Curved glass boxes on high end shopping street

The façade of P.C. Hooftstraat 138 – named The Looking Glass – features three glass and stainless steel boxes with curved glass and multiple glass joints. They start flush with the adjacent buildings and cantilever outward while moving upward. The fact that this gradual movement takes place within one shape is remarkable. It emanates a fluidity you wouldn’t associate with glass. The curved glass boxes are bonded with structural silicone. Slender stainless steel profiles in between te panels define the glass box shape. Octatube was invited as design and build partner to do the pre-engineering, technical design, production and assembly.

The P.C. Hooftstraat is one of the most elegant shopping streets of Amsterdam, located in the heart of the Museum quarter. This 19th century street, filled with traditional Amsterdam townhouses, has grown into a place where heritage and design meet each other. 

The recent, most remarkable developments in the P.C. Hooftstraat have taken place at eyelevel; with contemporary, fresh take on the streets’ façades. The latest addition is the showstopping façade designed by UNStudio and aptly named “The Looking Glass”.

For this façade, client Warenar Real Estate brought a team together of UNStudio, ARUP, bouwbedrijf Wessels Zeist and Octatube. It came down to meticulous detailing and a constant pursuit of quality. We organized several design workshops to collectively clarify wishes and visions.

Glass boxes
The three glass boxes cover both ground floor as well as the first floor, with a total height of 8.2 meter and width of 1.8 meter. These boxes have a twisted geometry and make the design of the façade of number 138 special and distinctive.

The boxes have precisely detailed glass connections and two of them have a custom non-orthogonal steel and glass door. They start flush with the brick façade but rising up they lean and cantilever outwards (250mm) to become more visible from a distance. The largest glass elements are the front panels of 6 meter in height, starting narrow at the bottom and become wider as they go upwards and protrude. They contribute their weight to the narrow base beneath. 

The glass panels are laminated, annealed low-iron glass with a SGP interlayer, making the façade transparant, strong and rigid. The glass is produced by our partner Cricursa in Spain.

Stainless steel
Slender stainless steel elements wrap around the glass, defining the shape of the glass boxes and provide extra stifness around door openings. They create a modern version of stained glass windows.

It was a true technical challenge as the tolerances were extremely tight and the glass had to fit perfectly. Octatube partnered with Dutch specialist Veratio. They made a mould for every individual profile due to the complex geometry.

To be able to make the stainless steel edge, the steel was cut out in curved way and then rolled so that it could be made double-curved. The resulting profiles were placed next to the glass that is curved as well, within a millimeter of accuracy in the corner detail. The edge between the glass and the stainless steel was structurally cemented.

A very precise job proving Veratio to be the most knowledgeable partner and craftsman. They developed an extensive checklist to make sure every steel element had the same finish. Quite a challenge, as everything was done by hand. It was an advantage that they had recorded everything well so that consistency could be achieved.

“We enthusiastically dove into this technical challenge. The bar was set high and top quality was pursued in all areas. The interaction between Veratio and Octatube led to a wonderful end result. We didn’t experience it as a customer-supplier relationship but really as a collaboration, which came about in a natural way.” 
Martin van Dam, Veratio.

Structural silicone
The three glass  boxes are entirely held together by structural sealant. Connections between curved glass elements are all glued in combination with stainless steel elements for edge protection and emphasis on the curved geometry. No bolts were used.

As the glass cantilevers outward, the silicone bonding has to withstand higher levels of tensile stress. By doing extensive iterative calculations the structural silicone is able to accommodate this as well as the tolerances in temperature differences and tolerances between the curved glass and stainless steel (around 2 mm in all directions).  

Horizontal pre-assembly
Due to the minimalistic detailing, desired quality and geometrically complex glass shape, all three glass boxes are preassembled in the factory of Octatube in Delft (Netherlands). By doing this, all glued details were applied under well monitored conditions.

Since each glass box has a width of 1.8 m and height of 8.2 m, it was no option to pre-assemble the units vertically. The glass boxes were therefore pre-assembled horizontally and rotated 90 degrees vertically on site. This resulted in extra analyses of stiffness of dead load supports. The two doors were separately produced and brought to site in a later stadium.

The stainless steel and glass boxes are connected to a so called picture frame made of rectangular hollow steel sections. The glass is connected (by silicone) to this frame and the frame is connected to the brackets. The supports and brackets for the doors are also connected to this frame. After positioning the frame at the construction site and connecting the brackets, a few extra connections in between ‘picture frames’ were made as well, to specifically increase stiffness for the door connections with long brackets.

Octatube designed a custom made steel auxiliary frame functioning as a stiff mould around the glass boxes during assembly and making sure that the structural sealant and glass did not experience too heavy loads during transport and hoisting and during the rotation from horizontal to vertical. The structure surrounds the glass boxes on all sides and is mechanically linked to the picture frame. The deflections of the glass boxes and the steel auxiliary frame had to be kept to a minimum. Therefore we calculate all stages of assembly. 
 

Rotating
The most spectacular part of the execution of this project was the rotation of the glass boxes during the installation on site. Two cranes handled the rotation of all three boxes with an elaborate pulley mechanism at the top and bottom of each box. The rotating had to be done slowly to gradually control the self-weight of the glass box: going from a horizontal load to a vertical load before being mounted in the superstructure. 

The rotation of the boxes only took a few minutes. For every box the total installation time including rotation, lifting and connecting to the main structure was just 1 hour. Once the glass boxes were connected to these brackets, the auxiliary frame was removed.

The glass boxes are surrounded by special brickwork. Up to the first soldier course of bricks we incorporated a stainless steel strip in the joints, that pulls the traditional architecture together with the contemporary façade. 

Custom made doors
Two of the glass boxes contain oversized doors made of laminated heat-strengthened glass with structurally bonded stainless steel framing. They are integrated into the main frame of stainless steel elements that support the glass façade.

To meet the building regulation, the curves of the façade were optimized for the required minimum free width and height to enter the store and the apartment above. The shop door measures 2.9 m high and 1.5 m width in closed position.

As minimalistic detailing was pursued by the design team, the door frame and all its hardware was integrated into the load-bearing stainless steel frame. Also, the top pivot is hidden in the 25mm thick horizontal plate which is connected through a glass joint of 10 mm to the main steel.

An important part of the hardware design is the custom-made door handle on both door leaves. The door handle is curved and varies in section size from start to end in fluent lines. It is custom-designed and produced for this project. 

Showstopping
After months of preparation, the entire assembly was done in just 1.5 days. In many ways “The Looking Glass” was a job for a Swiss watchmaker. It all came down to the exact millimeters – from the silicone bonding to the glass and steel connections as well as the steel frame and the rotation on site. 

The end result is a true showstopping addition to the P.C. Hooftstraat that we’re extremely proud of. This remarkable façade, for client Warenar Real Estate, came together in collaboration with UNStudio, ARUP, Brouwer & Kok and Wessels Zeist.

“I love combining hardcore calculating with project management. Not only is it an advantage for the client to have a partner that has technical knowledge, it’s also pleasant for yourself: you are involved in the project on a very broad level. Throughout the project, you remain informed. It feels very efficient to me and I get a lot of energy out of it!”  
Iris Rombouts, structural engineer and projectmanager Octatube

Photography by Eva Bloem