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Creating CopenHill

By CASPER HØJGAARD ARNDT, CENG, AND PETER MADSEN NORDESTGAARD, CENG

The design and construction of a new waste-to-energy plant in Copenhagen, Denmark, was meant to do more than just generate electricity. The unique structure also features a rooftop public park with an urban ski slope and other amenities.

There are no mountains in Denmark. In fact, the area around Copenhagen, and especially the region's island of Amager, is among the flattest parts of the country—which makes the idea of a ski slope in the Danish capital rather imaginative. Nevertheless, the plans for a new power plant at the north end of Amager Island gave rise to the idea of constructing an Alpine-style urban ski slope on the roof of the plant with an enormous ramp leading down to the ground.

Copenhagen has traditionally expanded into the waters of the adjoining Øresund strait by using excess soil materials from ongoing construction and infrastructure projects to create new land. Many of the urban areas enjoyed today by residents and tourists were created this way. Thus, it is not surprising that the site for the power plant/ski slope had been underwater until the mid-1960s, when a new district plan created the area now known as the power plant peninsula. By 1971, the first coal-fired power plant on the peninsula began to produce electricity, and since then several more power plant projects have been added and modernized on the reclaimed land.

But things are changing in Copenhagen, where the municipal authorities have set a goal of making the city the world's first carbon-neutral capital by 2025. A major factor in achieving this goal involves a transition to a greener, more efficient energy sector. The power plant/ski slope project—named CopenHill—is a key part of that plan. Also known as Amager Bakke, which translates as Amager Hill, the power plant is a new waste-to-energy facility that began generating electricity in May 2017. The public park on top of the power plant—which features the ski slope, running and hiking trails up and down the structure, and a cafe at the base—opened in late 2019. (At press time, these amenities were temporarily closed due to the COVID-19 pandemic. A climbing wall on the building's facade was scheduled to be completed early in 2020 but has been delayed by the pandemic.)

On Earth Day—April 22—CopenHill even made it onto CNN's list of "18 noteworthy green buildings from around the world." The new facility replaced an older, nearby waste-to-energy plant, scheduled for demolition by the end of 2020. The area will also house a new biomass-based combined heat and power plant, currently under construction, which will replace an existing coal-fired power plant scheduled to be shut down. Additional plans for the area include the construction of facilities that will recycle as much as 40 percent of the waste material delivered to the CopenHill power plant and the conversion of sites north of the CopenHill and the biomass facilities into one large, interconnected recreational space.

The 41,000 sq m CopenHill project was designed by BIG-Bjarke Ingels Group architecture firm, which has offices in Copenhagen and New York City. The engineering work was led by the MBG Joint Venture, which featured MOE Consulting Engineers, of Copenhagen, now part of the Artelia Group, and Geo, based in the Copenhagen suburb of Kongens Lyngby. MOE was responsible for the structural engineering, civil works, mechanical systems, project management, fire engineering, construction management, technical inspections, health and safety, design management, and acoustics; Geo was responsible for the geotechnical engineering.

The client for the CopenHill project was I/S Amager Ressourcecenter, which is owned by five municipalities within the metropolitan area of Copenhagen. I/S Amager Ressourcecenter wished to replace its old waste-to-energy plant for several reasons, including the fact that the environmental permit for the original facility had expired, though the plant had been given an additional period of five years to operate. The old plant was also inefficient and lacked the newest technology. Thus, any increase in its service life would be expensive and work against the goal of making Copenhagen carbon neutral.

So instead of trying to keep the original plant operating, the goal was to design and construct a new plant that would be the cleanest, most energy-efficient waste-to-energy facility in the world. In 2011, BIG-Bjarke Ingels Group won an international architectural competition that had among its goals increasing the recreational use of the area surrounding the power plant and ensuring that this area, as well as the plant itself, would contribute to the city in a positive way.

But the idea of designing the facility as a multifunctional building that would combine waste-to-energy treatment with public recreational use was not part of the competition. Rather, that idea originated with BIG-Bjarke Ingels Group, which sought to create a public space well out of the ordinary and change the perception of power plants as monstrous concrete blocks.

After the competition, the project was delayed as the owner had to rethink the entire procedure for waste management in Copenhagen and the future demand for combustion and the reutilization of waste. In March 2013, the CopenHill project broke ground and construction began on one of the largest and most complex aboveground construction efforts in Denmark in recent years.

The available space for the construction site was highly limited while the number of firms involved in the project seemed extensive, including three suppliers of boilers, turbines, and flue gas treatment equipment, and five firms working on concrete construction, steel structures, and the facade and installations. Hence, the engineering design and construction processes were challenging from start to finish. But the design team remained confident that it was possible to design an optimal plant layout and still create the mountain-like shape needed for the ski slope and hiking trails.

Oriented roughly southwest to northeast, the power plant building measures 207 m long and 60 m wide with a maximum height of 86 m. At the lower (eastern) part of the plant, the tipping hall—a large hall with 10 chutes down to the waste bunker—is elevated 8 m above grade. Trash trucks access this area via a small ramp at the south facade. This allows trucks to offload into a 75,000 cu m waste bunker at a lower level. The elevation of the tipping hall led to a more optimal and less costly construction solution for the waste bunker. The outer walls at the lower level were constructed with secant piles, and the bunker was excavated into the limestone layer.

Additionally, in new and cleaner waste-to-energy plants, the flue gas treatment process requires a larger space. During the design phase of the CopenHill building, three different locations were considered for the chimney: outside the building at ground level, inside the building, and elevated on the west facade above the administrative offices. The west facade site was the one selected for the chimney, which reaches a height 124 m above grade.

Space was used judiciously in the design. For example, an area between the horizontal top slab of the waste bunker and the ski slope was used to house various types of equipment, while excess space above the flue gas treatment system and the administrative offices provided room for efficient natural ventilation. The heat from the waste processing area is vented via a pair of 7 by 10 m ventilation boxes on the roof.

The power plant's foundations feature approximately 3,000 prefabricated reinforced-concrete piles that were driven into the soil to either a subsoil limestone layer or to the top of a moraine clay layer. The piles support the building as well as the access ramp along the north side of the facility, which measures 126 m long and 30 m wide.

The plant is constructed with cast-in-place concrete at grade (except for a portion of the waste bunker that is below grade) and is topped by a steel superstructure (see the illustration at right). To accommodate heavy equipment and machinery, the concrete portion was designed as a single structural section, but the superstructure is divided into three separate structural systems with distinct structures on each side of the waste bunker.

The concrete structure supports the superstructure, the brick facade, and the main equipment and machinery: the furnace, boilers, and flue gas treatment system. It also supports such facilities as the three lower stories of the administrative offices, various storage and processing rooms, and other spaces.

The steel frames above the concrete portion help support elements of the brick facade, the roof elements, the interior galleries, and the ducts.

The subbase of the roof consists of hollow-core concrete slabs while the facade system features aluminum and steel sandwich panels that span 10 m between the main columns. This system was installed using nut-free bolts in brackets that were welded to the main columns in the brick segments. Each constituent brick has a size of 2,833 mm wide by 1,250 mm high, with each brick overlapping the brick above it and beneath it by 500 mm.

The main static system of the steel superstructure is designed as a series of distinctive and unique steel frames and girders. Regular steel beams were used in just two locations—above the boilers, due to the limited space, and at the administration offices, due to the chosen grid system. In the tipping hall and the process area, a half-frame system and continuous girders supported on the facade and the inner columns were used, respectively, with a 10 m grid system. At the tipping hall, the tops of the half-frames are supported at the waste bunker with pin-bolt bearings. At the bottom, the half-frames are supported at grade to transfer the high horizontal reaction forces into the ground via steel anchors and sheet piles. The girders measure up to 5 m in height. The dimensions of the box profiles in the facade columns at the process areas reach as much as 750 mm wide by 1,600 mm high, with flange and web thicknesses ranging from 10 to 40 mm.

At the waste bunker there is only a minimal distance between the beams that support the bunker's crane and the sloping roof surface. The girders span 40 m with a height that ranges from 0.9 to 11.5 m. All girders have connections that were bolted together at the site and installed in one piece. In the waste bunker, a single girder weighs as much as 80 tons.

BIG-Bjarke Ingels Group did not want any trusses in the west facade that would disturb the views of the city for the plant's employees and visitors to the administrative offices. Thus, the facade was designed with single-span columns at each story and horizontal beams and girders between the columns. The horizontal forces are transferred to the process area by the concrete deck at each story.

The concrete deck is a composite structure of delta beams, hollow-core concrete slabs, and a layer of cast-in-place concrete. Because it is unusual to construct an administrative area adjacent to and on the same concrete substructure as mechanical equipment, the administrative area features a unique solution: an additional layer of floating concrete floor on each story, supported on rubber pads to mitigate vibrations.

The steel structure is braced to transfer all horizontal forces to the concrete portion of the building. In the longitudinal direction, this bracing consists of large truss girders in the north and south facades and inside the center of the plant. Across the building, stability is obtained through a large girder in the grid line that separates the administrative area from the process area. Every second girder in the process area (spaced 20 m apart) has two internal columns that support the continuous girder and ensure stability by a truss system that links the columns. To free space for equipment and machinery, every second girder was designed without any internal columns; instead, these girders are supported by diagonals from the internal columns at the neighboring grid line. The uppermost columns in the process area are supported, and the forces transferred by, three horizontal truss systems located at 30, 42, and 58 m above grade, respectively.

From inside the building, all the steel structures are visible, and one can see more than 120 different welded-box profiles. By looking up toward the roof, one can see the hollow-core concrete elements, the supporting girders, and the secondary support system that connects each girder to its two neighboring girders. The top chord of the girders are box profiles with a minimum width of 450 mm to support the concrete elements from both sides.

Because the slope of the roof structure reaches a maximum incline of 45 degrees, the box pro-files feature studs. All the elements are connected with reinforcement in both directions at the construction joints to prevent the elements from sliding. The secondary support system was built up with diagonal tension rods that connect the bottom chord of each girder to the top chord of the two neighboring girders at 10 m intervals. This ensures that no more than 200 sq m of the roof structure would collapse in case of a failure in any single element throughout most of the structure.

By the beginning of 2015, the concrete construction was well under way, the first steel structures were being erected, and the installation of the process equipment had begun. As many as 1,000 workers were at the site simultaneously, with up to 20 cranes in operation at any given time. In May 2017, the plant became operational, producing district heating for more than 72,000 households and electricity for more than 30,000 households. By October 2017 the new administrative offices were occupied.

When the power plant went into operation, the roof structure consisted of only the concrete elements and a protective membrane while the layout of the park continued to undergo various changes. Funding had also been an issue because the public owners of the power plant were prohibited by law from owning the park and financing its costs. Instead, a private fund was established in November 2015 and ultimately raised approximately 100 million kroner (U.S.$14.8 million) for the project. BIG, working with the project's landscape architects-Copenhagen-based SLA Architects-finalized the park design in 2016.

The park serves several functions. The main attraction is the ski slope, which takes up the majority of the area, covering 10,000 sq m of the approximately 17,000 sq m roof and ramp area. The recreational park accounts for another 4,000 sq m, and the final 3,000 sq m is devoted to the stairs and other purposes.

Designed to be used year-round, the ski slope features an artificial green plastic surface with silicone applied to help the skis glide. Because the roof is heated by the power plant below, large parts of the slope will always be free of snow.

The loads from the slope system and the skiers were considered within the structure's self-weight. Locally, the forces from the skiers were considered in the design of the system that anchors the skiing mat to the structure below by the use of steel screws and strips. The roof is drained by pipes integrated in the surface structure in a herringbone pattern that leads water to drains along the facade.

Stairs are located on both sides of the roof, and a recreational area with various hiking trails is located along the northern edge of the slope. The hiking trails were constructed with form able lightweight concrete that is anchored to the concrete slab of the main roof structure. With inclines of 5 to 35 percent, the trails pose a fun challenge to both walkers and runners.

Real grass grows up through the slope, and trees and other plants are located throughout the roof surface. The trees were anchored to the roof with a separate system, independent from the main steel structure, and are prevented from sliding down the slope by anchored concrete blocks. In addition, steel wires temporarily stabilized all the trees. The roots of the trees were also strapped to a temporary reinforcement net until the roots themselves could keep the trees stable, at which point the temporary steel wires were removed. Many species of bushes and flowers were also planted in a substrate soil material with crushed tiles to create the surrounding meadow.

The planned climbing wall will start 5 m above grade at the north facade. It will measure 10 m wide by 85 m high, making it the highest climbing wall installed on a building in the world. Supported by an interior steel framework and outer fiber composite plates, the climbing wall will feature three plateaus where climbers can change ropes.

The park can be accessed by two types of ski lifts, the stairs and the trails, and a glass-enclosed elevator that allows people to see the inner workings of the power plant as they ride up to the roof. The multifunctional use of the building will also be clear to visitors on the trails because they will feel the heat of the power plant from the two natural ventilation boxes they will pass on their way to the top. Similar relief valves for the boilers penetrate the roof structure, but visitors are shielded by a 5 m high protective wall that surrounds the valves.

The public reaction to Copenhagen's new mountain has been extremely positive. Skiers have said the slope's artificial surface mimics the feel of snow beneath their skis, and the varied terrain makes the course interesting and challenging for beginners as well as more experienced skiers.

Bjarke Ingels, founder and creative director of BIG, remarked on the studio's milestone project: "CopenHill is a blatant architectural expression of something that would otherwise have remained invisible: that it is the cleanest waste-to-energy power plant in the world. As a power plant, CopenHill is so clean that we have been able to turn its building mass into the bedrock of the social life of the city. Its facade is climbable, its roof is hikeable, and its slopes are skiable. A crystal-clear example that a sustainable city is not only better for the environment, it is also more enjoyable for the lives of its citizens." Honored with multiple awards, including the Scandinavian Green Roof Award 2019, from the Scandinavian Green Roof Institute in Malmö, Sweden, the design of the CopenHill project represents a unique accomplishment: turning a usually inaccessible industrial structure into a recreational area open to everyone. It is, in more sense than one, a towering achievement.

Casper Højgaard Arndt, CEng, is an engineer of steel structures and Peter Madsen Nordestgaard, CEng, is the technical director of steel structures in the Copenhagen, Denmark, office of MOE Consulting Engineers.

PROJECT CREDITS Owner: I/S Amager Ressourcecenter, Copenhagen, Denmark, region Architects: BIG-Bjarke Ingels Group, Copenhagen and New York City Lead engineering consultant: MBG Joint Venture, featuring MOE Consulting Engineers, Copenhagen, and Geo, Kongens Lyngby, Denmark Landscape architects: SLA Architects, Copenhagen Power plant equipment consultant: Ramboll Group A/S, Copenhagen Facade structures contractor and design: Sipral a.s., Prague Alpine ski slope design and engineering: Klenkhart & Partner Consulting ZT Gesellschaft m.b.H., Absam, Austria Artificial slope surface: Neveplast S.r.l., Albano Sant'Alessandro, Italy Concrete construction contractor: NCC Danmark A/S, Søborg, Denmark Steel structures contractor: Züblin Stahlbau GmbH, Senftenberg, Germany

Civil Engineering, May/June 2020, © American Society Of Civil Engineers. All Rights Reserved

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