The elliptical plaza of the Sony Center 'forum' is the centrepiece of the Potsdamer Platz redevelopment project. It is crowned by a spectacular and innovative roof.
The roof of the forum is designed to extend the use of the plaza during all weather. The space created under the roof is not air-conditioned, and the city’s building department demanded special requirements for ventilation and daylighting.
It was important to the architect Helmut Jahn that a roof should cover the plaza like an umbrella. The forum was not meant to be sealed, nor should it disconnect users from an outdoor experience.
The exceptional glass membrane roof rests on a ring beam weighing 500t. The alternating application of glass and fabric on a cable structure spans 102m. Structurally, the roof resembles a bicycle wheel in a horizontal position with a hanging central 42m-long king post, creating a column-free plaza.
Prior to the detailed design of the roof, Arup conducted an environmental study that used wind tunnel tests, dynamic thermal modelling and computational fluid dynamics analysis to assess comfort levels within the Forum space.
The forum roof is unique in its design, urban context, scale and structural use of materials.
Arup performed a daylight study using computer simulations. The Berlin building department needed rigorous technical proof that the plaza roof would meet Germany’s strict daylight requirements for the many offices and apartments that faced the plaza under the roof.
The challenge was to precisely model the complex passage of natural light through the roof and by inter-reflection into the offices and apartments. Daylight levels in individual rooms had to be predicted and compared with the code requirements.
Arup met the challenge by applying innovative ray-tracing computer techniques to support established daylighting design experience. Our engineers developed a methodology that determined light levels in a few key rooms, from which conclusions could be drawn on the conditions in all other rooms in each building.
A highly detailed 3D-computer simulation was built, including the complex geometry of the roof and physically-accurate modelling of the transmission and reflection of light from the roof and building facades.
The structural system
A continuous ring-beam was placed along the edge of a tilted cut through the hyberbolic cone, resembling the rim in a bicycle wheel.
The two top chords of the triangulated ring beam follow the surface of the hyberbolic cone. The ring beam is elliptical in plan with spans of 102m in the main axis and 78m in the minor axis.
A 42.5m-long kingpost was arranged in the tilted axle in the bicycle wheel. Two radial layers of cables connect the top and the bottom of the kingpost with the ringbeam. The top layer ridge and the valley cables create the folded surface of the roof. The bottom layer of the cables, the kingpost cables, suspends the kingpost over the forum.
The whole system is pre-stressed to stabilise the surface created out of cables, fabric and glass. The ring beam is supported vertically at seven points on the top of the roofs of the buildings surrounding the forum. Horizontally, the support configuration is structurally determined to avoid locking any forces from the roof into the buildings or vice versa.