Council on Tall Buildings and Urban Habitat
 

KfW Westarkade, Frankfurt
Posted December 2011
Other Featured Tall Buildings


The KfW Westarkade
was recognized as “Best Tall Building Overall” in the 2011 CTBUH Awards Program.

“The streamlined form integrates itself into its surrounding context, while simultaneously standing out through the playful use of color. Whereas many buildings use color as a way to mask an otherwise unremarkable building, here it contributes an additional rich layer to what is already a remarkable building.”
-
Peter Murray, CTBUH 2011 Awards Juror, New London Architecture Center

Location
Frankfurt
Completion
May 2010
Height
56 m (184 ft)
Stories
14
Primary Use
Office

Owner/Developer
KfW Bankengruppe
Design Architect
Sauerbruch Hutton
Architectural Coordination
Architekten Theiss Planungsgesellschaft mbH
Structural Engineer
Werner Sobek Engineering & Design
MEP
Reuter Rührgartner GmbH; Zibell, Willner & Partner
Energy Concept:
Transsolar
Contractor
Zublin

As we design for a sustainable future, we desperately need a new definition of beauty that goes beyond skin deep. Already being touted as one of the most energy-efficient office buildings in the world, KfW Westarkade stands out as a shining example of a truly environmentally-responsible project.

The building has been carefully integrated into its context, forming relationships with its neighboring buildings, streets and parkland, while simultaneously standing out through the playful use of color. Whereas many buildings use color as a way to mask an otherwise unremarkable building, here it contributes an additional rich layer to what is already a remarkable building. Germany already has a strong reputation for achieving natural ventilation in tall office buildings, and Westarkade can now be added most positively to that list.


One of the first office towers in the world predicted to run on less than 90 kWh/m² of primary energy per year, KfW Westarkade uses approximately half the average energy of European office buildings, and one-third of American. The building forms a 14-story extension to the KfW’s headquarters in Frankfurt, completing an existing ensemble of buildings from the last four decades by adding 700 new workplaces. Situated in the city’s West End, it lies adjacent to the Palmengarten park.
Figure 1. Building Envelope
The streamlined shape of the tower integrates itself into the cluster of existing buildings. It acts as a colorful interface between two distinct urban spaces: while it appears as a slim slab towards the city, it presents a discreet backdrop to the park and provides open sightlines for the existing ensemble. Furthermore it exploits the prevailing wind direction for controlled natural ventilation of the offices by means of its unique double layered wind-pressurized façade.

The new building extends the premises of the KfW Banking Group to the west with a four-story podium that clearly delineates the edge of the Zeppelinallee road. The tower above it is formed in such a way that it does not obstruct the view from the existing office floors of the main building. Together with the main buildings and the adjacent existing structure, the building’s south side creates a communal courtyard. The landscaped areas of the southern end of the Palmengarten are drawn through the site leading to this courtyard, resulting in a coherent open space.
Figure 2. Facade Detail
The various spatial effects of the building are enhanced by the polychromy of the façade’s narrow ventilation flaps, whose various colors address the different city spaces surrounding them: a family of green tones front the Palmengarten, while the hues of the red Main sandstone so prevalent in urban Frankfurt are interpreted along Zeppelinallee Road, and a group of blues complements the color scheme and materiality of KfW’s recently renovated main building.

The construction and use of the Westarkade is governed by numerous built and behavioral features to maximize sustainability, led by three factors: natural ventilation, activated slabs, and geothermal energy.

Figure 3.  Façade showing transition from park-facing to street-facing elevations

The building had as its primary aim to make a significant advance in the field of natural ventilation of tall buildings, as a significant component of their overall sustainability. The dynamically-controlled pressure-ring façade serves to neutralize external wind conditions which are otherwise too turbulent for operable windows, especially on higher floors. The façade’s outer layer contains sensor-controlled flap openings that maintain a constant and even air pressure within the ring. The inner layer has operable windows that allow the offices within to be ventilated. The air flow within the pressure ring is regulated to never exceed 6 m/s. The flaps are designed to adjust to five wind directions as well as outside temperature, solar radiation and pressure differences on the windward and leeward sides of the building.

Exhaust air flows through noise-attenuating overflow elements in the office partitions and along corridors until it reaches the cores, where the air, through stratification, is naturally driven upward to the roof through shafts.

As a result, the offices can be ventilated naturally for eight months of the year without creating drafts or undesired heat loss. Mechanical ventilation is required for less than 50% of all working hours. The double façade also functions as a passive thermal solar collector, as the flow of fresh air is pre-tempered by solar radiation within the double façade. In this way heat loss is minimized and heat energy is conserved. The outer skin of the double façade can be opened completely in order to avoid overheating of the building in summer.

The building employs thermally activated slabs, whereby a system of pipes built into the solid floors conveys water that serves as both a heating and cooling medium. This creates exceedingly energy-efficient, comfortable and constant room temperatures. Due to the high thermal storage capacity of the solid concrete floors, the rooms can be sustainably heated and cooled without the extremes in water temperature that are required with traditional radiators.

Figure 4. Street Perspective

For this sort of tempering, energy can be used that already exists in the building: the waste heat from the data processing center can cover half of this heating demand.

The construction of Westarkade lent itself to the installation of an underground duct that allows air that is largely contaminant-free to be drawn from beside the Palmengarten and fed into the office areas. From the shaft where the air is drawn, fresh air travels though ventilation tubes in the foundation slab and into the building’s ventilation system. The dimensions of this geothermal duct allow for the cooling of warm outside air in summer, while in winter the situation is reversed with the temperature differential becoming positive.

Figure 5. Facade
The architecture and technical facilities are configured to exploit natural daylight to the highest possible degree. Horizontal louvers in the space between the façades protect the building from high solar radiation and glare. An integrated sunlight redirection system in the upper third of the sun protection enables spaces to be comfortably shaded without becoming too dark.

Related Links
CTBUH Skyscraper Center Profile:
KfW Westarkade

KfW Westarkade was recognized as the Best Tall Building Worldwide Award Winner in the 2011 CTBUH Awards Program.
Download the KfW Westarkade 2011 CTBUH Awards Book section

2011 CTBUH Awards Book


Videos:
Best Tall Building Europe Conference Video: KfW Westarkade

The CTBUH would like to thank Sauerbruch Hutton for their assistance with this article. Photos © Jan Bitter.