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Sky Gardens
As the Commerzbank nears completion, its transparency and low-energy approach based on natural ventialtion, are emerging.
Frankfurt is a town committed to building towers that symbolise its hoped for future as a key European financial centre. Sir Norman Foster and Partners' Commerzbank is part of this agenda, in what is a relatively small and low-rise city. The Architects responds to the bank's four-to-seven-storey context by building to the site street line at that height before narrowing the tower.
This lower structure includes a banking hall, auditorium, shops, apartments and a covered plaza, an all around 35,000m2 of gross floor area with another 85,000m2 in the tower. The offices rise to 190m in 52 storeys, the whole building to 259m plus a 40m mast. The fitted-out building is due for handover in May this year.
The tower is triangular in plan, with a triangular atrium giving three arms of 16.5m deep office space linked by service spaces at the corners. This plan depth is partly a product of German regulations requiring occupants to be less than 7m from a window with normal ceiling heights. As is usual in Germany, much of the space will be cellular offices, although the combi office approach, with some shared spaces, is new for Germany.
Gardens in the sky are the other main ordering device for the tower. A garden takes up one arm of the plan for four storeys. People working in the inside offices of the other two arms can look across the atrium into the gardens, and beyond through the structural glazed screens to the city. And daylight flows in. The gardens will also be used as recreational and meeting spaces.
Every four storeys the gardens shift round to the next arm, spiraling up the tower in 12-storey loops. Their planting reflects the directions they face – Asian, Mediterranean or North American. For Commerzbank staff these spaces offer an extra dimension to the workplace.
However, to the untutored passer-by they may be near-invisible behind the glazed screens, except when lit. They have none of the on-street exuberance of Ken Yeang's aerial garden designs. Indeed, the whole building's client-approved colour palette is very sombre even by Foster's normally restrained standards.
Structurally the 12-storey repetition entails large concrete-cased steel corner columns and eight storey vierendeel steel framing bridging over the garden openings.
In fire terms the atrium is broken by a horizontal glass screen every 12 storeys. This works from an energy point of view too for a building using natural ventilation for some of the year. An unbroken atrium acting as a single stack could have pulled air too quickly through lower levels of the building and accumulated a lot of heat to be dispersed at its top.
The glazed garden screens have windows at the top for controlled natural ventilation. In summer, windows can also be opened at the bottom of the glazed screens. All these are under building management system control. Wind for cross-ventilation should be picked up from all directions. A minimum temperature of 5°C has been set for the gardens, which are warmed when necessary with exhaust air from the offices. There is also some underfloor heating locally to the serving bars. The maximum temperature is as outside air.
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