The transformation of a colossus
The Ambassador House is an impressive building: 150 metres in length, it is something of a landmark along the rail line between northern Zurich and the airport. It was opened as a hotel at the end of the 1980s, and is now being completely renovated. It is being transformed into a modern office complex with a striking overhang that will extend imposingly over the entrance area. This structure will be supported by a unique steel construction. We now take a visit to the construction site in Opfikon.
Until 2014, this was the entrance for hotel guests – now the Ambassador House welcomes visitors into an empty shell that is surrounded by scaffolding. Everywhere you look, there are workers scurrying around; the building is abuzz with drilling and the sounds of construction equipment. In the staircase, you can just about make out the stone foundation from the former hotel under a great deal of dust and debris. On the third floor, the building opens up to a platform made of yellow form boards. This will soon be the lowest ceiling of a new structure that will extend by 8 metres. Caution is required when walking through the formwork: reinforcements are sticking out of the concrete ceiling of the existing structure; the snow from the previous day is slowly turning to slush. Seven massive steel beams are resting on the formwork. A steel cable as thick as an arm travels diagonally from each beam over four stories into the air. The cables are currently hanging slack. Two workers are kneeling next to the head-high steel joints. Today the cables are being tensioned.
A “vendor’s tray” weighing several tonnes
Construction engineer Jörg Habenberger, who is responsible for the supporting structure, is also on site. “The structure’s load will be transferred via steel cables to the supporting framework of the existing building. This framework, of course, needed to be reinforced accordingly,” he explains. You can imagine it as a vendor’s tray – with the weight of a four-story office building. In order to reduce the load, lightweight concrete will be used for the ceilings. Hollow components embedded in the structure will help to further reduce the weight. Now we are in the calibration phase of the pre-tensioning process: the beams will rest directly on the formwork; the cables will be tensioned. How many tonnes of tension should be applied? At 5 tonnes, the cables are still sagging somewhat. The workers tighten the screws. Anchors in the concrete ceiling two floors down act as counterweights. The workers now switch to a hydraulic press, which they use to apply tension to the cables. The motor hums. At 20 tonnes, the steel cables become taut and the steel beams hover just above the framework. After around three hours of work in freezing conditions, all of the cables are stretched until they are parallel and run completely straight from the third floor to the upper edge of the sixth floor. The team packs up, satisfied with their work. In the next step, they will set the steel beams in concrete in the base plate.
An overhang as the defining structure
The new office complex, which has 57,000 square metres of floor space, is scheduled to open in spring 2017. The design by Stücheli Architekten consists of clearly structured components that appear to be stacked on top of one another. This effect is in large part thanks to the sophisticated overhang, which, together with the terrace below, make up the defining features of the building’s entrance.
Services provided by Basler & Hofmann:
structural engineering, earthquake retrofitting, service pipes and traffic planning
Steel construction: Baltensperger AG
Building contractor: Feldmann AG
Architecture: Stücheli Architekten
Total contracting: Halter AG Gesamtleistungen