Vierendeel girders as the key to success for the new Wädenswil-Au school

Image: Beat Bühler Photography

The new school complex Ort in Au Wädenswil is an architecturally and structurally challenging project - and at the same time a prime example of modern timber construction. The innovative timber construction solution is particularly noteworthy: school rooms were built above a sports hall divided into three sections, whereby the entire construction was consistently built in timber - without steel girders. At the heart of this solution are specially developed Vierendeel girders made of wood, which play a central role in load transfer and space creation.

The main challenge was to bridge the approximately 28 meter span of the gymnasium without resorting to conventional steel girders. This was achieved with a multi-layered structural solution: the ceiling construction above the gymnasium was designed with a camber of up to 9 cm. This geometric pre-curvature was designed in such a way that the structure levels out under dead and live loads after the subfloor is installed, creating a flat surface as the basis for the classrooms.

Images: Beat Bühler Photography

Vierendeel girders were used on this basis to support the roof loads on the school floor - a truss construction without diagonal struts that allows large openings with a high degree of rigidity. The beams were arranged in a grid of 7 to 8 meters and allow for spacious, column-free rooms.

The beams are made entirely of wood: the upper and lower chords are made of spruce glulam, while the vertical filler elements are made of spruce cross laminated timber (CLT). The latter act as shear plates that transfer the horizontal force between the chords. In order for the system to function as a unit, a high-performance connection was required - and this is where the real engineering skill of this project lies.

Images: Beat Bühler Photography and Timbatec Holzbauingenieure Schweiz AG

The connection between the CLT walls and the timber chords is made via so-called shear cams, which are milled directly out of the cross laminated timber in the longitudinal direction. These cams transfer the shear forces positively into the load-bearing system. The performance of these cams has been impressively demonstrated by extensive tests at ETH Zurich: A single cam can absorb shear forces of up to 30 tons.

Precisely planned, this connection solution enables a very high load-bearing capacity. The glulam beams were manufactured in the factory at Hüsser Holzleimbau AG, Bremgarten, and then installed on site in combination with the CLT panels. The precisely prefabricated components can be quickly installed on site. This significantly reduces the construction time.

Findings of the shear tests

• Ductile behavior
• Compression of the CLT panels (plastic behavior)
• Very rigid connection without slippage
• No visible damage or cracks from the outside
• Uniform load application in CLT shear cam

Images: Timbatec Holzbauingenieure Schweiz AG