Kopter Group AG formerly known as Marenco Swisshelicopter AG based in Wetzikon, Switzerland required a production hall for its helicopters. Initially, the company considered a steel construction. However, the architect, together with a construction firm specialising in timber buildings, convinced the board to go for a wide-span timber construction with BauBuche lattice girders – with an excellent energy rating into the bargain.
The SH09, a lightweight single-engine multi-purpose aircraft and the first Swiss-made helicopter, was due to go into series production in 2019. The fact that this took place in a large wooden hall is thanks to the convincing arguments and proposals of architect Renato Leuzinger, engineer Hermann Blumer and Franz Frevel from timber construction specialist Casa-Vita. In the run-up to Kopter Group AG’s decision at the beginning of 2016, the creative minds worked closely together to design a building and a suitable supporting structure and came up with an economically viable proposal that is very elegant and eye-catching.
Future-proof timber construction with production facilities at ground floor level and offices upstairs
The new building had to provide sufficient space to build more than 50 helicopters per year. At the same time, Kopter Group AG needed rooms to meet existing and new customers. The company was thus looking for a multi-function premises, which is now in place in the form of a 41 m wide and 60 m long structure of staggered height. It incorporates a hall that measures 32 m in width and 9.30 m in height, extending along the entire length of the building. There is also a 12.3 m high section of 9 m in width and 24.5 m in height that houses various workshops, the technical design department and various offices. This block is located along one of the long sides of the factory hall, and extends into a transition area.
Three of the four storeys of the building are made in reinforced concrete. The fourth floor stretches above most of the main hall and consists of a 3 m high timber construction placed on the solid concrete structure. Given its large floor space, the main hall ceiling had to be able to carry the extra load. The fourth floor right under the roof houses the administrative and sales offices as well as a training room for helicopter pilots and accommodation for three people.
In order to span the hall width of 32 m, the designers had to find a suitable support construction and roof geometry. From the very start, they considered lattice girders, as they offer the necessary load capacity and are both elegant and cost-effective. Made in conventional spruce glulam, such as construction would however have required element cross-sections and girder heights that would have been far too large. Taking into account that the construction also had to carry crane loads of up to 3.2 tons, the only material that ticked all the boxes was BauBuche, a modern timber material made from beech veneer with strength properties comparable with those of steel. Thanks to its excellent tensile and compressive strengths, BauBuche can not only withstand high loads, but is also able to carry the fourth floor construction resting on the girders without deformation. Given the exceptional material properties of BauBuche, lattice elements with small cross-sections could be used, so that the 31.7 m long lattice girders are only 2.6 m high and 40 cm wide.
As the girders become narrow towards the bearing points, or end in lattice diagonals, there are unobstructed pendentives catering for crane rails along the lengths of the building. The rails are suspended from top chords and secured with lateral steel plates to the diagonals.
The lattice girders installed at distances of 5 metres form the main roof supporting structure. At the free hall side, these girders are attached with special steel connector plates to a huge portal frame of a prestressed timber-concrete composite construction, spanning gate openings of 12.20 and 37.20 m. At the other side of the hall, some of the girders rest on steel plates embedded in the reinforced concrete walls of the solid structure, while others are attached to a 30.40 m long, 3.16 m high support lattice beam on posts – all made from BauBuche. The support lattice beam is positioned between the solid structure and the second reinforced concrete staircase tower at the end of the hall.
This means that the timber construction is supported directly or indirectly by the reinforced concrete elements of the building, which act like a “spine” for vertical bracing. The horizontal bracing elements are box girders placed on the lattice girders and joint together to form a roof diagram.
As there are building height restrictions in place near the airfield, the use of BauBuche had the added advantage that the girder height could be kept small so that the overall building height met the planning requirements. As a consequence of the compact design, considerable savings could be made with regard to construction materials and façade cladding.
During the evaluation phase, the possibility of a steel construction that had to meet the same challenging requirements had been examined. Although the structural strength properties of steel are equivalent to those of BauBuche , there would have been additional costs to fulfil the fire safety regulations. To meet the requirements of fire safety class R30, a steel construction would need a fire coating, so that the final costs would have been at par with those of the BauBuche construction. Also, a steel construction would probably not be as attractive as the implemented timber structure.
The twelve lattice girders and two support girders, each weighing 12 and 16 tons respectively, were designed and built by the timber construction specialists Casa Vita and SJB.Kempter.Fitze AG. Given their lengths, bringing them from the hills of Appenzell to Glarus required an oversize transport vehicle, an escort car and a special permit by the road authorities. The journey from the factory to Mollis airfield was completed without incidents, and the good weather conditions enabled the contractors to start with the installation of the elements on the same day, so that the extremely demanding work on site was actually completed ahead of schedule.
When choosing the material and colour for the façade, the planners took into account the environment into which the new building was to be placed. The office block protruding from the actual hall is clad in Ethernit elements, which provide a pleasing contrast to the sheet metal façade of the factory hall. With this inspired timber construction and its imposing lattice girders, the architect was able to come up with a design that truly stands out as a fit home for the unique helicopters that will soon be produced there.
-Text by Susanne Jacob-Freitag-
Advice for architects, building engineers, clients and timber construction companies
