Double-curved, Mirrored Façade of MVRDV’s Depot Boijmans Van Beuningen

As one of the most important cultural buildings built in Rotterdam this century, the new art storage facility for the Museum Boijmans Van Beuningen – known as the Depot Boijmans Van Beuningen – demands world-class architecture. For MVRDV it is a privilege and an honour to shape such a crucial building in our hometown. Due to complete construction next year with the building opening scheduled for 2021, our design, one that is simultaneously bold and unusual, but also respects the surroundings with its bowl shape that reduces the building’s footprint to the minimum possible size, and its mirrored façade that reflects the life of the park and museums around it.

© Ossip van Duivenbode

The combination of its shape and exterior finish, however, required an innovative solution: double-curved, mirrored glass façade panels, which meet all local regulations for glass safety, and which in places merge seamlessly with transparent glass sections to make the building’s windows blend into the rest of the façade. Achieving this effect required significant research and an innovative approach, not only from MVRDV as architects, but also from the project engineers, manufacturers, contractors, and even from the municipality of Rotterdam.

© Arjen Ketting

Origins of the Depot

The Depot Boijmans Van Beuningen will be the primary art storage facility for the Museum Boijmans Van Beuningen, Rotterdam’s premier art museum with a collection of 151,000 artworks, spanning all regions of the world and all artistic periods from medieval to contemporary art. Until now, this collection was housed in multiple locations around the city, including a large number of works that were kept in the basement level of the museum itself. Storing priceless art underground in a city that is, itself, below sea level, is inadvisable. The need for a new, more secure storage facility was first discussed in 2005, and discussions about the project were accelerated with every flood that threatened the artworks in the leaky storage spaces below.

MVRDV won the design competition for the new facility in 2013 with a proposal for a fully accessible art storage facility. The proposal not only provides safe, modern storage facilities, but implements open storage principles to allow visitors to browse among 70,000 artworks that are not included in the formal exhibitions at the museum. The building also offers the chance to see the research, conservation, and restoration work that takes place behind the scenes of the museum.

Architectural Design

Architecturally, the Depot comprises a 36.5-metre-tall, round, bowl-shaped volume with a public lobby space on the ground level and a dramatic central atrium. The public route zig-zags through the atrium as it rises, providing glimpses into the various storage spaces and work areas, and terminates in a steel-and-glass pavilion structure on the roof. This pavilion houses a restaurant and is surrounded by a rooftop sculpture garden complete with 75 birch trees planted on the building’s roof.

From street level however, the most striking aspect of the building’s appearance is its façade of 1,664 glass panels that are double-curved to give the building its smooth bowl shape, and given a mirror finish. Though considered ‘flashy’ by some, the mirrored façade and the shape of the Depot were primarily informed by a commitment to the social impact of the building. The building is located next to the Boijmans Museum itself, at the northern end of Rotterdam’s Museumpark – a park designed in 1994 by architects OMA and landscape architect Yves Brunier, and which is surrounded not only by the Boijmans Museum but also the Kunsthal, Het Nieuw Instituut, the Natural History Museum Rotterdam, The Chabot Museum, and Villa Sonneveld. Taking space away from the park to be replaced by a building the size of the Depot was, from the beginning, a contentious proposal.

The bowl shape of the Depot was therefore proposed to reduce the footprint of the building, preserving key routes through the park, and to ensure that the building has no ‘backside’, while the rooftop garden was proposed as a way to replace the park space appropriated by the building with an even larger, publicly accessible green space above with the added value of a panoramic view over the city. The mirror effect, meanwhile, ensures that the Depot emphasises the importance of its setting over its own presence in the Museumpark. It allows people to see into the park from further along the street, reflects the life and activity of the park back on itself, and celebrates the city by offering those on the ground a view of the skyline of Rotterdam.

© Arjen Ketting

While the Depot, as an art storage facility, does not require as many windows as most building types, it does still need openings in some key locations, and in this sense the mirror finish brought its own set of challenges. Windows are integrated into the design using a ‘dissolve’ effect, where a gradient is created between the mirrored glass and the transparent panels. This masks the edge of the windows; during the day, the natural reflectivity of glass windows means that these openings blend into the rest of the façade.

Glass Manufacture

The glass plates for the Depot are manufactured in China, by the only manufacturer we could find with the capability to carry out all three processes for mirroring, bending, and adding a gradient to the windows in the same factory. The panels are made from laminated, un-tempered glass, since the tempering process is not compatible with a mirror coating.

The panels are produced by first cutting flat glass panes to the correct size and shape. Because the curvature of the Depot is not continuous, as you might see in a shape based on a sphere, but instead curves more dramatically nearer the bottom, the panels have different curvature depending upon their eventual position on the building. Because of this, the building’s 3D model was used to calculate the correct 2D shape for each panel. The mirror coating is then applied to what will eventually become the inside of the outer laminate layer and the glass panes are formed in the oven, where gravity is used to form the heated glass into an appropriate mould. To minimise inconsistencies, both layers from each laminated panel are formed simultaneously, with a film between the two that allows the glass panes to slide against each other as they bend. The heat in the oven is slowly reduced so that the panes are cooled as slowly as possible, in an attempt to reduce any deformations during the cooling process. Once the panes have cooled and been removed from the moulds, they are laminated in the autoclave with a central PVB layer, before being shipped to the Netherlands.

Image courtesy of MVRDV

The double-glazed windows are made using a similar process, with the inner and outer panes laminated separately before being assembled into the window frame. The dissolve effect around the edge of the windows is achieved using a spotted ‘halftone’ gradient, which fades from a fully opaque mirror at the very edges of the windows to complete transparency over a distance of one metre.

No standards have been created that provide tolerance specifications for double-curved glass. For this reason, we imposed the strictest standard available for single curved glass (the German ‘F-merkblatt 009/2011 – Leitfaden für thermisch gebogenes Glas im Bauwesen’) which provides for size tolerances of +/- 4mm and a deviation in curvature of +/- 2mm. To ensure compliance with this, the panel tolerances are checked at three points: both before and after shipping, and before installation.

Panel Construction

To create a panel that can be installed on site, each glass pane is mounted to a steel frame that matches its curvature. Because the glass is not tempered, it cannot be drilled or otherwise mechanically connected, so this process is achieved using glue. A layer of glue 15mm thick is applied to the circumference of the steel frame, with small gaps at points to allow for water drainage from the panel. The glass is placed in a concave mould to ensure that it holds its shape as pressure is applied to glue the two parts together. The glue layer also helps to accommodate any minute differences between the curvature of the glass and its steel frame. For safety reasons, the Municipality of Rotterdam required a mechanical failsafe in the event that the glue attaching the glass fails. This was achieved with small steel ‘hands’ that are mounted to the steel frame after gluing and wrap around the edge of the glass, although they do not touch the glass, which would risk distortions in the mirrors. There are 12 of these per panel – four each on the top and bottom and two on each side – ready to catch the glass if the glue fails and hold it in position until the panel can be removed safely.

© Ossip van Duivenbode

Panel Installation

Anchor points for the façade panels were installed in the concrete structure of the Depot by being embedded in the concrete of the lowest two floors, which are formed of in-situ concrete, and drilled into place on the upper floors, which were made with concrete panels. Connection consoles are then attached to these anchor points, while the insulation panels for the façade are laser-cut to ensure that they fit over these consoles without any spaces for airflow.

The connection consoles are located at the corners of each panel, and therefore each supports a different corner of four different panels with four hooks in total. The panels are hung onto these hooks using steel rods embedded in the panel frame. Once mounted, the position of the panel can be adjusted both vertically and horizontally to provide optimal alignment. The vertical height of the lower hooks on the connection console (which support the top corners of the panels) are adjusted by simply turning a bolt, while the upper hooks (supporting the bottom corners of the panels) are adjusted using a specially designed key. Meanwhile, the horizontal position of the panel is adjusted by turning a bolt that moves a collar on the steel rod, moving the panel relative to the hook.

© Ossip van Duivenbode

The Limits of Tolerances

Despite the stringent tolerances imposed on the glass manufacture, absolute perfection is an unattainable goal and as such, we anticipated some imperfections. As expected, the panels did not give a perfectly smooth reflection, and the reflection did not always line up between two panels.

However, as more of the structure receives its mirrored finish, the spectacular effect we envisioned is gradually emerging. One could even say that the Depot is an art work in its own right, with the imperfections serving as a metaphor for the fragmented world we live in.

We are confident, therefore, that the finished building will be a sympathetic neighbour, adding value to the Museumpark, and a building that not only celebrates the existing skyline of Rotterdam, but contributes to it as well. We hope it will be worthy of the art it will keep safe, worthy of its location in Museumpark, and worthy to be mentioned alongside the many architectural icons of Rotterdam.