Structural Reuse of Decommissioned Ski Lift Steel Trusses for Load-Bearing Applications

Abstract

The ongoing effects of climate change have led to a rise in global temperature, significantly reducing snow cover and resulting in the abandonment of numerous ski areas across Switzerland. As a result, many ski lifts have been decommissioned and left to deteriorate due to lenient local regulations. To address this issue, this paper presents a case study approach to repurposing steel trusses from abandoned ski lifts for a new structural application within the building industry. The design, sourcing, and construction of a new load-supporting column are described, focusing on reusing the ski lift steel trusses as a whole, without dismantling them into their components. After collection, these elements are adapted to comply with current building standards. By pouring out the hollow structure with the recently developed building material Cleancrete ©, a new load-bearing structure is developed. A comprehensive life cycle assessment (LCA) demonstrates the environmental performance of the steel–Cleancrete hybrid construction, which achieves a global warming potential (GWP) of 536.58 kg CO₂-eq. In comparison, alternative designs using wood and concrete exhibited GWP values of 679.45 kg CO₂-eq, +26.6%, and 1593.72 kg CO₂-eq, +197.02%, respectively. These findings suggest that repurposing abandoned ski lift structures can significantly contribute to sustainable building practices, waste reduction, and the promotion of circular economy principles. The process outlined in this paper holds potential for future applications, particularly in the reuse of other steel components, ensuring continued circularity even as the supply of ski lift structures may dwindle.

1. Introduction

1.1. Context

Climate change and global warming have emerged as the pressing challenges of our time. Their far-reaching consequences can be felt anywhere, particularly in mountainous regions such as the Swiss Alps [1,2]. Globally, average temperatures have increased 1.1° Celsius since the 19th century [3], with average temperatures in the Swiss Alps increasing by 2.2° Celsius [4]. Various studies have linked the increased levels of CO₂ and other greenhouse gas emission with increased temperatures observed in the Alps [5]. One of the consequences of these higher temperatures is a reduction in snow cover, changing local environments and ecosystems [6].

Studies show that not only the extent, i.e., the area, but also the time span of snow cover in the Swiss Alps is affected, overall decreasing snow cover by between 20 and 30% [7]. Especially prominent is this reduction at lower altitudes, specifically below 1500 m, where snow is less frequent and melts earlier in the season [8,9]. These findings, among others, are supported by satellite data and ground measurements, which reveal a consistent trend towards shorter snow seasons and reduced snow depths [10]. This increase in temperature and the reduction in snow cover have various implications on different industries, one of them being the Swiss winter tourism industry, which is being hit especially hard. Challenged with shorter skiing seasons and less reliable snow conditions, many ski resorts struggle to remain profitable and eventually go out of business, with local authorities reporting more than 65 abandoned ski lifts. Due to the absence of local regulations mandating the deconstruction of ski lift structures upon decommissioning, these ski lift structures have been left to deteriorate within the landscape [11,12]. This discussion focuses specifically on the material consequences in ski areas rather than the touristic or societal impacts, which are the subject of an academic project by the chair of Langenberg and Theriot at ETH Zürich, aimed at rethinking and transforming these infrastructures.

1.1.1. Implications of the Build Industry on Global Warming

The building industry is a significant contributor to global CO₂ emissions, primarily due to the energy-intensive processes involved in the material production for buildings and their construction and operation [13]. These emissions can be broadly categorized into two types [14]. On one hand, there are operational emissions that occur while the building is operated, which include heating, cooling, and electrical applications [15]. On the other hand, there are embodied emissions, which occur during the building process and the production and transportation of the utilized building materials [16]. Most efforts in recent history have been directed at lowering operational emissions, with the result of switching from fossil to renewable energy sources and improving the insulation of buildings [14]. Reducing a building’s embodied emissions is crucial for reducing its overall environmental impact [16], particularly given that the steel industry, which relies heavily on fossil fuels for the energy-intensive production and forming of construction-quality steel, contributes approximately 7–9% of global CO₂ emissions [17], with nearly 50% of produced steel globally being directed towards the construction sector [18].

1.1.2. Circular Construction and the Process of Reuse

One way to possibly decrease greenhouse gas emissions is to apply the main principles of the circular economy to the building sector to develop a more circular construction approach [19,20]. The main principles, derived from waste management terminology, are expressed through the 10R structure [21]: refuse, reduce, redesign, reuse, repair, refurbish, remanufacture, repurpose, recycle, recover (energy). These actions are scaled from highest to lowest level of circularity and thus from highest to lowest level of greenhouse gas emission. Additionally, the lack of repairability in buildings and even in everyday objects has been a topic of concern and criticism for years, while resource consumption and waste generation continue to rise [22].

The practice of reusing components, in the case of the reuse of structural elements, is keeping their form and function intact, or only slightly modifying them to be used within a new function, therefore minimizing the need of sourcing new materials as well as energy intensive processing on primary and secondary materials [23]. Reuse strategies extend the life cycle of components and thus the raw materials within them, reducing construction waste and the demand for new raw materials [24]. However, this practice requires additional processes such as controlled disassembly, storage, possible modification and repair processes of components, and additional transport between these steps. It also advocates for a rediscovery of disassembly processes like design for disassembly (DfD) [25,26] and design for manufacture and assembly (DfMA) [27]. Case studies have however shown that these additional steps typically have a minimal ecological impact when compared to the savings caused by the avoided production of new components [28].

The reuse of building and structural components has always existed since ancient times as it allowed a great economic advantage due to the low labor costs and the intrinsic quality of the existing components, such as already-squared stones and wooden beams close to the new construction site [29]. Until the middle-ages, the reuse of elements was charged with additional meaning. So-called spolia, structural and decorative building elements, were collected after conquering new territories and used to embellish public buildings and show cultural encompassment within the local building culture [30,31]. The reuse of components can therefore comprise a stratification of meanings, on a functional, symbolic, and logistical level, to which an ecological component is added, making architectural design with these elements a holistic process.

1.1.3. Reuse of Structural Elements

With the onset of the Industrial Revolution, particularly around the late 19th and early 20th centuries, the value of building components initially plummeted in proportion to the labor costs for their disassembly and assembly on site. This shift was further accelerated by mid-20th century developments, where construction costs were reduced as unskilled workers began assembling more complex, prefabricated system components, significantly lowering the cost and perceived value of individual building elements [32]. This trend contributed to the exponential growth of a linear economy based on the extract–produce–dispose model, relegating the reuse of structural elements to a niche practice within the construction industry [31]. Additional factors such as supply chain dynamics, costs, limited demand, traceability, and quality certification have also played significant roles in limiting the broader adoption of reuse practices [24,33,34].

Particularly in the reuse of steel structures, there are studies which precisely trace all mechanical, chemical–physical and esthetic analyses and related tests to assess the reuse of elements [35,36,37,38].

The complete utilization of components without modifying their geometry, as illustrated in this article, is the most economically advantageous approach. This method avoids additional processes that increase design and specialized labor efforts, thus reducing overall costs. However, this approach can lead to the over-dimensioning of structures for the new functions they are intended for. Different construction contexts require distinct strategies. For some steel truss structures, it may be more efficient to dismantle individual metal profiles and reassemble them with a different geometry, thereby optimizing their static behavior. However, when the elements are connected through welded rather than bolted joints, it is advantageous to use the structure without dismantling the individual profiles [23,39,40,41].

In Switzerland, interest in the reuse of structural steel elements is growing, particularly due to case study examples such as the erection of the Kopfbau Halle 118, or K.118, designed by Baubüro in situ and built in Winterthur [42].

This trend is also evident across Europe and worldwide, with various projects being developed like the Holbein Gardens Project, the East Arkengarthdale Bridge, or the Port of Dundee East Redevelopment [43,44,45]. In the K118 case study, the entire primary and secondary steel structure from a distribution center building in Basel was dismantled and reused, making only minimal changes. The columns are HEA 200, the beams are IPE 400 and 450 for the primary structure, and IPE 270 and 330 for the secondary structure. In the process of reuse, the columns and the beams placed in the facade perimeter of the building were retrofitted by filling the flange cavity with concrete, fully encasing the web. This was carried out to comply with fire safety regulations. With a total weight of 63 tons, a saving of about 4.9 tons of CO₂ equivalent was achieved compared to using a new structure of the same size. In addition, some demolition companies, such as Eberhard AG, have already started to stock, analyze and catalog steel elements recovered from demolition to be sold and reused for new projects.

Much has been written about the advantages and disadvantages of the structural principles of the concrete-encased steel composite columns. In the case of K118 and in the subject of this paper, the use of concrete in combination with the steel structure primarily aims to meet fire regulations rather than to enhance the structural strength of the elements [46,47].

Rather than using concrete, the here-described prototype utilizes Cleancrete, a newly developed cement-free concrete developed by Suisse startup Oxara, to fill the ski-lift trusses. This material is also used in the environmentally friendly composite construction system “Stahlkammer”, a hybrid structure composed of steel frames filled with cement-free concrete, developed at the ZHAW and ETH. Environmentally, Cleancrete substitutes conventional cement with Oxacrete, a white powder that acts as a superplasticizer, which offers a low global warming potential (GWP) of 0.006 kg CO₂-eq, 47 environmental impact points (UBP), and an embodied energy requirement of 0.035 kWh oil-eq, which makes it a natural choice for this column design [48].

1.2. Objective

The main goal of this paper is to evaluate the feasibility of incorporating reused steel trusses within the framework of new construction projects. The proposed elements utilize abandoned ski lift steel trusses in combination with Cleancrete to form a load-supporting column which is to be used as a structural element in construction.

This paper aims to address what the methodology and process of such an undertaking would look like. In particular, the technical possibility of reusing these steel trusses as a whole, without the need to dismantle the structure into its pieces. It will explore whether the quality and condition of the reclaimed steel trusses can be assessed with sufficient confidence. Additionally, it will examine the techniques to extract, prepare, and reassemble these structures. This paper will also assess the environmental consequences of this construction method in terms of its impact on global warming potential.

2. Methods and Material

2.1. Methodology

The methodology of this study details the sourcing, design, planning, and construction processes for repurposing steel trusses from abandoned ski lifts to create a new structural application for a four-story building. The design process begins with a comprehensive inventory and assessment of the available ski lift structures, documenting their geometry and condition as well as assessing their structural properties and unique characteristics. This information is crucial for the conceptual phase, where the unique attributes of the ski lift components are not simply incorporated, but actively drive the building design. Rather than starting with a predetermined idea and trying to fit reused components into it, this methodology utilizes a regenerative design approach to maximize the potential of the ski lift structures for both their structural and esthetic qualities [49,50].

Following the conceptual design, material sourcing and stock identification are carried out. This involves identifying and documenting the available ski lift structures suitable for repurposing. The stock identification process includes a thorough evaluation of the physical and mechanical conditions, geometrical dimensions, and mechanical properties of the ski lift trusses. A special evaluation is conducted on the connection of each joint of the steel structure to assess potential challenges within the reuse process. Considering all ramifications, the workflow is presented in Figure 1.

The specific structural requirements are considered to ensure that the reused trusses can meet the requirements set by lawmakers. Initially, a structural design is developed, integrating the ski lift structures into the framework of a building. To assess the feasibility and structural integrity of using the ski-lift trusses as a whole, load simulations, fire safety simulations, and LCAs are carried out. To assess the performance of this prototype, regarding its global warming potential in comparison with similar constructions, the utilization of other, more conventional materials is explored. Finally, the results and potential future works are discussed in the last section of this paper.

2.2. Sourcing

The identification of potential structures is challenging due to the scarcity of detailed information on the status, condition, and political framework of decommissioned ski areas. For this study, potential structures were identified in a nonsystematic, pragmatic manner by validating information from newspaper articles, non-governmental organization reports, and discussions with local authorities.

To qualify as potential structures, the trusses had to meet specific criteria:

• All operations have ceased;
• Ownership rights are clear;
• The structures are visually free of structural damage;
• The structures are available and have not been sold to another party;
• The geometry is suitable for future usage;
• The type of connection details are available;
• Their ease of disassembly.

For this paper, the decommissioned ski lift in Hospental–Winterhorn was selected as an exploratory structure due to its accessibility, the local authorities motivation and willingness to provide information on these structures, its square geometry, and its screwed connection to the foundation.

The Hospental–Lückli (SLW1-SLW10) chairlift is a single-cable circulating lift with fixed clamps and two-seater chairs with its design shown in Figure 2. Although the installation is still in its original condition, it is no longer operational.

Built in 1980, the installation consists of the following parts:

• Hospental drive and tensioning station (valley station);
• Lückli reversing station (mountain station);
• Nine steel truss columns between the top and bottom stations (SLW2-SLW10).

After selecting the desired structure for reuse and confirming dependencies such as structural integrity, ownership clarity, and accessibility, each truss undergoes thorough conditional and structural visual inspections. Once approved, the trusses are dismantled and collected, beginning with the removal of the cable carrying the lift seats. Due to the challenging terrain, deconstruction by crane might not be feasible, and may require helicopter transport to the interim storage site at Hospental valley station. To address this uncertainty, both processes will be described in detail and will be the subject of the following LCA in Section 2.7, to evaluate their consequences on GWP.

Due to the ever-increasing distances of the lift structures from the valley station, ranging from 175.15 to 1111.50 m, calculations use an average distance of 602.89 m. Lifts 1 and 10 are not considered as their primary purpose is the rerouting of the steel cable to which the seats are connected, rather than contributing to the structural stability of the construction. Figure 3 illustrates the structures in their respective locations.

• SLS_1;
• SLS_2: 175.15 m;
• SLS_3: 216.95 m;
• SLS_4: 381.15 m;
• SLS_5: 486.45 m;
• SLS_6: 675.25 m;
• SLS_7: 791.53 m;
• SLS_8: 985.20 m;
• SLS_9: 1111.50 m;
• SLS_10.

2.2.1. Sourcing via Helicopter

The extraction process utilizing a helicopter involves several steps, beginning with the inspection of the bolts anchoring the structures to their concrete foundations, which are loosened but not removed. After securely connecting the ski lift to the helicopter using a steel wire, the sagging steel is tensioned, and the previously loosened bolts are removed. The structure is then lifted and flown to the base station at Hospental valley station, where the steel components holding the ski ropes are removed. Following the gathering of the structure, the steel truss is transported to a metalworker for reprocessing. Finally, after being filled with Cleancrete, the new column is transported to the new construction site. Removed parts which are not used are assumed to be recycled.

2.2.2. Sourcing via Crane

The extraction process using a crane involves several steps. Initially, a crawler crane or spider crane connects the truss columns to its cantilever arm. Following this, the bolts securing the column to its foundation are carefully loosened. To ensure safety and prevent a potential fall of the structure, the bolts are only removed once a secure connection between the crane and truss is established. Once securely attached to the crane, the bolts can be removed, and the column is lifted from the foundation and transported to the base station. At the base station, the head part of the truss is removed, with the detached elements going on to be recycled. The truss columns are then temporarily stored at the base station before being transported to a metalworker for reprocessing. Finally, the reprocessed trusses are transported to the construction site for integration into the new structural system.

2.3. Truss Details and Quality Inspection

The truss has a square layout, measuring 1.06 m in length and width, and a height of 12.25 m. It is composed of three standardized L-profile elements as shown in Figure 4. The truss consists of two parts connected via six M18 bolts and a top plate on each side of the L-profile. The largest profile, measuring 120 mm × 12 mm × 6620 mm, serves as the corner support and forms the outer profile. It is welded to a baseplate with dimensions of 320 mm × 200 mm × 30 mm, anchoring the structure to its foundation with two M32 bolts, creating a stiff unity. Between each corner profile, 60 mm × 6 mm × 1300 mm profiles are welded on the inside, forming the truss structure. The structure is stiffened by welding the 60 mm × 6 mm profiles diagonally at 45-degree angles, alternating between 45 and −45 degrees. This system is continued on all four sides to create a parallel structure. After reaching 6.16 m in height, the outer profiles transition to a 100 mm × 10 mm × 5600 mm profile, maintaining the same stiffening mechanism and spacing.

The connection between these profiles is reinforced with an external metal piece overlapping both structures, secured with three bolts in each profile. To achieve a continuous system, the diagonal profiles passing the connection are not welded but screwed to each profile using two bolts on each side. To provide a larger surface area for connection and allow the use of two spaced-out bolts, the profile changes its base form from a straight to an angled shape, thus increasing its surface area.

In assessing the condition of the reused steel trusses, an initial visual inspection was conducted to evaluate both the general state of the elements and the integrity of the galvanization. Special attention was paid to the welded joints between the anchoring plates and corner profiles, as these were identified as the critical components. While the visual assessment revealed no visible damage or deterioration. The operational history of the ski lift, which functioned for approximately 13 years with seasonal and limited daily use, suggests that the impact of cyclic loading is minimal. Following the guidelines from the Stahlbau Zentrum Schweiz, specifically the Steelaid Re-Use classification, the steel trusses are categorized under reuse class C-A [51]. To confirm this classification, material testing, including a 1:1 mockup, will be carried out in a subsequent phase to evaluate the fatigue strength of the welds and identify any potential signs of degradation.

2.4. Design

The design of the column aims to optimize the structural qualities of reclaimed steel trusses. Steel trusses offer a high strength-to-weight ratio, enabling them to support substantial vertical loads while maximizing the efficiency of the load-bearing structure. The truss design transfers load through its angled crossing members to the corners, making the corners the most suitable points for additional load bearing in a structural system. To leverage this characteristic, the column is oriented at a 45-degree angle rather than orthogonally, as in conventional systems. The truss has a square layout, measuring 1.06 m in length and width, and a height of 12.25 m making it suitable for buildings with at least four stories. This configuration results in a floor height of 3.0625 m, accommodating various typologies such as residential, office, and educational buildings. Figure 5 presents photographs of the truss in its installed location.

Steel by itself does not meet the fire resistance requirements set by lawmakers for load-bearing systems within the Swiss building context. To address this issue, steel trusses are filled with Cleancrete, which in case of a fire acts as a secondary load-bearing system, insuring the safety of the structure. Cleancrete exhibits distinct thermomechanical behavior compared to conventional materials. While a reduction in strength is typically expected for most materials under high temperatures, the burning of the clay components within Cleancrete leads to an increase in strength. When subjected to post-heating conditions, Cleancrete’s compressive strength more closely aligns with that of fired clay bricks, exhibiting a range of 2–5 MPa [48].

Once the steel truss arrives at the workshop, it is placed standing and secured to a formwork panel acting as a foundation. The truss is inspected for local damage or deformation. After a successful visual inspection, a steel hollow body measuring 516 mm × 516 mm × 12.25 m, with a wall thickness of 5 mm is inserted in the middle of the steel truss, maintaining a 260 mm distance to the outer edges of the diagonal L60 profiles, thus providing a vertical installation shaft for various utilities. To meet static and fire resistance requirements efficiently, only the necessary amount of Cleancrete is used, forming a 260 mm thick layer, except where the crossed diagonal steel elements reduce the cross-section. Additionally, a hollow core is incorporated within the column to accommodate HVAC systems, water, and electrical installations. This design allows direct access to these installations, eliminating the need for conventional pipe and cable routing, and integrating the necessary infrastructure within the load-bearing structure. In preparation for the Cleancrete infill, the hollow body is screwed to the foundation formwork panel, ensuring a leakproof seal. Before pouring the Cleancrete mixture into the steel truss, additional formwork panels are installed to ensure a leakproof structure and a smooth pouring process. To maximize efficiency, the panels are placed inside the L120/L100 corner profiles rather than encompassing the entire structure. Figure 6 outlines the remodeling process along with the adaptations made to the steel truss. This approach reduces the size of the formwork panels, optimizes the volume of Cleancrete needed, and leaves the diagonal L60 profiles visible after the formwork is removed. The formwork remains in place for 5 days to support the material during its initial curing process, followed by an additional 51 days of drying to achieve the desired properties [48]. After removing the formwork, the retrofitting process is completed, and the structure complies with all local regulations.

2.5. Integration in a Load-Bearing System

The design of the column and the intended structural system aims to optimize the structural qualities of reclaimed steel trusses, specifically their low GWP and their efficient load distribution through triangular units and load concentration at the corners. The horizontal load-bearing structure connects at the truss corners, where the structure is stiffest.

An embracing steel collar is welded onto the truss, incorporating beams that support prefabricated wooden slabs. The collar is equipped with two steel swords that are threaded through the diagonal steel profiles and cast into the structure. This ensures more efficient load distribution and maintains a functional system when exposed to fire. To achieve perpendicularity, the column is positioned at a 45-degree angle within the system, creating an orthogonal load-bearing beam system. Figure 7 illustrates the process of retrofitting the reused steel truss column within the new load supporting system. The adaptation to the structural system as well as the hollow body for the HVAC system in the center of the structure are kept as a constant within the frame of comparison of the design alternatives, outlined in Section 4.

2.6. Assessment

The structural and fire safety assessment of the reused steel truss columns, as well as the design alternatives, wood and concrete, is conducted performing calculations required by SIA Norm 261 (2020) to ensure compliance with safety standards and optimal design dimensions.

2.6.1. Load Testing

The permanent load exerted by the mass timber floor, including impact sound insulation and an underlay floor is being considered with qk = 5.8 kN/m², while the variable live load due to occupancy is pk = 3.0 kN/m². The vertical support force at the foundation level is 1167 kN, with a reduced load for seismic and fire safety scenarios of 614 kN.

Analysis indicates that seismic loads exceed wind loads and, following SIA Norm 261 (2020), the wind load is neglected and seismic impacts are calculated. Using the equivalent lateral force method, the seismic loads are calculated with material-specific ductility coefficients q = 4.0 for the reuse steel truss, q = 1.5 for wood, and q = 2.0 for concrete. The earthquake design moment (M_d) at the foundation level is 252 kNm for the steel truss, 672 kNm for wood, and 504 kNm for concrete. Maximum vertical loads (N_Rd) are 2128 kN for the steel truss, 5595 kN for wood, and 4726 kN for concrete, with respective safety factors of 1.82, 4.79, and 4.05 with ≥1.00 satisfying requirements of the norm. Combined earthquake and normal force moments (M_Rd) are 348 kNm for the steel truss, 1055 kNm for wood, and 523 kNm for concrete, with respective safety factors of 1.38, 1.57, and 1.04, with ≥1.00 satisfying requirements of the norm.

2.6.2. Fire Safety

Assessing the fire resistance of the columns for a duration of 60 min (R60) is required for vertical load-supporting elements. For the calculations, the exposure of all four sides of the column to fire, with the inner sides protected by the hollow cross-section, is assumed. This evaluation, conducted in compliance with Swiss standards, examined the reuse steel truss column as well as the design alternatives using wood and reinforced concrete.

The reuse steel truss filled with Cleancrete was evaluated in accordance with the standards outlined in SIA 263/2013 D. While Cleancrete provides thermal protection to the steel profiles, the steel reaches 600 °C after 16 min, reducing its strength by 50%. The load-bearing capacity is primarily provided by the Cleancrete core, which has a net cross-sectional area of 1.0 m², excluding the 516 mm diameter hollow body. The stress in the Cleancrete during a fire is σ_c,fi = 0.81 N/mm². Cleancrete’s compressive strength increases to 3 MPa at high temperatures, ensuring structural integrity and compliance with fire resistance requirements [48].

The wooden column was assessed using SIA 265/2021 D. After a 55 mm charring depth over 60 min, the residual cross-section supports a design fire load (N_d,fi) of 614 kN. The residual load-bearing capacity (N_dR,fi) is 13,000 kN, yielding a fulfillment ratio of N_dR,fi/N_d,fi = 21.2, thus meeting fire resistance standards.

Concrete columns were evaluated based on SIA 262 (2013) criteria. The required minimum thickness of 140 mm and reinforcement cover of 20 mm are met, ensuring R60 fire resistance compliance.

These calculations demonstrate that the reused steel truss columns effectively satisfy all structural, safety, and fire resistance requirements, ensuring compliance with the Swiss standard SIA Norm 261 (2020). The calculations serve as basis for the design of the reuse steel truss column in Section 2.4 as well as design of the material alternatives in Section 2.7.

2.7. Life Cycle Assessment and Comparison

The LCA method [51] is the primary tool for evaluating environmental impacts of products and processes. Utilizing the extensive literature that offers LCA comparisons of building structures with reused elements [52,53,54], this section initially looks at the environmental impact of a reused steel truss in comparison to a new steel component. Subsequently, it evaluates the sustainability of the finished, reworked reused column. This assessment aims to provide a detailed comparison to demonstrate the environmental consequences and potential trade-offs of using reused structural steel trusses in combination with Cleancrete as a load-supporting column within the construction industry.

To better understand the overall performance of the reused steel truss column, two widely used material variations, as seen in Figure 8, are considered for comparison:

• A hollow, monolithic reinforced concrete column—790 mm × 790 mm × 12.25 m, with a wall thickness of 140 mm
• A hollow, wooden cross laminated timber (clt) column—750 mm × 750 mm × 12.25 m, with a wall thickness of 120 mm

All design alternatives are engineered to meet loading requirements set by lawmakers within the Swiss SIA norms, fire safety standards, and internal hollow body dimensions. Any additional processes necessary for retrofitting or fitting specifications for integration into a holistic structural system are consistent across all three designs and are therefore not considered within this comparison.

In this study, the functional unit of the reused ski lift column encompasses the entire process beginning with the disassembly of the structural column from its existing site and finishes with its arrival at the new construction site. The comparison explicitly excludes the foundation and the node connection between the column and the floor system, as these elements are assumed to be identical across all alternatives. This focused approach allows for a precise assessment of the environmental and functional performance of the reused column in comparison to other standard industrial solutions. All non-structural elements are excluded from the comparison, except for specific additive processes such as coating or painting. A detailed breakdown of material quantities for all alternatives is provided in Figure 11.

To compare the different manufacturing phases between the reused column and the design alternatives, the method outlined in SIA 2032, which refers to the European standard SN EN 15804 [55], is used.

SIA 2032 defines the manufacturing phase of a new element and a reused one as “analogous”. Figure 9 illustrates this correlation: the sub-steps A1 to A3 are reinterpreted and renamed for clarification purposes: A1 Raw material procurement becomes R1 Component harvesting in the cycle of reuse, A2 Transport becomes R2 Transport, which refers to the transport to an interim storage facility or workshop, and A3 Production becomes R3 Preparation. A4 still refers to the transport of the building components to the construction site, and A5 continues to describe the construction phase of the building. Since the exact distances of transport are not known, R2 will be calculated with a 100 tkm distance each for the comparative LCA.

Figure 10 provides the system boundaries used within this LCA, include all processes related to the selective deconstruction of the obsolete structure R1, the storage, preparation and repair of the recovered existing components R3, the extraction of new resources A1, the production and manufacturing of new components A3, all transport between the various stages R2 and R4, but also A2 and A4, and the construction work at the new site A5 = R5. However, the column’s utilization, maintenance, and end-of-life phases B1–7 are excluded, as their inclusion would introduce additional uncertainty and require an in-depth sensitivity analysis beyond this article’s scope. Following this method, R1–R3 and, respectively, A1–A3 are calculated for the reused column and the design alternatives.

A cut-off approach, or 100:0, Refs. [56,57] is chosen to allocate impacts to the recycled and reused materials. In this allocation approach, all environmental impact from the extraction of raw materials and production of the component is fully attributed to its first life cycle and not repeated in subsequent life cycles.

Consequently, the impact of the reuse of the ski-lift steel truss is calculated as the original production of the raw material; the manufacturing and assembling of the structure is not considered. This environmental impact is already accounted within the structure’s first life cycle. At the same time, waste disposal costs (C1–C4) are also not taken into account because they are outside the boundaries of the system.

The design of this method incentivizes reuse processes, but does not provide an incentive to design buildings with new components designed to withstand multiple life cycles, e.g., with DfD techniques, since the resulting environmental impact is already being accounted for within the first lifecycle. De Wolf compared various LCA calculation methods and their effect on the environmental impact of reuse of components using K118 as a case study [58].

The Swiss LCA database KBOB [59] was used to calculate all steps for the construction of the reused column and design alternatives, providing data for A1–A3 and, respectively, R1–R3. For the disassembly and stocking phases of the existing elements, all necessary work and machinery was defined and the time required for the operations was estimated. Particularly in the disassembly phase of the pylons, two scenarios were calculated as mentioned in Section 2.2.1 and Section 2.2.2, comparing the usage of a helicopter or a mobile crane (such as a spider crane), as the use of a helicopter has a great environmental impact. As a unit of measurement for comparison, the GWP was used, which is expressed in kilograms of equivalent carbon dioxide (kg CO₂eq).

All data used to conduct the LCA are displayed in Figure 11 and are situated in relation to each other in Figure 12.

3. Results

Reusing the steel truss structure results in 1.46 tons per column of steel being retained within the reuse cycle, preventing 1365 kg of CO₂-eq emissions

The calculated GWP of the reused steel column collected by helicopter, crane, the wooden column, and the concrete column are represented in Figure 12. The reused steel column collected by crane shows the most competitive GWP, outperforming the helicopter-collected variant by 5.81%, the wooden column by 26.6%, and the concrete column by 192.7%. Cleancrete utilization contributes 37.9% of the total GWP. Transportation distances significantly influence GWP, with 0.232 kg CO₂-eq per tkm. In the wooden pillar design alternative, material production accounts for 94.4% of the GWP, representing most emissions. A similar pattern is observed in the concrete pillar design alternative, where concrete production contributes 65.0% and rebar production 29.3% of the GWP, together comprising 94.3% of the total GWP.

4. Discussion and Future Work

Global warming and emissions from the building industry are critical issues necessitating immediate attention and solutions. One way to decrease greenhouse gas emissions is to apply the main principles of the circular economy to the building sector to develop a more circular construction approach. This paper examines the potential of integrating circular economy principles into the construction sector by reclaiming obsolete steel trusses from decommissioned ski areas and repurposing them with Cleancrete for load-bearing purposes.

Efficient and systematic procurement and sourcing strategies are crucial for the sustainable reuse of materials in construction projects. The procurement for this project, which involved the reuse of ski lift trusses, was initially conducted pragmatically by establishing contact with local authorities to gather information on available decommissioned ski areas. However, this process could be significantly improved by developing an open-source database of ongoing dismantling projects. Such a database would minimize transport routes by offering a wider arrange of decommissioned structures and their availability and time of deconstruction, thereby reducing the GWP of the reused elements, while also ensuring that the materials meet the required geometric and mechanical properties. The research project Swircular, carried out by the ETH in Zürich, is currently developing such a database, aiming to streamline the sourcing of reusable materials. Collecting the desired structures from ski areas, typically located in mountainous terrain, presents logistical challenges. To optimize GWP during the deconstruction process, it is essential to avoid using helicopters, which generate substantial greenhouse gas emissions. Instead, alternative transportation methods with lower environmental impacts should be employed to ensure efficient and sustainable sourcing of the trusses.

The next steps involve developing a modular structural system utilizing the prototype steel truss column to its full potential. In the context of an entire building, as opposed to a single column, additional challenges such as connecting beams and floor slabs to the structure must be addressed in detail to comply with all regulations set by lawmakers. As the next step, a full-scale 1:1 mockup of the pillar will be constructed, in which both the steel and the welded joints will be tested. Ultimately, the goal of future works is to build a case study database utilizing both the prototype column and the developed structural system.

5. Conclusions

The reuse of steel trusses from decommissioned ski lifts significantly reduces the environmental impact of new construction projects. This paper demonstrates the effectiveness of a circular design approach, including procurement, assessment, extraction, and reassembly. The reuse of steel truss structure results in 1.46 tons per column of steel being retained within the reuse cycle, preventing 1365 kg of CO₂-eq emissions per collected ski lift column. The comparative LCA shows that repurposed trusses have lower GWP compared to new steel, wood, and concrete. The crane-collected steel column shows the lowest GWP, outperforming the helicopter-collected variant which emits +5.82% kg of CO₂-eq, the wooden column which emits +26.6% kg of CO₂-eq, and the concrete column which emits 197.0% kg of CO₂-eq. Cleancrete accounts for most of the new emissions within the reused steel truss column, namely 37.9% of the total GWP. Transportation plays a crucial role, contributing 0.232 kg CO₂-eq per tkm [59], which accumulates over the course of the logistic and fabrication process, resulting in significant emissions, especially within the reuse case due to the weight of the column.

The unorthodox use of decommissioned ski lift truss structures in combination with Cleancrete exemplifies how unconventional reuse strategies can significantly reduce greenhouse gas emissions in the building industry. This approach showcases the potential of repurposing structures that might initially seem unsuitable, demonstrating their effectiveness and environmental benefits in new construction contexts.