The facade facing the square is clad in spruce boards and twists along its length to create a curved entrance area, seemingly drawn down into the interior of the library. The timber surface, tilted almost to the horizontal, becomes the ceiling of the foyer behind the curving line of the glass facade. A restaurant, cinema and multipurpose hall are intended to entice more of the passing public to enter. Visitors ascend a double spiral staircase set into the glass facade or an escalator in the rear part of the building to reach the upper floors. In contrast to the column-free foyer, the whole of the storey above is defined by the building’s structural elements. The size of the rather small, enclosed rooms determines their type of use. A curving, stepped sitting area has the timber-clad profiles of the load-bearing structure informally superimposed upon it and forms the internal face of the curved facade.
The reading room, another space glazed on both sides under an organically curved, white, cloud-like ceiling is found on the more orthogonally laid out roof storey. Circular skylights transition gently into the ceiling surface and contribute to the calm, bright atmosphere of this space, a welcome place of withdrawal from the commercial bustle of the city. A balcony terrace, cantilevering far beyond the western side of the reading room, offers views onto the busy city and creates an inviting, weather-protected forecourt at the edge of the square.
Structure for a complex shape
The new building for the Oodi Library resulted from an international competition, which was won by ALA Architects in 2013. The main characteristics of the design are the open, column-free foyer with a curved ceiling and the balcony terrace with the large cantilever towards the west. The structure behaves as a bridge spanning over a future tunnel planned to run directly under the building.
The task for the engineers from Ramboll charged with the structural design in 2014 proved to be complex: the space available for the load-bearing structure was quite tight due to the architectural requirements placed on the room-defining surfaces. The structural depth available for the bridge construction at the vertex over the foyer was hardly more than two metres, while the end supports were restricted to two relatively small areas. Loads from the upper storeys had to be transferred into the main structural members from over more than two-thirds of the depth of the building. In addition, the balcony, which projects up to almost 14 m beyond the west facade, creates a highly asymmetrical loading pattern. In response to these requirements, the engineers devised a system of two outwardly tilted arches with a span of 109 m (Fig. A).
The 12.5° tilt of the front arch reduces the balcony cantilever, while that of the rear arch at 22.5° minimises the transverse deck span distances. There remained enough room in between them for the feature suspended spiral staircase that defines the foyer. The longitudinal profile of the arches deviates slightly from the ideal line; their cross sections result the axial forces and bending moments arising from the imposed loads. Thus the depth of the hollow box profiles varies over their length, with the more heavily loaded front arch also being wider (Fig. D). In order to resist the considerable horizontal forces from the arches, their bases are tied together by 17 tendons designed for a total tensile force of 115 MN and cast into the ground floor slab. The arches and tendons terminate in special box-slab structures made from welded steel plates, which transfer the vertical forces into the foundations (Fig. C).
Balanced system
The asymmetric system consists of two vertically tilted arches, which take all the vertical loads but have a natural tendency to overturn outwards. For this reason, the arches are connected by 4.50 m deep truss girders at 6 m centres and tied back to the cores in the back of the building, which in turn are tied down to rock. The truss chords support the floor slabs on both upper storeys. Tendons cast into these slabs connect the arches to three reinforced concrete cores in the part of the building to the rear and to two further cores at the building ends. On the outside of the front arch, the truss girders continue as welded I-beam cantilevers and support the balcony. The balcony projects up to 13.50 m in front of the west facade and therefore 17 m in front of the vertex of the arch. In this area specifically, the cantilevers are only 2 m deep and transfer considerable tension and compression forces into the truss girder chords.
The steel construction of the irregularly undulating roof surface forms an independent structure of welded I beams and steel hollow profile columns supported by the arches and truss girders. Bracing is provided at the ends and in the rear area of the building by diagonals. Because some columns are omitted in the area of the arch vertices, the span of the roof girders varies between 12 m and 24 m, and their height also varies accordingly. Pin-jointed, single-span beams mean the roof structure is relatively flexible in order to avoid imposed strains in the roof surface caused by vertical movements of the arch construction.
Construction process
The deformation of the arches and cantilever in relation to the building size is relatively high from a structural design point of view. The vertical deflection of the arches under self-weight and live load is up to 80 mm, while that of the balcony front edge can be as much as 200 mm. So that these components conform with the design geometry when the building is completed, they were designed and manufactured to have an appropriate precamber.
The arches each comprise six 18 m long segments. Each segment was lifted into position by crane and welded on site. In order to be able to continue steelwork erection while the arches were being constructed, temporary steel truss towers with reinforced foundations were erected under the five butt joints of each arch.
Minimising risk through simplification
The whole structural design process was defined by tight cost and time budgets and a mandatory architectural form. The complex arch construction, which was the first of its type, equated to over 40 % of the 2,400 t erected steelwork and represented not only a considerable part of the material costs but also a challenge for the local construction contractors – and therefore a risk factor that could attract a corresponding contract price surcharge. As much simplification as possible became the strategy for the design and fabrication of the arch structures and for the selection of all other components of the whole structure. Therefore, proven components and methods were used, such as orthogonal reinforced concrete cores in the rear part of the building, deck support systems out of ordinary commercially available WQ system steel beams and prestressed concrete hollow-core deck slabs.
The trusses connecting the arches run in a flat plane. The hollow box sections of the arches consist of 60 mm thick, flat S355 steel plates, while the flange plates of the front arch are 100 mm thick. The load capacity of the arch is defined exclusively by the size and thickness of the steel plates: plate stiffeners were largely omitted on the grounds of cost. The simplicity of the arch profile in general is in contrast to the complicated individual connections of the truss girders in detail. A digital BIM planning model simplified the design of these connection points, no two of which are the same.
