Framework for Designing Sustainable Structures through Steel Beam Reuse

Abstract

The architecture, engineering, and construction sector requires carbon-intensive materials, such as steel, in the construction process and generates a large amount of waste in the life cycle. This causes global warming and waste problems. The demand for the reuse of construction materials is increasing, although it is not the convention, to reduce the environmental impact. Although the sustainable effect of the reuse of materials has been proven in several studies, materials are not always reused in practice, owing to the lack of an information system for reusable materials and the economic uncertainty. In this study, we propose a framework for designing structures using reusable steel beams. The design framework consists of a material bank and a design support tool. The material bank provides information on reusable materials based on the building information modeling. The design support tool generates efficient material procurement plans and provides information about the environmental and economic impact of the project. In a case study used to verify the framework, CO₂ emissions were reduced by up to 77% through material reuse, which was consistent with the results of previous studies. However, owing to the cost of processing reusable materials, the overall cost was found to increase by up to about 40%. Therefore, an economic analysis over the entire life cycle when using reusable materials needs to be done.

1. Introduction

The demand for greenhouse gas (GHG) reduction has increased due to rapid climate change caused by global warming. In particular, the architecture, engineering, and construction (AEC) sector is a resource-intensive industry sector accounting for about 40% of the energy consumption and over 32% of the CO₂ generation in the U.S. and Europe [1,2]. There is, therefore, a great need for the AEC sector to reduce its generation of GHGs. In addition, the AEC accounts for 51% of the total steel resource consumption [3] and up to 30% of the global waste generation [4,5,6], including Europe and most developed countries. The material-related energy consumption accounts for 10–20% of the AEC’s total energy consumption [7], and this proportion increases with the type and life span of the structure. As such, resource consumption in the AEC sector poses a serious threat to the environment, and according to the Organization for Economic Co-operation and Development (OECD), the use of construction materials is expected to increase further in the future [8]. Accordingly, calls for the reduction of both waste and production of construction materials are increasing worldwide. In Korea, the AEC sector accounts for about 40% [9] of the energy consumption, which is two times greater than that of the transportation section [10], and most of the generated waste and GHGs owing to the use of carbon-intensive materials. In 2020, the annual steel consumption of the AEC sector in Korea was estimated to be over 20.7 million tons [11]. In response to this situation, the Korean government is attempting to reduce GHG emissions by 50% [12] and CO₂ emissions from AEC by approximately 60% [13] by 2050.

Countries around the world have introduced the concept of a circular economy (CE) to reduce the environmental impact caused by the production of construction materials and the discharge of waste, with waste management being a key strategy for a CE [14,15,16]. The CE concept is used not only for construction but also for responding to resource depletion and environmental issues and other sustainability issues across all fields. This concept is particularly important in the AEC sector, which has a significant influence on the environment, owing to its large resource consumption and waste generation. The European Union action plan for CE actively encourages the recycling and reuse of construction materials and provides guidelines [17,18]. Reduce, recycle, and reuse are recommended as major strategies for waste management [19]. In particular, the reuse of materials extracted from structures has proven to have a high potential to improve resource efficiency, energy use, and carbon emissions of the AEC sector [20]. When steel scrap is processed using an electric arc furnace for recycling, which is considered an eco-friendly strategy, 0.15–1.03 tons of CO₂ are generated in the process of producing 1 ton of steel [21]. In Korea, the electric arc furnace method generates only about a quarter of the CO₂ generated by the basic oxygen steelmaking method, in which iron ore is processed using a blast furnace [22,23]; the amount of CO₂ generated can be further reduced by the reuse of materials. In addition, the reuse of construction materials reduces material production costs. Reuse is attracting attention as the most promising alternative for enhancing the sustainability of the AEC sector by replacing the resource and energy-intensive material production process and reducing waste.

Research has been conducted on design methods for efficiently extracting materials from structures for reuse, or using materials already extracted to apply reuse in the AEC sector. Research on design for deconstruction has developed strategies for both deconstructing structures from the design process, and evaluating tools for ease of disassembly [24,25]. Research related to design for reuse (DfR) has investigated the environmental effects and design strategies of new structures using reusable materials [26,27]. Nevertheless, materials are rarely reused in the AEC sector, and there is a lack of information on reusable materials and their properties. There is no official service that provides a list or status of reusable materials, and even if reusable materials are sought, it is difficult for designers to grasp information on their properties [28,29,30]. Therefore, the case for construction using reusable materials and the process of design are not well-defined. In addition, project stakeholders, including the owner, are concerned about the economic uncertainty of using reusable materials [28,29,31]. Designers, in particular, are reluctant to reuse because they are concerned that their designs and material procurement strategies may be compromised by limitations in the shape and quantity of available reusable materials [28,29,31,32]. The opportunity to enhance the sustainability of the AEC sector is therefore lost because reusable materials are not used in practice, despite the environmental benefits and existence of policy incentives for reuse.

In this study, we propose a framework for designing structures with reusable steel beams to reduce CO₂ emissions. The framework consists of a material bank for managing reusable material information and a design support tool to increase the efficiency of reusable material use. The framework supports the design process by providing information on reusable materials, efficient material usage plans, and information on the environmental and economic impact of the project for designers and stakeholders. In this way, it facilitates the process for stakeholders to use reusable materials to improve the sustainability of the AEC sector. Sustainability is a broad term that comprehensively considers the economic and social performance and environmental resilience to balance the interests of current and future generations [14,33]. In this study, we focused on the environmental aspect of the various elements of sustainability.

The subject of the proposed framework is a steel beam structure. This is because the demand for steel beams in the AEC sector is very high; 60 million tons of sections are used worldwide every year [34]. In Korea, about 7.9 million tons of steel sections and I beams (or H beams) are produced every year [35]. Not all the steel components produced in Korea are used in the AEC sectors. Nevertheless, the proposed framework is expected to allow an amount of steel sufficient to cover a significant portion of the Korean and global demand for steel beams. Steel beams are easy to reuse [36,37], and although reuse is not a common practice at present, the steel beam recovery rate in the AEC sector is about 85% [3], making it easy to secure inventory. The environmental benefit that can be obtained through reuse is significant because steel beams require a lot of energy in the production process. In Korea, 67% of the total steel is produced through the basic oxygen steelmaking method using a blast furnace [38]. With this method, about 2 tons of CO₂ are generated for every 1 ton of iron produced [39]. In this study, the amount of CO₂, which accounts for over 60% of the GHG emissions [40], was calculated to measure the environmental impact of construction projects. The framework of this study focuses on the process of creating design and material procurement plans using reusable materials. Therefore, this study does not include a method of extracting materials from a structure to be deconstructed, or a design method to facilitate extraction.

The rest of this paper is organized as follows: Section 2 explores previous studies on DfR, material banks for managing reusable materials, and methods for assessing the environmental and economic costs of construction projects; Section 3 proposes a design framework for promoting steel beam reuse based on previous research considerations; Section 4 describes a case study to verify the effectiveness of the proposed framework; Section 5 analyzes the results of the case study.

2. Literature Review

2.1. Design for Reuse (DfR)

DfR means the design of a new structure using reusable products or components such as steel beams [29,37]. Previous studies related to DfR have suggested ideas for designing new structures using reusable construction materials, and design methodologies and strategies to create new design proposals [26,28,29,31]. In addition, these studies have verified the effect of material reuse by measuring the environmental impact of structures created through DfR [41,42,43,44]. Studies related to design methods have optimized the design of specific types of the structure considering constraints on the shape or quantity of reusable materials. In particular, previous studies have shown a specific interest in the reuse of steel. They have generated new designs using reusable steel extracted from general buildings [41,43], structures such as electric pylons [42], and manufacturing waste [44].

There are barriers to reuse in the AEC sector despite the research and proposed strategies related to DfR. First, the systems for managing and providing information on reusable materials have not been clearly defined [28,29,30]. Although material information is essential for creating a design and planning a project, data exist individually or are not managed, so project stakeholders cannot access them in practice. In addition, the absence of material information leads to concerns about the additional costs of reuse. Although the review of economic feasibility is one of the most important aspects to determine before proceeding with the project, reuse is not considered in practice due to uncertainty about the cost [28,29,31]. Most of the previous studies on DfR have ignored the measurement of economic cost [41,42,43].

Therefore, to make the reuse of materials achievable, a system is needed to gather and manage information on reusable materials [45]. To this end, a database to integrate and manage material information, such as a material bank, must be created and made accessible to designers and project participants. In addition, designers need a tool to predict the environmental and economic impact of using reusable materials. In particular, decisions made at the design stage have a great influence on the overall cost of the project, so the designer should be able to determine costs easily and modify the plan iteratively. In addition, the constraints on the shape and quantity of reusable materials available must be overcome using an efficient material procurement plan to fulfill the design. In the case of the new production of materials, the required shape, quantity, and procurement plan are fixed, but when reusable materials are used, the number of possible procurement alternatives increases. A reasonable material procurement plan is needed through the optimization of cutting stock [43,46] because reuse may not be possible due to the cost, which varies depending on the procurement plan.

2.2. Material Bank for Managing Reusable Material Information

A material bank refers to a system and database for managing material information for recycling or reuse of construction materials [47,48,49]. The material bank provides project stakeholders with access to material information, thus facilitating the application of reusable materials to construction projects.

Bertin et al. proposed a building information modeling (BIM)-based framework and material bank to reuse the load-bearing elements of buildings [47], and Honic, Kovacic and Rechberger proposed a material passport for recycling wood and concrete [50]. Jayasinghe and Waldmann developed a framework to extract components that make up the entire building and convert them into a database for recycling and reuse of construction materials [49]. Arora et al. proposed a method of quantifying the material flow by type of building material in order to predict the material flow in a material bank [51]. As a result of an analysis of urban areas in Singapore, approximately 6.5 million tons of steel can be reused, and the effect of steel reuse is expected to be significant.

Previous studies have indicated high levels of interest in storing the information about reusable materials as a key function of a material bank. Therefore, these studies mainly used the BIM data of the structure to be deconstructed [48,49,50,51]. BIM is a standard method for managing project information in the AEC sector and an effective means of managing information on reusable materials because it can include attribute information of buildings and components, which can be expanded as necessary.

Previous studies have defined the type of material properties to be stored together with BIM data. The items of the previously proposed material bank consisted of the shape, quantity, type, structure, and chemical properties of the materials [47,49,50]. The capital of the material bank database becomes the material information extracted from the BIM model of the building to be deconstructed [48,49,50,51]. Information from the database is provided to designers and project stakeholders through the application of a BIM authoring tool or a web-based service [47,49].

It was found that the material bank can assist in the reuse of materials, but in most cases the cost is still not considered. In addition, processes such as material modification and transportation are not considered in the environmental impact analysis, which focuses on the production of materials. For this reason, sufficient information on the economic impact, such as additional costs due to reuse, is not provided to the stakeholders. According to Smeets, Wand and Drewniok, the management of data of reusable materials can lead to project cost reduction [52]. However, the use of material banks is still not a common practice, and the lack of economic data prevents the reuse of materials.

Therefore, a review of the economics of reuse for those involved in the project is needed in practice to operate the material bank. Items needed for measuring the costs of individual processes should be defined in the material bank. Costs can then be measured over the entire life cycle, including the modification and transportation of materials, to respond to concerns about additional costs.

2.3. BIM-Based Life Cycle Assessment and Life Cycle Cost

It is important to accurately grasp the impact of the project by evaluating the entire life cycle, including modification and transportation, as well as the production process of materials [47]. Life cycle assessment (LCA) and life cycle cost (LCC) methodologies measure the environmental impact and cost of a product throughout its life cycle with an analysis of the flow of resources and materials [53,54,55]. LCA and LCC are the representative methods for the assessment of the environmental and economic impact of AEC projects.

Evaluation using LCA and LCC requires information on the life cycle of the structure. Accumulating life cycle information is one of the main roles of BIM [56]. In addition, BIM can contain information in units of materials that are components of a structure [57]. Material unit information is also essential information in LCA and LCC processes. Therefore, BIM is an information management strategy suitable for projects considering LCA and LCC as it provides life cycle information and material unit information necessary for life cycle evaluation. In addition, because BIM can be used for material banks [48,49,50], it can integrate the information needed to perform an environmental and economic evaluation of construction projects using reusable materials.

The core principle of LCA is to measure the total environmental load of the project by analyzing the materials and work required at each stage throughout the life cycle of the building, and summing the environmental loads for each stage [58]. ISO 14040 [59] and ISO 14044 [60] explain the implementation of the LCA through four phases: goal and scope definition, inventory analysis, impact assessment, and impact interpretation. The amount of material required is estimated using the BIM model. The environmental load data generated for each unit of material and work are provided as attribute information for each object in the BIM model. Recently, it was also provided in a standardized form through national Life Cycle Inventory Databases (LCI DBs). As representative examples, there are the U.S. Life Cycle Inventory Database [61] and the Korea Life Cycle Inventory Database (Korea LCI DB) [39]. In the case of economic cost evaluation through the LCC, the cost of each step and the total cost are calculated by multiplying the unit costs by the total material or work cost [62].

It was found that BIM’s management function of life cycle and attribute information can be effectively used in the material banks, LCA, and LCC processes. The LCI DB can provide environmental load data for each element of work and material. Therefore, BIM data needs to be linked to the LCI DB for the life cycle evaluation of reuse projects. Additionally, system boundaries need to be established according to the type of structure during the evaluation process throughout the life cycle. For example, in the case of civil engineering structures, unlike buildings, cooling and heating costs may not be required during the operation process.

The current status of the reuse is summarized as follows. The reuse of materials can effectively reduce the environmental impact of the AEC sector. In particular, steel beams, which are carbon-intensive materials, are very likely to be reused owing to their high demand and high recovery rates. Nevertheless, the following problems are still faced in the reuse of steel beams. First, data on the status and attribute information of reusable materials are not managed. Designers cannot consider reuse at the design stage because data on reusable materials are not provided. In some cases, although reuse is economical, it is avoided owing to the lack of data and economic uncertainty [52]. Another problem is that the use of reusable materials increases the design difficulty. There are strong restrictions on the form and quantity of reusable materials. For these reasons, it is more difficult to create designs and material procurement plans when considering reuse. Therefore, in order to overcome the barriers for reuse, a material bank for managing reusable material data and a design support tool for creating an efficient alternative and assessment for environmental and economic feasibility are needed.

3. Method

This section describes a design framework for structures with reusable steel beams. Figure 1 shows the structure of the proposed framework, which consists of a material bank and a design support tool. The designer inputs the design proposal and material reuse settings into the design support tool. The design support tool requests a list of available materials for reuse from the material bank based on the information entered by the designer. The design support tool creates efficient stock cutting and material procurement plans that can be fulfilled using reusable materials. Finally, an environmental and economic cost assessment is performed based on the LCA and LCC. The designer iteratively revises the design based on the assessment results and determines the final solution.

3.1. Material Bank Database

The material bank performs the role of a database to store and manage information on reusable materials. Designers can search for available materials for reuse in the material bank and then proceed with the design, depending on the materials available. Alternatively, they can establish a material procurement plan by referring to the material bank after the design proposal is created. Gathering material information is the most important activity when managing a material bank. BIM is a standard for managing information on structures and materials in the AEC sector, so it is the most appropriate source of material bank information. In this study, information about reusable materials is extracted from the BIM model of the deconstructed structure and saved in the material bank. In addition, to provide information on a large number of reusable materials for project stakeholders, the material bank proposed in this study can extract data from other material banks. Accessibility to reusable materials is enhanced by integrating data in the material bank with similar data generated from previous studies related to material banks.

For designers to decide whether to use reusable materials, the material bank must be able to provide information on the appropriate attributes of the material. These attributes should include information about the steel beams required to design the general structure. They should also include information needed for the analysis of environmental and economic costs and information on the remaining life of reusable materials. Consideration of attributes used in material banks and general BIM models is also needed for consistent material bank data collection. Therefore, in this study, the material bank database items are organized as shown in

Table 1. Attributes of steel beams in the proposed material bank.

Categories	Attribute	Description	Reference
General information	Identifier	Identification code for each beam	[47,49]
	Usage	Previous usage of steel beam (ifcColumn/ifcBeam)	[47,49]
	Specification	Specification code (American Society for Testing and Materials, Korean Standard, etc.)	-
	Reusability	Purchase availability of steel beam (True/False)	[49,63]
	Location	Current location of steel beam (longitude and latitude)	[49]
	Unit cost	Unit cost of steel beam	[47,63]
Geometry (shape)	Type	Shape of profile (Asymmetric/I/Circle/C/Ellipse/I/L/Rectangle/ Trapezium/T/U/Z)	[64]
	Dimensions	Width x Height x Web thickness x Flange thickness	[47,49,63]
	Radius	Radius of circular profile	-
	Length	Length of steel beam	[47,49,63]
	Curvature	Curvature of steel beam	-
Physical properties	Material	Type of material	[47,49,50,63]
	Connection type	Type of connection before disassembly (Rivet/Bolted/Welded)	[25]
	Unit weight	Weight of steel beam per 1 m	[47,49,50]
	Yield strength	Yield strength of steel beam	[47,49,63]
	Tensile strength	Tensile strength of steel beam	[47,49,63]
	Extension	Extension value of steel beam	[47,49,63]
	Remaining life	Predicted remaining life of the steel beam	[50,63]
Chemical properties	Chemical composition	Chemical composition data table of steel beam	[49]

. Based on the attributes commonly used in previous studies and mill certificates of practice, attributes needed for steel structure design, such as curvature and section profile, were added.

The designer selects and uses the attributes of the materials depending on the need. The general information category provides basic information about the condition and cost of the material. Depending on the supplier of reusable materials, the cost of the steel beam may vary, so the cost of the project may change. Therefore, information on the cost must be provided to the designer from the design stage. Geometry is the information that has the most direct influence on the design and material procurement plans. In particular, the type, dimensions, and length of the section are the most important for the designer. The connection type is also necessary information to construct a structure. Physical properties are related to the capacity of the structure. For example, the yield strength is one of the values needed to determine the size of a steel beam suitable for a column. The remaining life is a factor that has a great influence on the life of the planned structure and maintenance requirements. However, the life prediction of materials was excluded from the discussion because detailed methods for life prediction were not within the scope of this study. In the case of CO₂ emission information, it is provided through linkage with the LCI DB managed by the government.

3.2. Design Support Tool

A design support tool for generating plans using material bank data is another key component of the framework for the reuse of construction materials. The design support tool proposed in this study creates a material procurement plan according to the design plan, depending on the available reusable materials. After the material procurement plan is created, the environmental and economic impacts of the project are evaluated. Evaluation of the design support tool enables designers to iteratively revise and develop the design.

3.2.1. Solution Generation

After the BIM model of the new design is created, the designer sets the scope to apply the reusable material. Designers can apply reusable steel beams to the entire steel structure or designate only some parts. For example, uncertainty about the quality and certification of reusable materials [28] leads designers or structural engineers to hesitate in the use of reusable materials in areas where loads are concentrated. After the scope is set, the attribute information about the selected material is extracted. The search for the reusable material to be used is performed through the process of finding the same material as the attributes of the previously extracted material. However, materials that do not match the design in the material bank database are also included in the search results. For example, if the length of the reused material is longer than the design length, it can be cut, used, and included in the result. However, reusable materials shorter than the designed materials are excluded from the search. In addition, the extracted information attributes are used in cutting stock and material procurement plans for reusable materials and in environmental and economic evaluation processes.

Next, the designer can set constraints on reusable materials. These constraints determine the range of reusable materials required to search for in the material bank. For example, the designer can retrieve only materials whose remaining life is longer than the target life of a new structure. In addition, even if the specifications of the design and the reusable material do not completely match, an alternative to applying the reusable material with the difference in specification within a certain range can be considered. For example, if there is no reusable material with the same sectional specifications as the design material in the material bank, the designer may request the material bank to search all sectional specifications within a certain size range, even if the sectional size of the design is changed.

A material procurement plan is created after the designer determines the scope to which the reusable material will be applied and the available material for reuse. However, it is difficult for designers to create efficient procurement plans directly because there are many types of reusable materials to be considered, and many possible alternatives. The problem of finding an efficient alternative in a situation where resources are limited is dealt with in operational research. In particular, a situation where you want to cut while reducing the loss of an object such as a steel beam is called the cutting stock problem (CSP) [43,46]. In Figure 2, a total of six materials are required for the design. The solution to the CSP is to replace the materials required for construction by using reusable materials (hatched in Figure 2) and minimizing the amount of newly produced materials. The case of cutting steel beams to fit the shape of the design is the typical type of CSP. Efficient cutting plans for the CSP can be derived using mathematical models or heuristic searches such as meta-heuristics [65,66]. However, implementing an algorithm to solve the CSP was not within the scope of this study, and commercial programs to solve the CSP have already been developed. In this study, Gurobi [67], a commercial tool, was used to solve the CSP. Information on the type, quantity, and length of the material extracted, based on the previous designer’s settings, is used to define the CSP problem. Finally, all possible cutting patterns and plans with the least material waste are created, and based on the cutting plans, a material procurement plan is created, including a production plan for new materials.

3.2.2. Evaluation of a Solution through LCA and LCC

Finally, environmental and economic impacts are calculated according to the design and material procurement plan through the LCA and LCC. Assessment throughout the life cycle is necessary to measure the impact of the project and to make informed decisions. To this end, it is necessary to define the system boundary, the CO₂ emissions generated in each life cycle, and the cost.

In this study, the LCA is performed based on the following four phases of ISO 14040 and 14044: (1) Goal and scope definition: In this study, we aim to evaluate the environmental impact of structures using reusable materials. The environmental impact is assessed through the calculation of the CO₂ emission, and the assessment proceeds from the process of mining raw materials to the deconstruction of the structure. (2) Inventory analysis: Figure 3 shows the general life cycle of a steel structure when using reusable materials. The life cycle can be classified as product and construction, in use, and end of life [68]. The process of extracting raw materials and producing materials is generally a process that generates a large amount of CO₂. In construction sites, CO₂ is generated when using machinery and energy and in the production of additional materials during construction, maintenance, and deconstruction. At the end of the structure’s life, materials are transported to landfills, recycling plants, or material banks. Reusable materials are processed with modification before being used in new projects. In the case of steel beams, cutting, bending, and punching are mainly performed. The scope of this study is from mining raw materials to the deconstruction of structure. If data on the flow of materials after the deconstruction, such as landfill, recycle, and reuse, can be obtained, life cycle evaluation from cradle to grave can be performed. However, in this study, the process after deconstruction and the benefit beyond life cycle (which includes load and benefit of reuse and recycle) were not considered, owing to the high data uncertainty. The processes of summing up CO₂ (G) and cost (C) generated at each stage of the life cycle are performed as shown in Equations (1) and (2):

$$G=G_{raw}+G_{mfg}+G_{mod}+G_{const}+G_{maint}+G_{deconst}+G_{trans},$$

(1)

$$C=C_{reuse}+C_{mfg}+C_{mod}+C_{const}+C_{maint}+C_{deconst}+C_{trans}.$$

(2)

$G_{raw}$ and $G_{mfg}$ are the CO₂ amounts generated during raw material extraction and manufacturing, respectively. The steel beam manufacturing process is the most energy-intensive. Specifically, using a blast furnace generates a large amount of CO₂. $G_{mod}$ is the CO₂ generated using bending, cutting, and punching machines in the process of processing reused materials. $G_{const}$, $G_{maint}$, $G_{deconst}$ are the amounts of CO₂ generated during construction, maintenance, and deconstruction, respectively. $G_{trans}$ is the amount of CO₂ generated primarily from the combustion of diesel fuel during transportation. The LCC, like the LCA, is performed by summing the costs incurred in each step [62]. $C_{reuse}$ and $C_{mfg}$ are the purchase costs of reuse and newly produced steel beams. $C_{mod}$, $C_{const}$, $C_{maint}$, $C_{deconst}$, $C_{trans}$ are costs for modification of reusable materials, construction, maintenance, deconstruction and transportation, respectively. (3) Impact assessment and (4) Interpretation: In this process, the CO₂ emission generated by the structure in each life cycle and activity is calculated and analyzed using data from the design alternatives, material procurement plans, and LCI database. Each project has different points of focus in the life cycle. For example, buildings generate a lot of CO₂ during operation, but simple structures do not. In addition, at the end of life stage, landfill, recycling, and reuse processes are generally excluded from analysis due to high uncertainties [69]. Therefore, the designer must adjust the system boundary for LCA and LCC analysis to be suited to the characteristics of each project.

4. Case Study

A case study was used to verify the proposed design framework and to confirm the effect of material reuse. The case study consisted of a scenario based on a situation in Korea. In this scenario, reusable materials were procured from the steel structure of a deconstructed building, and the newly constructed structure was a noise barrier tunnel (NBT). In Korea, many thousands of kilometers of sound barriers, including NBTs, have been built and their demand is constantly increasing [70]. In addition, even for a short NBT, the length is several hundred meters, so it requires a lot of materials and has a great impact on the environment. However, as shown in Figure 4, the main structures of NBTs are all made of steel beams, and the layout is similar among structures, so it is easy to use reusable materials and highly likely that sustainability will be enhanced.

The NBT in the case study was to be installed on a 200 m straight road. Figure 5 shows the system boundaries of the material flow and life cycle analysis in the scenario. The analysis was performed on the main steel frame of the NBT only. The evaluation was carried out from the manufacturing process of the material to the deconstruction of the NBT. This case study excluded the iron ore mining process in the evaluation process, owing to the absence of clear data. In addition, the process after the deconstruction of the NBT, which reached its target life, was not considered, owing to the uncertainty in the material flow. Reusable materials were included in the analysis from the process after the deconstruction of a five-story steel frame building.

To measure the CO₂ emissions and costs generated during the life cycle of the NBT, the CO₂ emission and cost, according to the unit work required, were set, based on the Korea LCI DB [39] and the data of an NBT company (

Table A1. Unit CO₂ emission and cost.

Work	Emission Source	Description	Value	Unit
Construction, deconstruction	Crane car	Work time per station	0.536	h/station
	Crane car	5-ton crane car fuel efficiency	5.1	L/h
	Diesel fuel	CO2 emissions of diesel fuel	2.677	kg/L
	-	Construction work cost	429	USD/ton
	-	Deconstruction work cost	383.47	USD/ton
Modification	Band saw machine	Power consumption of band saw machine	7.2	kW
	Band saw machine	Work time per unit	1/4000	hour/ea
	Electricity	CO2 emission from electricity use	0.495	kg/kWh
	Band saw machine	CO2 emission per cutting	0.0007974	kg/ea
	-	Cutting modification cost	289.02	USD/ton
	Bending machine	Power consumption of bending machine	8.4	kW
	-	Work time per unit length	1/1080	h/m
	Bending machine	CO2 emission from 1 m bending	0.00385	kg/m
	-	Bending modification cost	784.48	USD/ton
Manufacture	Blast furnace	CO2 emission from component manufacturing	2340	kg/ton
	-	Component purchase cost	689.95	USD/ton
	-	Purchase cost of reusable steel beam	429	USD/ton
Transportation	Truck	25-ton truck fuel efficiency	23	L/h
	Diesel fuel	CO2 emission of diesel fuel	2.677	kg/L
	-	Transportation cost per hour	84.89	USD/h

in the Appendix A). The Korea LCI DB was built based on ISO 14044 and provides values measured in Korea. The remaining life of reusable material and NBT target life were set to 50 and 40 years, respectively, based on consultations with a working-level NBT construction company. Based on a previous study [71], the life of new steel beams was set to 100 years. For the construction and deconstruction process, CO₂ due to the use of the crane car was measured, and the maintenance of the NBT included only the process of producing and replacing materials that exceeded their life. It was assumed that the transportation of materials was completed at a speed of 60 km/h using a 25-ton truck. The modification of the material included cutting and bending processes.

Figure 6 shows the BIM model of the building to be deconstructed and the material bank database for managing information on reusable materials extracted from the building. The BIM model was constructed using Rhinoceros [72], a 3D modeling tool, and VisualARQ [73], a plugin for BIM authoring and exchanging BIM data according to the IFC 2 × 3 standard. Attribute information of the material was extracted using Rhinoceros, VisualARQ, and Grasshopper [72], a visual scripting environment in Rhinoceros. The database was built using Excel. In the Grasshopper environment, a component is a function that performs a specific role. A component that accesses the attribute information of each object in the BIM model was used, and only information corresponding to the predefined items listed in Table 1 was extracted. Once all attribute information was extracted and combined as a table, it was saved in an Excel file. Each column in the database was an attribute item of the previously defined material bank. Information on over 500 steel beams was extracted from the BIM model of one building and saved into the database.

As shown in Figure 7, the designer created the NBT design and BIM models. The designer entered the BIM model into the design support tool, and at the same time set the scope to apply the reusable material and the constraints on the reuse. The design support tool extracted information such as the length and cross-sectional dimensions of the material (yellow beams in Figure 7) selected by the designer among the BIM models in the input design. The extracted attribute information was used as criteria for searching for applicable materials from the material bank. For example, because the length of the column in the design was 4500 mm, reusable materials shorter than this were not retrieved from the material bank. In addition, the designer was able to search for reusable materials by setting the allowable range, even if the specifications or conditions of the steel beam in the material bank did not exactly match the design.

The design support tool received the initial design and constraints and asked the material bank for information on available materials for reuse. Subsequently, the design support tool defined the CSP by extracting information on type, length, and quantity from the list of available materials received from the material bank and the list of materials required for the NBT construction. The solution for the defined CSP was explored through the commercial program Gurobi [67], and a material procurement plan was created based on this (Figure 8).

Finally, the environmental and economic impacts of the initial design and the generated material procurement plan were evaluated through the LCA and LCC (Figure 9). Alternatives (A) and (D) represent the initial alternative and the case that did not consider reuse at all, respectively. Alternatives (B) and (C) are amendments to Alternative (A). (B’) and (C’) refer to the revised alternatives of (B) and (C). Alternative (D), a case without reuse, generated approximately 420 tons of CO₂ over 40 years, at a cost of USD 270,000. Alternative (A) was found to generate approximately 95 tons of CO₂ over the same time, at a cost of USD 378,000. This represented a reduction of approximately 77% of CO₂ through reuse; however, the cost increased with the amount of material reused.

Therefore, the designer created modified alternatives to reduce the high cost of the initial alternative. Initially, it was decided to apply the reused material to all of the NBT columns, adjustable beams, and rafter beams, but in the cases of B and C, the adjustable beam and rafter beam were excluded from the scope of reuse. In addition, Alternatives B’ and C’ were created in which the length of the column, which was 4200 mm, was increased to 4500 mm, which was the length of the reusable material. As the size of the material increased, the CO₂ and cost increased during the production of new materials, but the cutting process of materials for reuse was omitted, reducing the overall cost.

5. Discussion

The results of the case study show that the proposed design framework can provide designers with an alternative applying reusable materials. Information on reusable materials was extracted from the BIM model of the building to be deconstructed and stored in the material bank database. The design support tool extracted information based on the initial design proposal and constraints, and then requested a list of materials conforming to it from the material bank database. Finally, an efficient material procurement plan was created, using the list and information about available materials delivered from the material bank, and environmental and economic analysis results were provided to the designer. Through iterative design, the designer was able to create alternatives that generated less CO₂ than alternatives without reuse.

The results of the case study are analyzed as follows:

• The use of reusable materials can reduce the CO₂ emissions of construction projects. This case study shows that CO₂ can be reduced by up to 77%. Although they will depend on the type of construction project and the assumed situation, the results of this study support the conclusions of previous studies [37,63] that material reuse is one of the most effective strategies for reducing CO₂ emissions. In particular, most of the CO₂ generated during the life cycle of the NBT is generated during the manufacturing process of the material, and reuse is effective because it can most directly reduce the CO₂ generated from manufacturing (Figure 10). In the case of the initial Alternative (A), the column, adjustable beam, and rafter beam were replaced with 175, 200, and 200 reusable materials, respectively (Figure 8). The use of reusable materials caused CO₂ to be generated due to the modification and longer distance for transportation, but the production of new materials was reduced. Eventually, Alternative (D), with no reuse, generated about 420 tons of CO₂, whereas Alternative (A) generated 95 tons of CO₂, resulting in a CO₂ reduction effect of about 325 tons. In this study, the use of carbon-intensive steel beams reduced carbon emissions significantly, however, the reduction in the CO₂ production may vary depending on the type of material. For example, when less CO₂ is generated during manufacturing or recycling, the effect of reducing CO₂ that can be obtained through reuse may be lower than that obtained with steel beams.
• Material reuse can increase the cost of the project. As a result of this case study, the cost of the alternatives created through the proposed framework was higher in all cases than when no reusable materials were considered. The project costs of Alternatives (A) and (D) were about USD 378 and 270 thousand, respectively. The case study shows that the application of reusable materials increases the cost by up to 40%. In some cases, CO₂ and cost reduction cannot be achieved simultaneously when reusable materials are applied. This is consistent with the designers’ concerns [28,29,31] regarding the economic uncertainty of reusable materials. It was proven that reuse is sometimes more expensive than manufacturing new materials [52,74]. Previous studies generated optimal solutions by focusing primarily on the production process of the material without considering the cost of modification for reusable materials [41,42,47]. However, the results of this study show that it is necessary to review the economic feasibility of the previous optimal solutions throughout the life cycle. For example, if the scope of the cost assessment is limited to the material production process, it appears that the cost of the project can be reduced through reuse. However, there are several items that incur costs when reusing materials, such as transportation and modification.
• The process of modifying reusable materials must be included in the cost assessment process. The cost of purchasing reusable materials is generally lower than that of new materials. However, the cost of the entire project may increase due to the cost incurred in the modification process of reusable materials into the desired shape, size, and quality [44,75]. Processing costs can vary greatly, depending on the type, region, and labor cost of the project. In this study, it was observed that as the amount of reusable material increased, the cost for modification increased, and eventually the whole cost of the project increased, compared to the case without reuse (Figure 11). In addition, since it is not easy to determine the remaining life of reusable materials in practice, inspection costs for this may be added. As such, the incidental costs incurred by using reusable materials act as barriers to reuse [29,30,31]. Therefore, from the planning stage, the modification and inspection costs of reusable materials must be considered. In addition, research on automation and simplification of reusable material inspection should be carried out to reduce the cost of reuse projects.
• The most promising strategy to reduce CO₂ and cost at the same time is to use the reusable material without changing its shape. However, excessive constraints on the shape of the material can increase the design difficulty and cost. In fact, designers are hesitant to use reusable materials due to concerns about the increase in design difficulty [28,29,31,32]. Therefore, it is necessary to assist the designer through the development of a design tool to generate a design in a required form while minimizing the processing of reusable materials.
• Above all, data on deconstructed structures and reusable materials should be obtained for the operation of the proposed framework. This is because the entire framework will not work without data on reusable materials in the material bank. In particular, materials used in buildings that are about to be deconstructed are not likely to be digitized in the form of a BIM model. Therefore, research should be conducted on a method for automatically constructing existing building and material data and so creating a comprehensive database containing significant amounts of material bank assets. The use of visual data [76], such as photographs or laser scanning data [77], enables the automatic generation of BIM models of existing buildings and will help to increase the number of BIM models, which are assets of material banks.

The scope of this study did not include the development of methods for predicting the life of reusable materials. However, the service life of the material must be determined to compare it with the target life of the structure to apply the reusable material. The life prediction of materials or structures has been performed mainly through long-term aging tests, laboratory tests, and theoretical/analytical methods [78]. The long-term aging test is employed to predict the lifespan through tests and observations of historical data or current buildings. The theoretical/analytical method has the potential to automate the life expectancy process because it determines the life span through a mathematical model or simulation. ISO 15686 [79] also provides a method and a simple formula for predicting the lifetime. Akanbi et al. [80] developed a whole-life performance estimator based on BIM and mathematical models and determined whether or not the material could be reused. Recently, a study was conducted to determine the reusability by attaching a sensor directly to the material [81]. The important point in material bank is that information must be managed on a per material basis. David et al. showed that it is possible to manage material-level information through radio frequency identification (RFID) technology and BIM [3].

6. Conclusions

This paper proposed a design framework for using reusable steel beams in structures to promote sustainability. The proposed framework consists of a material bank and a design support tool. The material bank manages the information on reusable materials. The design support tool generates a material procurement plan and evaluates the project’s CO₂ emissions and costs through LCA and LCC methodologies. All information on the framework is managed based on BIM data.

In a case study to verify the proposed framework, an alternative was created to reduce the CO₂ of construction projects by up to 77% through material reuse. However, it turned out to be impossible to create alternatives that reduced both CO₂ and costs simultaneously. Therefore, the designer explored a compromise between CO₂ and cost through an iterative design process using the framework.

This study provides the following contributions:

• This study proposed a generalized design framework for reusing steel beams that have a significant environmental impact, thereby enabling the creation of a steel structure design plan and a material procurement plan using reusable materials.
• The case study shows that the reuse of materials is an effective strategy to reduce the CO₂ generation of construction projects.
• The results of the case study also show that practical concerns about the economic uncertainty of material reuse are valid. The framework of this study can help project stakeholders overcome this through economic evaluation over the entire life cycle.

There are several possible avenues for future research:

• For the application of reusable materials, methods to measure the life and quality of materials with low costs are required. Concerns about the quality of materials and rising costs due to inspection act as barriers to the reuse of materials [28,52]. Therefore, a method for tracking and ensuring material quality and inspecting materials with low costs is also required. Structural health monitoring technology using sensor and vision technology [82] and material monitoring information management technology using RFID, BIM, and digital twin can be considered for automated inspection and reduction of inspection costs.
• The material bank should contain detailed construction process information. For example, the material bank of this study only contains information about the type of connection of the steel beams. However, to construct a structure using steel beams, information on the shape of the connection and the specification of the connecting member is required. In particular, if reusable materials were connected by bolted joints in the past and there is perforation at the joint, the information must be provided to the designer in advance. In addition, information on the connection of materials is needed to determine the reusability of structures and materials. This is because the reusability is affected by the ease of disassembly of the structure, and the difficulty of disassembly varies depending on the connection type of the material [74]. The evaluation of the difficulty of disassembly will help determine whether it is possible to actually extract and use reusable materials from the structure to be deconstructed.
• Optimization of the design plan using reusable materials and the material procurement plan needs to be performed. Because the number of alternatives that can be explored through the designer’s iterative design process is limited, many alternatives need to be explored through the computational optimization process and optimal alternatives derived.
• The precision of LCA and LCC analyses needs to be improved. For example, catering to the inflation rate in the cost evaluation process improves the accuracy of the analysis. Limitations of this study arise owing to the experimental assumptions and limitations of the scope of the LCC analysis. In addition, if data on the flow of materials after the deconstruction of the structure are obtained, it will be possible to evaluate all stages of the life cycle and accurately analyze the benefits of reuse and recycle. Therefore, a more precise analysis will need to be performed in the future.