Archive for ‘cepezed architects’

July 31, 2011

Westraven, Utrecht | cepezed architects

In a world fixated on the sustainable possibilities of new construction, the far greater environmental benefit of upgrading our existing building stock is often ignored. Westraven indicates what is possible if we address this issue.
– Antony Wood, CTBUH 2009 Awards Juror, CTBUH

Figure 1. Building exterior at night.

85.1 m (279ft)
Primary Use

Government Buildings Agency
Design Architect
cepezed architects
Structural Engineer
Grontmij Technical Management
Construction Combination Westraven
Other Consultants

The Westraven Office Complex in Utrecht, the Netherlands, is a combination of a renovated existing building and a new extension for the use of several divisions of the Dutch Department of Public Works. The program includes office space, conference facilities, a national meeting center, a communications center, and a “future center” named LEF for the Department of Public Works. This existing 85-meter (279-foot) tall construction has been radically renovated and reorganized, and a stretched out four-story podium has been designed around the base of the building. Various functions and facilities are based in large, open spaces in the podium, which are inviting for both meeting places and casual encounters. Much attention has been devoted to obtaining perfect equilibrium between low energy consumption and an optimum working climate. The architecture, technical installations and construction physics are fully integrated, which significantly contributes to the overall sustainability of the complex.

The Original Westraven

Westraven is an area to the South of the centrally situated Dutch city of Utrecht, located between the Amsterdam-Rhine Canal and the intersection of two major motorways, the A2 and A12. It was built in the early 1970s, utilizing a so-called jack-block system, which at the time was a cutting edge construction technique of British invention. This method required that the roof had to be constructed immediately after the foundations and basement were completed. Once the roof was completed, the core, which was made out of prefabricated concrete blocks, was constructed. This means that the entire unit was jacked up to create the space needed for a new layer of blocks and the top floor.

Figure 2. Existing high rise tower

As a result, the construction of the building was gradually elevated at a rate of two floors every three weeks.

Regardless of its revolutionary construction, Westraven received a generally negative critical response, which over the years only became stronger. One reason was the strong winds at the main entry which frequently hindered people and an alternative entry had to be used frequently. Also, the monolithic office block appeared distant and the working conditions inside were below par. The substandard climate control and a dark, stuffy atmosphere also played an important role in this negative criticism.

Because of the negative feedback, but also its central location, it was decided that the building was to be renovated and that it would be expanded to accommodate additional divisions of the Departments of Public Works. In addition to improving climate and the working environment, the new complex had to be open and transparent while also reflecting a certain prominence in the city. Moreover, Westraven had to set an example as a landmark of sustainability.

Opening up

The tower has been stripped to its concrete skeleton, which has been completely reused. Five large voids, of 6 meters (19.7 feet) wide and 5 meters (16.4 feet) deep, were created by removing floor segments of three successive floors, alternatively along the western and eastern façades. Now users no longer enter a dark and nondescript space when they exit the lifts at the core of the tower. Instead, they are met with an abundance of light and splendid panoramas over the Utrecht area. This not only improves spaciousness and scale, it also helps users to orientate themselves. Moreover, because of the newly created voids, the different office floors are interconnected, which contributes to both internal communication and spatial diversity.

Figure 3. Transparency and daylight in the lobby

From the outside, the voids are large rectangles of solar-control glazing, which immediately grabs  attention because it visually stands out amidst the horizontal rhythm of the rest of the façade. As such, they serve as an important identifying element of the building.

The new thermal façades are made of floor-to-ceiling glass. This not only improves the views and the amount of daylight into the offices, but it also ensures that the occupants will always move towards light when they pass through the hallways. The interior arrangements are fully flexible, comprised of different types of rooms using a partitioning system that is largely transparent as well.

Textile Skin

In order to facilitate natural ventilation, façade panels are operable. This has been made possible because the tower has been given a second skin, which eliminates possible inconvenience caused by the wind.

At the time of the preliminary design, the idea was that this outer facade would be made out of silk-screened glass, but during development the architect came up with the idea of a light-weight and consequently more cost-effective skin of Teflon-coated and open-weave fiber-glass textile.The soft turbulence behind the textile would neutralize the forces of the wind, so that the cavity between the textile and the thermal façade would be relatively tranquil and would enjoy a constant supply of fresh air. As a result, it would become possible to ventilate the offices directly with fresh air from the outside without any wind pressure problems. But there would also be another important advantage as the textile skin would also function as a sun shade without impeding the views.

Figure 4. New textile skin

The façade screen was inspired by textile applications in horticulture and is extremely light-weight in comparison to other outer façade solutions. At Westraven, the textile is attached to steel balusters by means of aluminum tubes, sharing the existing construction with the window-cleaning equipment.
Because such a solution has never been attempted before, the façade screen has comprehensively been tested against a large number of technical and functional criteria. These showed that the screen reduces the forces of wind and the sun adequately, but it is also sufficiently transparent and translucent. The colorfast fabric is unaffected by water, grease and dirt, is resistant to the weather, fungus, rust and insects, and is easily cleaned if necessary. Moreover, the screen does not flutter or vibrate and has been extensively tested to address the potential icing issues of the system.

The translucence and transparency can be attributed to a combination of factors; the difference between the light intensities inside and outside, the color and degree of reflection of the textile, and the angle of the sun all play a role. In addition, the distance of the textile to the building, the mesh-size, and the thread thickness of the fabric in relation to the resolution of the human eye are also relevant factors. Because the screen is so thin, good views remain possible across a very wide angle.

Both the Government Buildings Agency and the Department of Public Works went along with the textile skin, but opted for alternation of the fabric with strips of glass at eye level. Because the textile façade has no sound-insulating properties, the building has an outer façade of glass on the north side, which is closest to the motorway.

Mechanical Installations

To further improve the interior climate, all stories have been equipped with climate ceilings. Since the construction was erected using the jack-block system, it was designed to be as light as possible at the time. Therefore, the floors were relatively thin and had been reinforced with concrete beams. The spaces in between these beams were perfect for accommodating climate-control units suitable for both heating and cooling the building by pumping hot or cold water, respectively, through the pipes. For Westraven, the supplier customized the units so that several other components, such as the sprinkler system, intercom speakers and light fittings could also be integrated; thus contributing to a tidy, clear and visually unobstructed atmosphere.

Figure 5. Section through offices showing junction of tower and low rise structure and double skin façade


New low-rise sections have been added on either side of the tower. This added 24,000 square meters (258,334 square feet) of space to the existing 27,000 square meters (290,626 square feet) in the tower. These extensions have a V-shaped floor plan with rounded edges and are enclosed by a conservatory made of transparent material.

The sections are linked by an intermediate zone, with a large entrance area and two indoor gardens, while their hallways form an extension of the entrance. As a result, the ensemble, as a whole, appears as an indivisible unit in which the transition from old to new is hardly perceptible.
The structure of the extension is nearly all steel and has been dimensioned as slender as possible for both aesthetic and sustainability reasons: a slender construction of steel provides both lucidity to the building and reduces the use of material. Moreover, because of its relatively low weight and its excellent thermal conductivity, it absorbs and emits warmth faster than the more traditional and heavier concrete structure. As a result, less energy is being used for heating and cooling.

Sustainable Climate Concept

As mentioned, much attention has been devoted to obtaining perfect equilibrium between low energy consumption and an optimum working climate. For this purpose, a sustainable climate concept has been developed. The high degree of transparency of the design, for example, is not only based on architectural and aesthetical principles, but also by maximizing the use of daylight so less artificial lighting is needed. Also, a dynamic lighting system has been installed, which automatically adapts to the color and intensity of the natural light available.

Figure 6. Diagram of the building’s climate zones

Four different climate zones have been developed in order to adjust the climate concept to the actual day-to-day use of the building. The offices and workspaces are provided with full service climate control facilities. The floors of the new office wings have been rendered thermally active by means of a water network system that has been cast in the concrete slab components of the floors. Through this network, the structure is kept at a constant temperature. In the long term, this system uses less energy than an ongoing alternation of heating up and cooling down.

Since each of the conservatories is used for completely different functions (such as the restaurant, gatherings or workshops), the standards for these particular areas are less demanding. The bulk of these enormous spaces are heated by return air passing through the edge of the office floors.

At the ground level, this is complemented by floor heating. Since the conservatories serve as a buffer between the new office wings and the direct influences of the weather outside, the demands upon the office façades are relatively low. Therefore, an indoor partitioning system could be used for these, which saves considerably on construction costs. Perforated panels in the façades also contain sound insulation and can be opened, which allows for extra ventilation through the atrium.

During the warmer periods, an opening is created in the conservatories through a sun screen system for which the same fabric has been used as applied in the greenhouse industry. When the temperature rises, the screens automatically come down. The warmth is retained in the cavity between the screen and the façade and is naturally ventilated. The same technique is used in order to obtain the warmth entering through the transparent roofs between the office wings. An additional cooling installation is integrated into the floor.

Figure 7. New extensions at podium level


The Energy Performance Coefficient (EPC), which is a figure used in the Netherlands to measure the energy efficiency of a building, was 33% better than required by the Dutch Buildings Decree for the tower and even 50% better for the podium. Westraven has won the Daylight Award for its exquisite balance between the use of daylight, artificial lighting and other architectural aspects and was granted the Dutch Construction Award with much praise for its sustainability on all levels. Recently, Westraven has been shortlisted for the Prime Property Award 2010, which is a European prize for sustainable real estate investment.

The Westraven was recognized as the Best Tall Building Europe Finalist in the 2009 CTBUH Awards Program.