The new library will occupy a high-profile site in Helsinki, with a large plaza in front to create a dynamic public connection. © ALA Architects
The new central point of the city’s library system will feature dramatic arches, cantilevers, and an ethereal “cloud roof.”
July 9, 2013—City officials in Helsinki, Finland recently selected the winning design from among 544 entries in an international competition for a new central library that will attract an estimated 5,000 visitors per day. The graceful, arched structure will feature an ethereal undulating roofline evocative of clouds.
The winning design is titled Käännös—a Finnish word meaning “to turn” or “to translate.” It was developed by ALA Architects, in Helsinki. The firm teamed with the Ireland offices of Arup for engineering support on the design, which features sweeping arches to create a welcoming ground floor level and extensive cantilevers to maximize floor space above.
The city had requested a design that is energy efficient, dynamic, and creates a public space for the center of Helsinki, according to Paul Dunne, a director in Arup’s Dublin office. The structure will serve as a signature location for the city’s impressive public library network.
“The winning entry is based on the idea of dividing the functions of the library into three distinctive levels: an active ground floor, a calm upper floor, and an enclosed in-between volume containing the more specific functions,” said Dunne, in written comments to Civil Engineering online.
To create the dramatic opening on the ground floor, engineers
employed a series of arches to transfer the loads while minimizing
the need for interior columns. © ALA Architects
“This concept has been developed into an arching form that invites people to utilize the spaces and services underneath, inside, and on top of it,” he added. “The building will be an inspiring and highly functional addition to the urban life of Helsinki.”
The design features a large plaza in front of the building, linking indoor and outdoor spaces. The ground floor is envisioned as a busy, public space, while the top floor will be the calm, quiet space typical of libraries. The building will occupy a high-profile site in the city, with the upper floor providing views of the Finnish Parliament House.
The design specifies a large inviting space on the building’s ground level—free of columns—and an impressive 27 m deep cantilevered space above with a back span of 18 m. That combination drove the design decision to employ a series of large arches for structural support.
“Traditional arches support gravity loads by acting in as much in complete compression as possible and typically support a flat deck directly above,” Dunne said. “However, three arches are used to share the gravity loads so that the load can be distributed more effectively. To add strength and stiffness under gravity loads, each arch has been trussed above its height.”
Lateral stability is provided through the floor plates, which act as horizontal girders that carry lateral loads via trusses back to vertical braced bays, Dunne said. The vertical braced bays will be situated around the perimeter of the building, hidden within the facade.
The serene upper floor features a large, cantilevered deck and an
ethereal “floating cloud” roof. © ALA Architects
The floating cloud concept for the building’s roof presented another challenge for the structural engineers, who devised a system to directly support the irregular shape and variations in depth. Increased depths are employed in areas of the largest deflection and stress, Dunne explained. The use of a space truss for the roof allows the structure to span large distances back to the column supports while maintaining a light, economical, and rigid structure.
“In addition to the gravity loads, this building experiences a torsional effect due to the cantilever trying to twist about its point of support,” Dunne said. “To counteract the twisting effect of the cantilever, the arches are braced together on plan and elevation to create a stiff torsional box. The arches are also braced at their ends to carry the torsional forces down to foundation level.”
The 16,000 sq m structure will feature a sculptural façade of wood and high-performance glass, the extensive cantilever functioning as an entrance canopy facing a large plaza in front of the building.
The city set ambitious energy targets for the new library. The design team employed thermal modeling to inform both the facade’s geometry and also material selections for enhanced performance.
“Focusing on achieving the best passive energy contribution has allowed the team to minimize the environmental systems within the building,” Dunne said. “The high-performance facade, combined with the low energy mixed mode systems, allows natural ventilation of the building while providing comfortable conditions year round.”
The library is scheduled for completion in 2017.