Pilatus is the leading aircraft manufacturer in Switzerland and has gained a solid international reputation as an innovative aerospace engineering company. Founded in 1939 in Stans, Central Switzerland, the company manufactures training and utility aircraft. To keep pace with demand, Pilatus extended its production capacity in 2017 with a new 120 m long assembly hall known as hall 25. Shortly afterwards, a second, 200 m long hall was built. It houses the new competence centre for structure assembly operations, mainly for the PC-24 and PC-21 models. In the past, the production and assembly of fuselages, wings and tail units had been outsourced to companies in Switzerland and abroad. This is no longer the case, and they are now built in the new hall. Pilatus is thus bundling its core competences in airframe design and production, strengthening its market position. The new hall cost around 45 million Swiss Francs and went into operation in June 2019 and has helped eliminate bottlenecks at the main factory in Stans.

Timber everywhere – and BauBuche for high loads

The two assembly halls are of a similar design. From the outside, they resemble the wings of an aircraft. They also share many internal features, such as a supporting structure made in timber. As an innovative firm, Pilatus operates a ISO 14001-certified environmental management system. That is why Oscar J. Schwenk, chairman of the Board of Directors of Pilatus Flugzeugwerke AG, opted for a timber construction for both new halls. The planners chose locally sourced spruce and fir, as well as beech and BauBuche, a high-strength beech veneer laminated timber. The new hall with a production area of approximately 10,000 m² (52 m x 193 m) is 69 m wide and 17 m high. From the very start, it was clear that the hall had to be free of columns, as the crane system needed access to the entire assembly area to ensure efficient material and work flows. This required a roof construction with main and auxiliary girders able to carry very high loads, and BauBuche proved the best material for such a structure.

Six arched glulam girders with steel ties at distances of 34.50 m from each other form the main supporting structure. They weigh up to 90 tons and span the width of the assembly hall, resting on prestressed reinforced concrete posts that form part of the outer shell and on steel plates in the longitudinal reinforced concrete partition wall that separates the production area from the adjacent office space. At right angles to these main beams, we find 4.50 m high lattice girders made in BauBuche spanning the length of the building. Along the external wall, the lattice girders are divided into 34 m long individual elements, serving as single-span girders installed between the reinforced concrete posts. In the centre section of the hall, the lattice girders are however designed as continuous beams. Here, the prefabricated 34 m elements are installed between the glulam girders. Vertical loads are transferred through the steel connections to the arched girders as well as to the posts at the gable ends of the hall, while the tensile and compression forces acting on the chords are transferred through the arched girders. At the gable ends of the hall, the lattice girders are borne on concrete posts and form a frame that transfers wind and earthquake loads directly to the foundation.

Thanks to the high load capacity of BauBuche, the designers came up with rather elegant, filigree lattice structures: The upper and lower chords of the girders in the centre of the hall measure 42 cm x 40 cm, while those of the lattice girders along the outer wall have cross-sections of only 32 cm x 36 cm. The cross-sections of the braces vary depending on the load requirements and measure between 24 cm x 24 cm and 36 cm x 36 cm. The lattice girders carry tertiary arched binders with roof ribs placed at distances of 4.30 m from each other. OSB panels act as stiffening elements for the construction.

Snow and variable loads pose greatest challenge to designers

The suspended crane in the structure assembly hall, known as hall S, has an approved load capacity of five tons and runs along four crane tracks. The I-beams of the two inner crane tracks are partly attached to the steel elements of the primary girder underspan construction and partly to the continuous lattice girders. Where the steel elements and the secondary girders cross, they form fixed points braced in four directions. Along the lattice girders, the crane tracks are vertically suspended without additional bracing.

Catering for the crane load was however a minor issue for the designers, as the real challenge was posed by the snow load. The engineers needed to make sure that the large roof area would be able to withstand changing loads, allowing for minor deformation. That is why the crane tracks were attached to the arched girders in a manner that allows for some movement and play. In addition, the crane features a joint at the centre, compensating for minor deviations in the crane track geometry.

Assembly of prefabricated elements

Strüby Konzept AG based in Seewen, Switzerland, acted as the main contractor for the construction of hall S. The 34 m long elements of the BauBuche lattice girders weighing up to 16 tons each were produced by Holzbau AG n’H Lungern and installed on site by the Strüby timber construction team.

The fact that there were no columns inside the hall posed a huge challenge to the construction team, as it was not possible to pre-assemble the lattice girders at the factory. The individual arched girders were transported to the site in two parts, assembled by the team on site and eventually installed with the help of a mobile crane.

For the construction of the impressive assembly hall, the contractor used 2,920 m³ spruce/fir timber, 270 m³ BauBuche, 27,000 m² timber panels and 135 tons of steel cables.

Green façade for perfect integration into landscape

The hall roof and parts of its façade are clad in aluminium and painted in a green that matches the colours of the countryside around the new Pilatus competence centre. The white epoxy resin floor finish gives the interior an atmosphere of lightness.

Since the middle of 2019, Pilatus Flugzeugwerke AG has been using hall S for the assembly of wings, tail units and fuselages. With a length of 200 m, the hall had to be constructed as a column-free space to facilitate the operation of the crane system. This was achieved by a supporting roof structure of steel-tied arched girders combined with suspended lattice girders made from BauBuche.

_{-Text by Susanne Jacob-Freitag-}