, 7], among others. Studies have been conducted on various stages of construction, encompassing the materialization phase [8], the construction process [9], and the steel production stage [10]. Presently, there seems to be a relative dearth of research concerning carbon emissions specifically related to prefabricated steel structures. With the rapid development of prefabricated steel structure buildings, the extensive use of steel will intensify the energy consumption of prefabricated steel structure buildings, resulting in an increase in carbon emissions [11]. Prefabricated steel structures will become an important source of greenhouse gas emissions. Therefore, it is of great and urgent significance for the low-carbon development of prefabricated steel structure buildings to fully study the carbon emission problem of prefabricated steel structure column connection joints, and to explore the carbon reduction development advantages of prefabricated steel structure construction compared with traditional steel structure construction.

The construction industry is recognized as a significant contributor to global carbon emissions, often ranking among the top three sectors in this regard [12]. Scholars worldwide have extensively researched methodologies for calculating carbon emissions in buildings. These methodologies primarily encompass the input–output method, process-based method, and hybrid approaches [13]. Scholars summarize the current research hotspots of carbon emissions from public buildings (CEPB) into five categories: (1) theoretical research and simulation modeling; (2) energy systems; (3) materials; (4) public building renovation; (5) The main factors leading to the decrease of CEPB [14]. Huang et al. [15] conducted research on carbon emission policies and carbon emission calculations in different countries around the world and across the country. Panagiotis Chastas et al. [16] towards zero emission and zero energy buildings, literature reviews highlight the importance of embodied energy and embodied carbon emissions. There are also studies discussing how to reduce carbon emissions in the manufacturing and construction industries by applying carbon reduction technologies [17, 18], analyzing the carbon footprint of manufacturing systems and construction processes [19], exploring appropriate carbon calculation tools [20], reviewing environmental impact reduction strategies [21], the carbon reduction potential of implementing digital (Industry 4.0) concepts in manufacturing [22]. Steel is the main material of prefabricated steel structure buildings, and it is also the main source of carbon emissions in building materials. Bernardino D’Amico et al. [23] used a computational tool based on an optimization framework that considered the embodied carbon in the amount of steel used in building structures. Li et al. [24] and Ren et al. [25] researched and analyzed the influencing factors of carbon emissions in the iron and steel industry and low-carbon technologies, and put forward the development approach for the reduction of greenhouse gas emissions in China's iron and steel industry. Chen et al. [26] explored how the existing literature can effectively reduce the carbon emissions of steel structure building products from a cradle-to-site perspective. Due to the increasing use of steel structures in modern buildings, the demand for steel is also increasing. How to reduce the carbon emissions of steel has attracted the attention of more scholars.

In recent years, most of the research on carbon emissions from prefabricated buildings has focused on prefabricated concrete, precast and cast-in-place components, and high-rise and low-rise assembled buildings, etc. Emanuele Bonamente et al. [27] and Susan Abed Hassan et al. [28] conducted research on greenhouse gas emissions (carbon footprint) and primary energy consumption (energy footprint) of multi-storey buildings in Italian companies and Iraq, respectively. Dong et al. [29] used a life cycle assessment (LCA) model to calculate the carbon emissions of a private residential building in Hong Kong, and concluded that precasting can reduce carbon emissions by 10% per cubic meter of concrete. Mo et al. [30] analyzed an office building with an assembly rate of 96.8% in Hangzhou, Zhejiang. The total measured carbon emissions of the research object was \(2265.73 t C{O}_{2}e\), which was \(22 kg C{O}_{2} e/{m}^{2}\) lower than that under the non-prefabricated method. Guo et al. [31] concluded that when enterprises determine the prefabrication range of 35–40% of prefabricated buildings, they can obtain the maximum carbon reduction effect at the minimum cost. Jack C. P. Su et al. [32] developed a system to evaluate the carbon emissions and costs of product designs, helping companies optimize assembly processes to minimize carbon emissions. However, there are very few studies on carbon emissions based on fabricated steel structures. Patricia González-Vallejo et al. [33] studied the carbon footprint of metal structure projects in Romania and concrete structure projects in Spain, and concluded that metal structure buildings have a greater impact on the economy and the environment than reinforced concrete buildings. You et al. [34] compared the differences in CO2 emissions between brick-concrete structures and steel–concrete structures in urban residential buildings. Zhu et al. [35] calculated the carbon emissions of the steel structure residential project of Hangxiao Steel Structure Co., Ltd. in the whole life cycle of the building. Xu et al. [36] conducted a life cycle assessment of the carbon emissions of classical types of long-span structures, considering both space frame structures with bolted spherical joints and space frame structures with welded hollow spherical joints.

To explore the potential advantages of energy efficiency and emission reduction in prefabricated steel structures, this study examines prefabricated steel structural column connection joints. Currently, the primary connection methods for these joints include flange connection joints and full penetration welding connection joints. For steel building joints, welding can provide some strength and stiffness, but the construction time is long and generates large carbon emissions and pollution. For steel–concrete structures, steel–concrete bonding is generally used [37], which provides better integrity, and concrete bonding usually requires less energy than welding, which may reduce carbon emissions during construction. The choice of the specific connection method has to take into account the design specification, assembly process and environmental impact. The innovative flange connection joints eliminate on-site welding, reducing both welding material usage and mechanical energy consumption. This addresses the inefficiency and high energy consumption associated with on-site welding, presenting significant carbon emission reduction benefits. Conducting a carbon emission assessment for these joints quantifies their energy-saving effects, providing empirical support for the sustainable development of prefabricated steel structures. A comprehensive exploration of the structural aspects, performance, and construction techniques of steel column connection joints contributes to improving the construction efficiency of prefabricated steel structures, reducing energy consumption and material wastage, advancing the industrialization of steel structures in China, and fostering sustainable development. In this team, Yanxia Zhang et al. [38] proposed a box-shaped column flange connection achieved by plug welding-core sleeve. The term "plug welding" refers to the plug welding of the lower column during the factory processing of prefabricated components, there is no welding on the construction site, all connections are bolted. Additionally, Zhang et al. [39] compiled Technical Standard for Fully Bolted Assembled Steel Structures of Multi-High-Rise Storey Buildings (T/CSCS 012–2021), and this paper is permitted to conduct carbon emission-related research on this joint. In the actual fabricated steel structure project, the connection joint has the advantages of ensuring the rigid connection performance at the joint, realizing high-efficiency assembly of box columns, saving engineering costs, and has the advantages of low pollution, green energy saving. Wu et al. [40] studied the carbon emission characteristics of various welding processes. In contrast, the traditional full penetration welding joints have disadvantages such as high energy consumption and large pollution, which leads to large carbon emissions and low efficiency in the construction of traditional steel structures.

As one of the construction industrialization development paths, prefabricated buildings are the primary means by which the construction industry can achieve its carbon peaking and neutrality goals. Therefore, it is necessary to conduct a comprehensive study on the carbon reduction of fabricated steel structures in actual projects. This paper focuses on the dormitory building project of the Tongzhou Campus of the Affiliated High School of Capital Normal University, and adopts the quota-based carbon emission factor method to calculate the carbon emissions of the steel column connection joints at different phases of the assembled steel structure building. It also compares the carbon emissions in the construction phase with the traditional full-fusion welded connection joints, provides technical support for the subsequent energy saving and emission reduction of assembled steel buildings, and promotes the sustainable development of assembled steel buildings.

The research object has the advantages of typical application, suitable conditions, convenient analysis, and practicality that it reflects. At the same time, based on the concept of comparative analysis, from the perspective of different phases of the entire life cycle of steel column connection joints, different types of joints, and the entire building, different calculation ranges are established and the differences in joint member data are compared and analyzed in depth. At last, the advantages of low-carbon construction of flange connection joints of fabricated steel structures are discussed to serve as a reference for the development of low-carbon fabricated steel structures.

2 Backgrounds

The construction project at the Tongzhou Campus of the Affiliated High School of Capital Normal University represents Beijing's initial adoption of fully bolted prefabricated steel structures without welding. Therefore, selecting the flange connection joint as the subject of study holds paramount significance. Given the widespread application of prefabricated construction in the building industry and its pivotal role within the overall prefabricated construction system, this joint has also been applied in the new outpatient and emergency medical technology building of Shougang Hospital of Peking University and the second phase of Shenzhen New Energy Vehicle Industrial Park, improving the overall performance, construction speed, and energy efficiency of prefabricated steel structures. This choice aligns with national policy directives aimed at achieving industrialization and sustainability goals, while also bearing practical relevance for various construction endeavors.

In this paper, the dormitory building of Tongzhou Campus of the Affiliated High School of Capital Normal University is used as the calculation model (Fig. 1). The main structural members of the dormitory building all adopt the steel frame structure system. The assembled steel building adopts box-shaped column flange connection achieved by plug welding-core sleeve, the splicing position is at the top of the 1st floor underground, the top of the 2nd floor above ground, the top of the 4th floor and the top of the 6th floor, and contains a total of 160 box-shaped column flange connections achieved by plug welding-core sleeve. Since the joint installation components have been processed and fabricated in the factory, only manual bolt installation is required during the on-site construction phase, which reduces the material consumption of welding and mechanical fuel consumption compared to the conventional steel column connection joint full penetration welding process, resulting in greater carbon reduction benefits.

Fig. 1
figure 1

Dormitory building calculation model

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Therefore, this study utilizes the dormitory building as a model project for carbon emission analysis. Considering the construction involving the box-shaped column flange connection achieved by plug welding-core sleeve employed in the current project development, this joint configuration was selected as the focus of this research. By collecting statistical data concerning construction, materials, and equipment, carbon emissions are accounted for in various stages of building. By comparing the data results from different scenarios, suggestions conducive to energy-saving, emission reduction, and sustainable development in prefabricated steel structures are proposed.

The flange connections achieved by plug welding-core sleeve is located at the beam-column joint connection, and the specific structure is shown in Fig. 2. Among them, the upper column and the lower column are equipped with flange plates and connected by high-strength bolts; a core cylinder is set in the column at the splicing of the upper and lower columns, and a spacer is set in the corresponding position. The cross section of the core cylinder is octagonal, and the thickness of its wall plate is greater than the thickness of the column wall plate according to the force at the joint. The core sleeve is manufactured in the factory with a negative tolerance of 1 mm ~ 2 mm and preassembled in the factory to ensure smooth installation of the connection joint in the field. Additionally, to ensure a reliable connection between the core sleeve and the column wall, tapping bolt holes are located on the upper column, while corresponding plug welding holes are created at the respective positions on the lower column.

Fig. 2
figure 2

Detail of flange connection achieved by plug welding-core sleeve

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The prototype was taken from the steel column splicing joint of the dormitory building of Tongzhou Campus of the Affiliated High School of Capital Normal University. In this paper, a two-story high steel column is analyzed as a unit, and the calculation is conducted for the building's typical size joint, with a total height of 7400 mm, of which the upper column is 3700 mm, the lower column is 3700 mm, and the cross-sectional size of the column is 500 × 500 × 22 × 22 mm. The size of the flange plate is 740 × 740 × 30 mm, the upper and lower flanges are connected by 20 M24 high-strength bolts. The detailed drawing of the core sleeve is shown in Fig. 3.

Fig. 3
figure 3

Detailed diagram of core sleeve /mm

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3 Methodology

3.1 Parameters affecting carbon emissions

3.1.1 Activity level data and sources

Activity level data, often referred to as carbon source data, represents energy consumption [41]. The prototype specimen for this study is derived from the steel structure column connection joint within the dormitory building at the Tongzhou Campus of the Affiliated High School of Capital Normal University. The required activity-level data collection encompasses machinery usage and labor statistics necessary across various phases, including steel column fabrication, installation, transportation, and dismantling involved in the construction process of the dormitory building. Since some of the activity data such as construction material transportation are not suitable for use on to individual objects, the uniform specification data in the Consumption Quotas for Building Construction and Decoration Works [42] are used for calculation. In this paper, data such as activity level and construction of production process of flange connection joints are compiled by collecting project purchase list and settlement information. The statistical indicators of the project are shown in Table 1.

Table 1 Statistical indicators
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3.1.2 Carbon emission factor data

The carbon emission factor refers to the emission of some greenhouse gases, mainly CO2, per unit of activity, and also includes the emission of methane (CH4) and nitrous oxide (N2O). The International Energy Agency (IEA) unifies the emissions of various greenhouse gases as carbon dioxide equivalent CO2e. There is no measured carbon emission factor in this study, so carbon emissions are represented by carbon dioxide equivalent CO2e. For the steel production phase, the carbon emission factors for steel components in this study were determined according to the activity data collection and calculation methods provided in the Methodology and Reporting Guide for Greenhouse Gas Emissions of Chinese Steel Producers (for Trial Implementation) [43]. In addition, for the activity level data of construction material transportation, building construction and dismantling phases, this paper adopts the carbon emission factors provided by the Standard for Building Carbon Emission Calculation (GB/T 51366–2019) [44], as well as the data from foreign studies such as the IPCC Guidelines for National Greenhouse Gas Inventories, and the carbon emission factors that can be referred to in published domestic papers, which are more relevant for this assembled steel column splicing joint.

3.2 Calculation range

3.2.1 Carbon emissions greenhouse gas range

According to the International Energy Agency (IEA), the emissions of various greenhouse gases are unified as carbon dioxide equivalent (CO2e), so the carbon emissions in this study are also quantitatively analyzed according to CO2e, and the carbon emission factors are also expressed as carbon dioxide equivalent for greenhouse gas emissions.

3.2.2 Accounting boundaries

For the carbon emission calculation of the box-shaped column flange connections achieved by plug welding-core sleeve, the scope of the accounting system includes the joint in each phase of building material processing and transportation, building construction and construction, and demolition, with the carbon emission calculation time boundary of the construction phase from the start of the project to the completion and acceptance of the project, and the carbon emission calculation time boundary of the demolition phase from the demolition to the demolition dismemberment and transportation out from the floor. Using CO2e as the greenhouse gas accounting boundary, the carbon emission size of this steel column splicing joint was calculated and evaluated. The building material processing to construction phase of this joint belongs to the materialization phase of the assembled steel structure, which is the whole process of the bill of quantities from the extraction and processing of materials, transportation, construction and building to the completion of delivery. Since the calculation of carbon emission for the box-shaped column flange connections achieved by plug welding-core sleeve is too frequent for multiple sub-component projects one by one, the quantity consumption quota for the work of assembled construction is referred to, and the sub-component projects are grouped into component fabrication, component installation, personnel activities, and mechanical equipment energy consumption.

3.3 Calculation method

The 2006 IPCC Guidelines for National Greenhouse Gas Inventories issued by the Intergovernmental Panel on Climate Change (IPCC) proposed three carbon emission calculation bases [45], and this paper adopts the emission factor method proposed therein. According to other studies related to carbon emissions of buildings in the context of carbon neutrality target, there are also input–output method, material balance method and other calculation models.

The input–output method is a top-down calculation approach suitable for predicting the direction and extent of changes in targets based on inputs and outputs over a period. It measures direct and indirect carbon emissions. However, this method lacks in-depth analysis of specific processes and faces challenges in data collection, resulting in significant uncertainty in the calculated results. These factors render it unsuitable for carbon emission calculations in this study.

The material balance method involves a comprehensive analysis of carbon material flows, requiring thorough tracking and assessment of inputs and outputs. This method is intricate and involves substantial work due to its complexity. It is suitable for calculating carbon emissions across entire ecosystems or specific processes. However, the characteristics of connections in prefabricated steel structures pose challenges when employing this methodology.

The carbon emission factor method refers to a technique where the quantity of emitted gases per unit product in normal technical, economic, and managerial conditions is statistically calculated to derive the total emissions using average values. This method requires clear data on carbon emission activities and emission coefficients, often focusing on activity level data and emission factors. In the case of connections in prefabricated steel structures, where the primary material is steel, the involved processes are relatively straightforward. The input–output material types are singular, and the carbon content remains relatively stable, resulting in fewer uncertainties. Compared to other methods, this approach is more conducive to evaluating the individual contribution of specific joints within a building towards energy conservation and emission reduction. It offers a more objective reflection of the actual carbon emissions from steel column joints to a certain extent.

For the carbon emission calculation of the box-shaped column flange connections achieved by plug welding-core sleeve, which is the subject of this paper, the emission factor method is adopted as the carbon emission calculation model commonly used at present. China's Standard for Building Carbon Emission Calculation (GB/T 51366–2019), which was promulgated and implemented based on IPCC guidelines, has a higher value of use for China's existing buildings and energy carbon emission factors in line with China's national conditions. In addition, this paper also refers to the Assessment Standard for Green Building (GB/T 50378–2019) [46] promulgated by the Ministry of Housing and Urban–Rural Development, the Guidelines for the Preparation of Provincial Greenhouse Gas Inventories (for Trial Implementation) [41] promulgated by the National Development and Reform Commission and other relevant national standards and specifications that are in line with China's greenhouse gas inventory. The specific formula and symbols are as follows:

3.3.1 Component fabrication phase

The steel structure column joint serves as a prefabricated component. This study refers to the engineering quantity list of the steel column production phase in the quota, hence, the carbon emissions in this phase stem from emissions produced by materials, labor, and machinery. According to the Standard for Building Carbon Emission Calculation and relevant research papers, the carbon emissions in the steel column production phase should be calculated according to the following formula:

$$C_{zz} = \sum\limits_{i = 1}^{x} {C_{jci} + C_{zp} }$$
(1)
$$C_{jci} = T_{i} \times MF_{i}$$
(2)
$$C_{zp} = T_{r} \times RF_{i} + \sum\limits_{i = 1}^{n} {E_{zz,i} \times EF_{i} \times Q_{zz} }$$
(3)
$$E_{zz,i} = \sum\limits_{j = 1}^{m} {T_{i,j} \times R_{j} }$$
(4)

where,

\(C_{zz}\) ——Carbon emissions from individual joint during the fabrication phase of steel columns;

\(C_{jci}\) ——Carbon emissions of \(ith\) building materials production;

\(C_{zp}\) ——Carbon emissions from steel column assembly phase;

\(T_{i}\) ——Consumption of \(ith\) material in the fabrication phase of steel columns;

\(MF_{i}\) ——Carbon emission factor of the \(ith\) material;

\(T_{r}\) ——Consumption of man-days during the fabrication phase of steel columns;

\(RF_{i}\) ——Carbon emissions per unit workday of labor;

\(E_{zz,i}\) ——Total energy use of the \(ith\) type in the construction phase of the building \((kWh \ or \ kg)\);

\(EF_{i}\) ——Carbon emission factor for energy type \(i(kgC{O}_{2}e/kg \ or \ kgC{O}_{2}e/kWh)\);

\(Q_{zz}\) ——Quality of individual joint in the fabrication phase of steel columns \((t)\);

\(T_{i,j}\) ——The \(jth\) type of construction machinery shift consumption consuming the \(ith\) type of energy;

\(R_{j}\) ——The \(jth\) type of construction machinery unit shift consumption energy usage.

3.3.2 Construction phase

The carbon emission in the construction phase of the building includes the carbon emission generated from the completion of the construction of each sub-component and the carbon emission generated from the construction process of each measure project. For the construction phase of steel splicing joint, according to the Standard for Calculation of Carbon Emission of Buildings and relevant research papers, the calculation of carbon emission in the construction phase of buildings should be calculated according to the following formula:

$$C_{jz} = T_{r} \times RF_{i} + \sum\limits_{i = 1}^{x} {T_{i} } \times MF_{i} + \sum\limits_{i = 1}^{n} {E_{jz,i} \times EF_{i} } \times Q_{jz}$$
(5)
$$E_{jz,i} = \sum\limits_{j = 1}^{m} {T_{i,j} \times } R_{j}$$
(6)

where,

\(C_{jz}\) ——Carbon emissions from individual joints during the construction phase of steel columns;

\(T_{r}\) ——Consumption of man-days during the construction phase of steel columns;

\(RF_{i}\) ——Carbon emissions per unit workday of labor;

\(T_{i}\) ——Consumption of material \(i\) in the building construction phase;

\(MF_{i}\) ——Carbon emission factor of the \(ith\) material;

\(E_{jz,i}\) ——Total energy use of the \(ith\) type in the construction phase of the building \((kWh \ or \ kg)\);

\(EF_{i}\) ——Carbon emission factor for energy type \(i(kgC{O}_{2}e/kg \ or \ kgC{O}_{2}e/kWh)\);

\(Q_{jz}\) ——Single joint volume in building construction phase \((t)\);

\(T_{i,j}\) ——The \(jth\) type of construction machinery shift consumption consuming the \(ith\) type of energy;

\(R_{j}\) ——The \(jth\) type of construction machinery unit shift consumption energy usage.

3.3.3 Building material transportation phase

The transportation of materials in prefabricated construction can be categorized into the transport of prefabricated components and other building materials. For building materials, short to medium-distance transportation commonly utilizes road transport. Due to the fewer construction materials involved in prefabricated steel structure connection joints, it's challenging to individually assess the transportation distances of specific building materials based on actual circumstances. Hence, this study calculates the transportation of steel and metal components based on the Consumption Quotas for Building Construction and Decoration Works metal transport table. For other (non-metal) construction materials, the transportation distances primarily rely on average distances from referenced research outcomes and national-level average transportation distances for major construction materials [47].

The primary focus of this study is on construction materials such as steel, bolts, and non-metals. Due to the fact that the equipment required for construction is not only used in the fabrication and installation of steel column joints but also in steel beams and other sub-projects, it is challenging to differentiate the carbon emissions from transportation of this equipment. Furthermore, due to the absence of relevant quota data, this aspect has not been taken into consideration.

According to the Carbon Emission Calculation Standard for Buildings, the carbon emission of this connection joint in the transportation phase of building materials should be calculated according to the following formula:

$${C}_{ys}=\sum_{i=1}^{n}{M}_{i}\times {D}_{i}\times {T}_{i}$$
(7)

where,

\(C_{ys}\)—— Carbon emissions from the transportation of building materials \((kgC{O}_{2}e)\);

\(M_{i}\) —— Consumption of major building materials \((t)\);

\(D_{i}\) —— The average transportation distance of the \(ith\) building material \((km)\);

\(T_{i}\) —— Carbon emission factor per unit weight of transportation distance under the transportation mode of the \(ith\) building material \([kgC{O}_{2}e/(t\cdot km)]\).

3.3.4 Building demolition phase

The carbon emissions during the demolition phase encompass building dismantling and transportation of debris. As the current status of the project's buildings remains undemolished, data pertaining to the demolition processes are unavailable for collection. Therefore, the carbon emissions for this phase will be estimated through simplified calculations.

According to Ju et al. [48, 49], the energy consumption during the demolition phase of a building constitutes 90% of the energy consumption during the construction process. Therefore, the carbon emissions during the building demolition phase contribute 90% of the carbon emissions during the installation and construction phase. The carbon emissions from the transportation of demolition debris are estimated to be 90% of those generated during the transportation of construction materials [47]. Carbon emissions from the demolition phase should be calculated according to the following formula:

$$C_{cc} = C_{jz} \times 90\% + C_{ys} \times 90\%$$
(8)

where,

\({C}_{cc}\)—— Carbon emissions from steel column joints during the building demolition phase \((kgC{O}_{2}e)\).

4 Calculation results of emission factor method

With reference to the specified parameter range and calculation method, carbon emission calculation and result analysis are carried out for the box-shaped column flange connection achieved by plug welding-core sleeve using the collated member material, dimensions and other data.

4.1 Calculation of carbon emissions by phase

4.1.1 Steel column joint fabrication phase

The subject of this study is the internal sleeve-type flange connection joints of steel box columns, primarily composed of upper and lower columns, flange plates, high-strength bolts, and octagonal inner sleeves. The inner sleeve flange joints and their connection to the steel columns are factory-processed, with on-site construction focusing solely on the installation of high-strength bolts at the flange plates of the upper and lower columns. This process achieves efficient vertical assembly and promotes green construction practices. This section focuses on the fabrication process of steel structure column joint connections, primarily involving: plug welding between the core sleeve and the lower column wall, tapping bolt alignment between the upper column and the core sleeve, and initial fastening using high-strength bolts (manually). Specific data regarding activity levels were referenced from the Consumption Quotas for Building Construction and Decoration Works steel column fabrication consumption table. The calculated total mass of the finished steel column joint in the model amounts to \(2808.05 kg\), with the steel column itself weighing \(2443.50 kg\).

As the Consumption Quotas for Building Construction and Decoration Works already factors in material wastage, this study references the consumption data from its steel column fabrication table. This is coupled with the volumetric details and material quantities from the structural diagram of the column joint components for computation. According to Yang [50], national standards specify a \(6\mathrm{\%}\) wastage rate for steel material production, while finished steel structures do not account for wastage. Hence, this paper calculates the carbon emissions of steel material during the building material production stage by multiplying the finished quality of steel used in both types of joints by \(1.06\). Based on a company's standard Welding Material Consumption Quota Standard (Q/HZ MB103-79) and the research conducted by Huang et al. [51], calculations were made concerning the plug welding process. The quantity of plug welding holes and the required volume of deposited metal were used to calculate the necessary materials such as welding rods, wires, and associated mechanical energy, which were then incorporated into the carbon emission inventory for this phase.

In accordance with the Consumption Quota for Assembly Building Construction [52] among the steel column installation quota, the amount of installation work is calculated by the quality of the finished components in accordance with the size of the design drawings, and does not deduct the quality of a single area \(\le {0.3m}^{2}\) of the hole, welding, rivets, bolts do not increase the quality. Therefore, the fabrication process of single steel structure column splicing joint is calculated according to \(\le 3t\). The material is Q345B steel, M24 high strength bolts. Utilizing Eqs. (1)- (4), the carbon emissions inventories for steel structure column joint fabrication phase are detailed in Tables 2 and 3.

Table 2 Carbon emission inventory of labor and materials during the steel column fabrication phase
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Table 3 Carbon emission inventory of machinery during the steel column fabrication phase
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According to the report published by the IEA in 2022, our annual per capita carbon emissions in 2017 were about 7.56 t. Therefore, the artificial carbon emission factor at that time was calculated to be 6.90 kgCO2e/workday [56]. For fossil fuel carbon emission calculation, the fuel consumption needs to be multiplied by the default net calorific value of that fuel, i.e., the average low calorific value (e.g., the default net calorific value of diesel is \(42522 kJ/kg\)), and then multiplied with the unit calorific value carbon emission factor to find the carbon emission. For power-consuming machinery, the total power consumption needs to be multiplied with the power carbon emission factor of the regional power grid data built at the time of construction, where the power carbon emission factor is taken from the North China regional power grid \(2017\) annual emission factor of \(0.9680 tC{O}_{2}/MWh\), to find the carbon emission of power-consuming machinery. From the aforementioned carbon emission inventory, it can be deduced that the total carbon emissions for the steel column fabrication stage amount to \({C}_{zz} = 1911.22 kg\). This includes \(167.02 kg\) from manual labor, \(1049.10 kg\) from materials, and \(695.11 kg\) from machinery. It is evident that material-related carbon emissions contribute to over \(54.8\mathrm{\%}\) in this phase, while machinery accounts for \(36.4\mathrm{\%}\) of the emissions.

4.1.2 Steel column joint construction phase

The construction project of Capital Normal University's Tongzhou Campus adopts prefabricated steel structures with fully bolted connections, ensuring efficient assembly of the steel framework. The construction phase for steel column connection joints involves solely the use of high-strength bolts at the flange of the upper and lower columns. Reference was made to the bolt installation data table in the engineering consumption quota [42] to calculate the consumption of labor, materials, and machinery, considering the required quantity of high-strength and tapping bolts for a single joint. According to Eqs. (5)- (6), the carbon emission inventories during the construction phase of a single steel structure column joint are presented in Table 4 and 5.

Table 4 Carbon emission inventory of labor and materials during the steel column construction phase
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Table 5 Carbon emission inventory of machinery during the steel column construction phase
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According to the above data, the total carbon emission in the construction phase of steel structure column joint is \(155.90 kg\), of which the mechanical carbon emission is \(137.41 kg\), the artificial carbon emission is \(8.70 kg\), and the material carbon emission is \(9.79 kg\). From the data, it can be seen that the mechanical carbon emission in the construction phase of a single steel structure column joint accounts for the carbon emission of the whole phase is more than \(73\%\).

4.1.3 Building material transportation phase

This study is based on the actual transportation distance of building materials, and the list of work volume in the transportation phase is referred to the quota for the transportation phase of metal components in the Consumption Quotas for Building Construction and Decoration Works. Considered by the processing plant to the construction site, the metal structure transportation is divided into category I (steel column) and category II (high-strength bolts and other fragmentary components) for this joint. The origin of category I components is Hangxiao Steel Structure Co., Ltd., and the transportation distance is 1264 km. The origin of category II components is Handan, Hebei, and the transportation distance is 452 km.

The inventory measurement unit is 10 tons, and there's no need to factor in losses for the finished prefabricated components. Therefore, the consumption for an individual steel structure column joint is calculated based on the quality of finished steel materials. Furthermore, the carbon emissions from the transportation of non-metal materials were also taken into consideration. Regarding the transportation of construction equipment, as the torque wrench used for this steel column joint is challenging to investigate in terms of actual transportation method and constitutes a small proportion, it has not been taken into account. Due to the fact that cranes are not limited to lifting steel columns and the challenges in accurately quantifying carbon emissions from actual transportation data, this aspect has not been taken into consideration.

Based on the average energy consumption data for Chinese road transportation [57] and Yang's study [50] on the diesel truck's energy consumption per ton-kilometer, the diesel road transportation carbon emission factor is determined to be \(0.176 kgC{O}_{2}e/(t\cdot km)\). According to Eq. (7), the carbon emission list in the transportation phase is shown in Tables 6 and 7.

Table 6 Carbon emission inventory during the transportation phase
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Table 7 Carbon emission inventory of non-metallic materials during the transportation phase
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Based on the above inventory, the total carbon emissions for the transportation phase of the steel structure column joint can be calculated as \({C}_{ys}=1517.02 kg\). Among them, the carbon emission in the transportation phase of the first class member steel column is \(1513.76 kg\), accounting for more than \(99.9\%\). Due to the minimal proportion of miscellaneous components such as bolts within the studied components and their varying transportation distances, the carbon emissions during the transportation phase for these secondary components are exceedingly low. The carbon emissions from other non-metal materials are also very minimal.

4.1.4 Building demolition phase

Because the prefabricated steel structure building was not dismantled, relevant data for the demolition phase could not be gathered. Hence, carbon emissions for this phase were estimated in a simplified manner. Based on the calculations above, the carbon emissions for the steel column flange joint during the construction and transportation phases are \(155.90 kg\) and \(1517.02 kg\), respectively. According to Eq. (8), the calculated carbon emissions for the dismantling phase of a single steel structure column joint amount to \({C}_{cc}=1505.63 kg\).

4.2 Carbon emissions from single steel column connection joints

The carbon emissions at various phases of the box-shaped column flange connection achieved by plug welding-core sleeve are illustrated in Fig. 4. Combining the calculated data analysis, the total carbon emissions for a single steel structure column joint amount to \(5089.77 kg\), primarily stemming from the steel column fabrication phase. This is attributed to the significant carbon emissions from material consumption during this phase, accounting for over \(54.8\%\) of the emissions in that phase. The carbon emissions during the transportation phase constitute the second-largest contribution to the overall carbon emissions. This is attributed to the extensive transportation distance and the excessive material requirement for individual prefabricated steel structure column joint components. The proportion of carbon emissions in each phase is depicted in Fig. 5. Concerning the carbon emissions of this box-shaped column flange connection achieved by plug welding-core sleeve joint, the carbon emissions from the steel column fabrication phase constitute \(67.5\%\) of the total. The contributions to carbon emissions, from greatest to least, are as follows: steel column fabrication phase > transportation phase > dismantling phase > construction phase.

Fig. 4
figure 4

Carbon emissions at each phase of the box-shaped column flange connections achieved by plug welding-core sleeve

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Fig. 5
figure 5

Carbon emission ratio of each phase of box-shaped column flange connections achieved by plug welding-core sleeve

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5 Discussion

5.1 Comparison with full penetration welded joints

In this section, the model for calculations is based on the Sunshine Hall project in Nangang District, Harbin [58]. The structure of this project comprises entirely of steel, with the primary steel columns being box-shaped and constructed using Q235B steel plates designed for high-rise buildings. The on-site installation of steel columns involves fully penetration welded joints. We proceed to calculate the carbon emissions associated with the construction data of these joints. The steel column quantity for full penetration welded column joints remains consistent with flange joints. Additionally, the full penetration welded joints lack a core sleeve and flange plate, featuring an additional partition plate, ear plate, and connecting plate. The dimensions of the ear plate and connecting plate are referenced from the 16G519 Multi-story Civil Steel Joint Detailed Drawings [59]. The calculated total steel mass for the finished full penetration welded steel column joint amounts to \(2573.14 kg\). The specimen joint configuration is shown in Fig. 6:

Fig. 6
figure 6

Full penetration welding box column connection joint structure [58]

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5.1.1 Steel column joint fabrication phase

In the carbon emission calculations for the full penetration welded joint, the primary difference compared to the carbon emissions of the box-shaped column flange connection achieved by plug welding-core sleeve joint lies in the welding process. This specifically involves the mechanical usage and welding material quantity during the steel structure construction phase. The carbon emission inventories for the steel column fabrication phase are shown in Tables 8 and 9.

Table 8 Carbon emission inventory of labor and materials during the fabrication phase of full penetration welded steel column joints
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Table 9 Carbon emission inventory of machinery during the fabrication phase of full penetration welded steel column joints
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Similarly according to Eqs. (1)-(4), the total carbon emissions for the full penetration welded steel column fabrication phase amount to \({C}_{zz}=1832.40 kg\). This includes \(167.02 kg\) from labor, \(995.32 kg\) from materials, and \(670.06 kg\) from machinery. Materials accounted for \(54.3 \%\) of the carbon emissions from the fabrication of the steel columns, and machinery accounted for \(36.6 \%\) of the carbon emissions from this phase.

5.1.2 Steel column joint construction phase

The carbon emission inventories for the construction phase of the full penetration welded steel columns are shown in Tables 10 and 11.

Table 10 Carbon emission inventory of labor and materials during the construction phase of full penetration welded steel column joints
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Table 11 Carbon emission inventory of machinery during the construction phase of full penetration welded steel column joints
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Based on Formula (5)- (6), the carbon emissions for the full penetration welded steel column joint during the construction phase amount to \({C}_{jz}=308.84 kg\). This comprises \(69.84 kg\) from labor, \(79.35 kg\) from materials, and \(159.65 kg\) from machinery. Machinery contributes the most to the emissions in this phase, accounting for \(51.7 \%\) of the total emissions.

5.1.3 Building material transportation phase

Following the same transportation method as with flange joints, the carbon emissions during the transportation phase for steel and fragmentary metal components were computed using engineering quotas. Similarly, other materials were calculated based on average energy consumption and transportation distances for road transport. The carbon emission inventory for the construction phase of full penetration welded steel columns is illustrated in Tables 12 and 13.

Table 12 Carbon emission inventory of machinery during the transportation phase of full penetration welded steel column joints
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Table 13 Carbon emission inventory of machinery during the transportation phase of full penetration welded steel column joints
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Based on the inventories above, utilizing Eq. (7), the total carbon emissions during the transportation phase for full penetration welded steel column joints amount to \({C}_{ys}=1388.06 kg\). Among these, the carbon emissions for Type I components in steel column transportation phase amount to \(1387.12 kg\), constituting over \(99.9\%\) of the total.

5.1.4 Building demolition phase

From the calculations above, it's evident that the carbon emissions for the steel column flange joint during the construction phase and transportation phase are 308.84 kg and 1388.06 kg, respectively. According to Eq. (8), the calculated carbon emissions for the dismantling phase of a single steel structure column joint amount to \({C}_{cc}=1527.21 kg\)."

5.2 Comparison of carbon emissions under individual components

Based on the carbon emission inventories of the two steel column joint types, the carbon emissions during the fabrication phase of the full penetration weld joints amounted to \(1832.40 kg\), with a final joint mass of \(2573.14 kg\). Hence, the average carbon emission rate per 1 kg of this joint stands at \(71.21\%\). Meanwhile, the carbon emissions during the fabrication phase of the box-shaped column flange connections achieved by plug welding-core sleeve amounted to \(1911.22 kg\), with a final joint mass of \(2808.05 kg\), resulting in an average carbon emission rate per 1 kg of the full penetration weld joint at \(68.06\%\).

Comparing the carbon emission rates per unit of individual components reveals that during the steel column fabrication phase, the carbon emissions per 1 kg of full penetration welded joint exceed those of flange joints by 4.6%. This is attributed to the slightly higher steel usage during the fabrication of flange joints, despite the additional plug welding process considered in their design at the prefabrication plant, resulting in a lower average carbon emission rate. Specific data regarding carbon emissions during this phase is detailed in Fig. 7.

Fig. 7
figure 7

Comparison of carbon emissions of two types of joints during the steel column fabrication phase

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During the steel column construction phase, the carbon emissions for the full penetration weld joints amounted to \(308.84 kg\), whereas the box-shaped column flange connections achieved by plug welding-core sleeve emitted \(155.90 kg\) of carbon. Similarly, the average carbon emission rate per \(1kg\) full penetration welded connection joint can be found to be \(12.00\%\) and \(5.55\%\) for flange connection joint.

After standardizing the two components to the same unit measurement of 1 ton, the carbon emission per unit during the construction phase amounted to \(120.02 kgCO2e/t\) for the full penetration weld joints, and \(55.52 kgCO2e/t\) for the box-shaped column flange connections. The carbon emissions from full penetration welded joints exceed those from flange joints by \(116.2\%\). This is due to the fact that during the construction phase assembly, the flange steel column connection joints only require the installation of high-strength bolts at the upper and lower flange plates. In contrast, the full penetration welded joints necessitate on-site welding of the upper and lower columns, resulting in increased material consumption and mechanical energy consumption, leading to higher carbon emissions. The specific carbon emission data for this phase are illustrated in Fig. 8:

Fig. 8
figure 8

Comparison of carbon emissions of two types of joints during the steel column construction phase

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During the transportation phase of steel columns, the carbon emissions for full penetration welded joints amount to 1388.06 kg, while for the plug welding-core sleeve flange connection joints, the carbon emissions are 1517.02 kg. Similarly, the average carbon emission rate per 1 kg of full penetration welded joint is calculated at 53.94%, whereas for the flange connection joint, it stands at 54.02%. It is evident that the carbon emissions during the transportation phase of both types of joints are nearly identical, primarily sourced from steel column transportation, accounting for 99.9%. Specific data regarding carbon emissions during this phase is detailed in Fig. 9.

Fig. 9
figure 9

Comparison of carbon emissions of two types of joints during the steel column transportation phase

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During the demolition phase of steel columns, the carbon emissions for full penetration welded joints amount to 1527.21 kg, whereas for the plug welding-core sleeve flange connection joints, the carbon emissions are 1505.63 kg. Similarly, the average carbon emission rate per 1 kg of full penetration welded joint is calculated at 59.35%, while for the flange connection joint, it stands at 53.62%. The flange connection joint exhibits a lower carbon emission rate during the demolition phase, primarily due to fewer emissions during the construction phase.

By comparing the above data, it is evident that the flange connection joint exhibits a lower average carbon emission rate during both steel column fabrication and demolition phases compared to the full penetration welded joint. Both joints show nearly identical carbon emission rates during the component transportation phase. Furthermore, during the steel column construction phase, the flange joint demonstrates significantly lower carbon emissions compared to the full penetration welded joint. As depicted in the graph, this difference in construction phase emissions is primarily due to variations in material consumption and mechanical energy consumption. So the use of assembled steel plug welding-core sleeve flange connection joint in the actual building construction effectively follows the energy-saving and emission reduction policy, not only the performance has been improved, the steel structure components have been improved and optimized, the energy utilization rate has also been enhanced, and further developed into green buildings.

5.3 Analysis of carbon emission results for different joints under the whole building

The construction plan of the dormitory building of Tongzhou Campus of the Affiliated High School of Capital Normal University is shown in Fig. 10, and the section drawing is shown in Fig. 11. Given that the analysis is conducted on a per-unit basis considering two-story high steel columns, the carbon emissions calculation for the joint connections is uniformly performed based on the selected joint dimensions. The splicing locations are at the top of the first basement floor, the top of the second floor above ground, the top of the fourth floor and the top of the sixth floor, containing a total of 160 box-shaped column flange connections achieved by plug welding-core sleeve.

Fig. 10
figure 10

Plan drawing of dormitory building of Tongzhou Campus of the Affiliated High School of Capital Normal University

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Fig. 11
figure 11

Section of Tongzhou Branch of Capital Normal University Affiliated High School Dormitory building

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For the entire dormitory building with box-shaped column flange connections achieved by plug welding-core sleeve, the carbon emissions from the steel column fabrication, component transportation, steel column installation, and building demolition phases for all joints were calculated. The total carbon emissions for the flange connection joints amounted to \(814.36 t\).

If the entire dormitory building were to utilize full penetration welded joints, the carbon emissions for these joints in the dormitory would be calculated similarly. It's noteworthy that based on practical application in this dormitory project, considering the interruption in welding due to inclement weather, as well as actual efficiency gains, the full bolted connection method saved a total of 179 days. Within these additional days, the full penetration welded joints would generate extra carbon emissions, including the usual electricity and water consumption for regular living and office purposes, and notably, the carbon emissions generated by three tower cranes used in the actual project. Thus, focusing solely on the electrical consumption of these crane units, the calculated additional total carbon emissions for the three cranes over 179 working days amount to \(138.29 t\). Consequently, if the entire dormitory building were to adopt full penetration welded joints, the total carbon emissions for these joints would be \(947.33 t\).

If we disregard any potential delays in the construction period and focus solely on the entire dormitory building's construction phase, the total carbon emissions from all box-shaped column flange joints achieved by plug welding-core sleeve connection amount to \(155.90 t\). If all individual steel column joints in the units were replaced with full penetration welded steel column joints, the carbon emissions for these joints during the dormitory building's construction phase would reach \(308.84 t\). This reflects an additional \(152.94 t\) of carbon emissions compared to the box-shaped column flange joints achieved by plug welding-core sleeve connection, which accounts for an increase of \(98.1\%\) in carbon dioxide emissions. Hence, the adoption of box-shaped column flange joints achieved by plug welding-core sleeve connection in prefabricated steel structure buildings instead of traditional full penetration welded steel column joints has significantly contributed to energy conservation and emission reduction.

Regarding the carbon emissions from the steel column joints of the dormitory building, the total carbon emissions for the plug welding-core sleeve flange joint amount to \(814.36 t\), while the total carbon emissions for the full penetration welded joint reach \(947.33 t\). The carbon emissions from the full penetration welded joints surpass those from the flange joints by 132.97 tons. This discrepancy is primarily due to the lower carbon emissions during the construction phase of the flange joints and the indirect carbon emissions saved from the reduced construction period.

6 Conclusions and future outlook

6.1 Summary of carbon emissions from prefabricated steel structure connection joints

This study concludes that in prefabricated steel structures, the carbon emissions during the construction phase from box-column plug welding-core sleeve flange joints reduce by 49.52% compared to the traditional full penetration welded joints. The carbon emission accounting data demonstrates the green and low-carbon advantages of flange connection joints in actual projects, which not only have a greater energy saving and emission reduction effect, but also a higher energy utilization rate, indicating a shift in the direction of green buildings for prefabricated steel structures.

This paper takes the box-shaped column flange connection achieved by plug welding-core sleeve in the dormitory building of Tongzhou Campus of the Affiliated High School of Capital Normal University as the research object. Based on the consumption quota of prefabricated construction projects and the actual project quantity, the carbon emissions of steel structure column splicing joints in steel structure buildings at different phases are calculated by the emission factor method. Concluded as follow:

  1. 1.

    The carbon emissions for a single unit of the box-column plug welding-core sleeve flange joint are \(1911.22 kg\) during the steel column fabrication phase, \(155.90 kg\) during the joint construction phase, \(1517.02 kg\) during component transportation, and \(1505.63 kg\) during building demolition. The contribution of carbon emissions from a single steel structure column joint decreases in the following order: fabrication phase > transportation phase > demolition phase > construction phase.

  2. 2.

    In the steel column fabrication phase, the carbon emission rate of the flange connection joint is \(68.06\%\), while the full penetration welded joint registers at \(71.21\%\). Despite the additional plug welding process in the design consideration of the flange joint, its average carbon emission rate remains lower.

  3. 3.

    During the steel column construction phase, the carbon emission per ton for the plug welding-core sleeve flange joint is \(55.52 kgC{O}_{2}e/t\), while for the full penetration welded joint, it stands at \(120.02 kgC{O}_{2}e/t\). The carbon emission of the full penetration welded joint surpasses that of the plug welding-core sleeve flange joint by \(116.2\%\). This increase is primarily attributed to the welding materials required for construction and the mechanical energy consumption, whereas the box-column plug welding-core sleeve flange joint only necessitates bolt installation during construction.

  4. 4.

    Regarding the carbon emissions during the transportation phase, the carbon emission rates of the flange connection joint and the full penetration welded joint are nearly identical.

  5. 5.

    Regarding the demolition phase, the carbon emission rate of the flange connection joint is \(53.62\%\), while that of the full penetration welded joint is \(59.35\%\). The flange connection joint exhibits a lower carbon emission rate during the demolition phase, mainly due to reduced carbon emissions during the steel column construction phase.

  6. 6.

    For all steel column joints in the entire dormitory building, the total carbon emissions for the plug welding-core sleeve flange joints are \(814.36 t\), while those for the full penetration welded joint amount to \(947.33 t\). The total carbon emissions for the flange connection joint are reduced by \(132.97 t\) compared to the full penetration welded joint. This reduction is primarily due to lower carbon emissions during the flange connection joint construction phase and the indirect carbon emissions saved by reducing the construction duration.

6.2 Limitations of the study

Incomplete database of carbon emission factors and evolving trends. This insufficiency in the carbon emission factor database, coupled with the diverse range of building materials and machinery, signifies a continual evolution in the magnitude of carbon emission factors. Thus, there is an urgent need for a comprehensive and adaptable database of carbon emission factors to enhance the efficiency of energy conservation and emissions reduction in prefabricated construction.

Limitation in research scope. The study's focus on steel structural column connection joints within dormitory buildings limits its breadth, preventing comparative analysis of carbon emissions from these joints across similar structures. Expanding the number of research cases is imperative to analyze the favorable trends in energy conservation and emissions reduction associated with these joints in diverse prefabricated construction projects.

Data collection challenges and authenticity. The study conducted carbon emission calculations based on engineering consumption norms, complemented by some verified actual consumption data. However, the collection of on-site construction data posed significant challenges, impeding the precision and clarity of carbon emission calculations. Thus, a more focused effort on data collection is necessary to enhance the accuracy and lucidity of carbon emission assessments.

Shortcomings in demolition phase estimation. The calculation of carbon emissions during the demolition phase relied on empirical estimations from other studies due to the absence of actual building demolition. Consequently, there exists a slight discrepancy between these estimations and the real scenarios. Considering steel's high recycling coefficient as a construction material, additional empirical data is imperative at this stage to refine and render the carbon emission calculation methodology more reasonable and accurate.

6.3 Future prospects

The future focus of this study will center on the following aspects:

Optimization of carbon emission factors and methodology. Continued attention will be given to the selection of carbon emission factors, integrating existing software standards to refine and improve the efficiency and outcomes of the carbon emission factor methodology.

Expanding the scope of study. The future endeavors will not only focus on optimizing the efficient energy-saving construction of steel structural column connections but will also explore broader potential carbon reduction advantages in other aspects of prefabricated steel structural buildings. For other structural buildings such as steel- concrete structures, the selection of building material quality can also be optimized to extend the structural life cycle; the use of renewable materials as building materials is recommended to reduce carbon emissions from construction waste; and different structural construction solutions can be optimized to improve the efficiency of the construction process and to reduce unnecessary construction carbon emissions.

Enhancement of data collection. Future efforts will be directed towards refining the collection of data, particularly focusing on acquiring actual consumption data related to labor and machinery during both the factory prefabrication phase and on-site construction. This emphasis aims to ensure the authenticity and rationality of the data acquired for analysis.

Optimization of demolition and recycling phase calculation. Future endeavors will focus on exploring more rational and accurate methodologies for calculating carbon emissions during the demolition and recycling phases. This exploration will heavily rely on additional empirical data to enhance the precision of estimations and refine the calculation process.

These future endeavors will contribute to furthering research in the realm of low-carbon development in prefabricated construction, enhancing the feasibility and accuracy of energy-saving and emission reduction methods.