Op de innovatieve en constructieve Booosting glasdag vertellen de onderzoeksters Faidra Oikonomopoulou en Telesilla Bristogianni van TU Delft over het onderzoek en de ontwikkelingen van de glazen 'bakstenen', de glazen kozijnen en de glazen deurstijlen van Crystal Houses in Amsterdam (Chanel Store, P.C. Hooftstraat 94, Amsterdam). 26 september 2016 - Van Goghmuseum Amsterdam.

1. Structural Transparency: The Crystal Houses façade
Texts and photographs by Faidra Oikonomopoulou
THIS WALL IS TRANSPARENT!
Located in the heart of Amsterdam, the Crystal Houses façade, designed by MVRDV Architects, sets a
great example of the structural potential of cast glass. The new facade is an accurate yet entirely
transparent reproduction of the building’s original 19th
century masonry elevation. Solid glass bricks
reinterpret the standard brickwork and the typical architraves above the openings, even the traditional
wooden frames of the openings are translated into massive cast glass elements. As the facade ascends,
conventional clay bricks intermingle in between the glass ones to create a gradient transition to the normal
brickwork on the top, residential floor. The end result is a building that stands out and at the same time
blends naturally into the urban fabric of the historic street.
The Crystal Houses façade, P.C. Hooftstraat 94, Amsterdam
Based on the structure of the former masonry façade, the elevation of 10 by 12 meters employs more than
6500 solid glass bricks, each 210 mm thick and 65 mm high. Glass casting was the only solution for
producing glass components of this cross-section. Each brick was manually cast obtaining every time a
distinctive inner flow pattern as the liquid glass solidifies, revealing the hand-made process of the elements.

2. The desire of the architects to attain transparency at its purest did not allow the use of any visible
supporting structure, rendering the choice of an entirely self-supporting glass block system necessary.
Since this was a novel structural solution, the challenge of the materialization as well as of the fabrication
techniques of the solid glass block wall were assigned to the Delft University of Technology and in specific
to researchers Ir. Faidra Oikonomopoulou and Ir. Telesilla Bristogianni under the supervision of Prof. Rob
Nijsse and Dr. ir. Fred Veer.
In principle, a glass masonry wall of the abovementioned dimensions is plausible thanks to the high
compressive strength of glass and the 3-dimensional nature of the masonry wall that makes the
construction buckling-resistant. Indeed, just one glass brick can carry the complete dead load of the façade!
The lateral stability of the glass façade was further enhanced through its geometry. Four buttresses,
formed towards the interior of the façade by interlocking bricks, result in a continuous relief of increased
rigidity. To obtain an all glass structural system a colourless adhesive should be applied for bonding the
bricks. The lack of standardized strength data and building guidelines for such an application of structural
adhesives necessitated the research and testing on various different adhesives at the Glass and
Transparency Lab at TU Delft, in order to find one that would fulfil both the visual and structural
prerequisites. Actually the mechanical properties of the adhesive are equally critical to the ones of the glass
blocks for the developed system, as it is their interaction as one structural unit that defines the structural
capacity and properties of the glass masonry. The most favourable structural performance is when the
adhesive and the glass blocks fully cooperate, allowing the masonry wall to function as one rigid unit
against loading, resulting in a homogeneous load distribution.
Most adhesive candidates were discarded already in the early stages of research due to their coloration,
insufficient shear strength or too high flexibility. Eventually, visual prototypes and structural tests directed
the research to a photo-curing family of clear adhesives especially designed for glass to glass bonding, with
high shear stiffness and good long and short term compressive behaviour. Visually, the adhesive has a
similar refraction index to glass and does not discolour when exposed to sunlight. Another important feature
is its photo-catalytic curing which allows for fast construction: it takes less than a minute for the adhesive to
cure under UV-radiation, acquire its full structural capacity and become moisture- and water- resistant.
Series of 4-point bending tests on 1.2 m long glass beam specimens, each comprising 3 arrays of glass
bricks, proved the monolithic behaviour of the system when bonded by this adhesive. All specimens acted
as one single rigid unit under loading by failing with a straight, clear cut in the middle.
However, the structural tests also revealed that the chosen adhesive reaches its optimum (and desired)
bond strength when applied in a layer of a mere 0.3 mm thickness. The low viscosity and effectively zero
ideal thickness of the adhesive together with the inelastic nature of glass generated several implications
concerning its homogeneous application that resulted in exceptionally strict allowable tolerances regarding
not only the brick’s dimensions but also the overall façade. Not only the size of each brick unit, but even
each layer of the construction of the glass wall had to be confined within a tight dimensional precision of a
quarter of a millimeter. This demand of an unprecedented high level in accuracy and transparency in
construction introduced various challenges during the engineering and construction of the Crystal Houses
façade, calling for innovative solutions.
The required ±0.25 mm tolerance necessitated the post-processing of the manually cast glass units: during
the cooling of the molten glass, natural, inevitable shrinkage occurs to all surfaces of the glass component.
After the annealing of the bricks, this shrinkage was removed from the bonding surfaces by a CNC
polishing machine to provide glass elements of the desired precision.
A completely transparent wall bears yet another challenge: Any small defect in the adhesive layer, even an
air bubble three meters above the ground, is entirely visible. To minimize inconsistencies in the glue layer,
a special bonding technique for the uniform distribution of the glue was developed, utilizing specially
designed PURE® moulds for controlling the flow, spread and amount of the adhesive.

3. The desire of the architects to attain transparency at its purest did not allow the use of any visible
supporting structure, rendering the choice of an entirely self-supporting glass block system necessary.
Since this was a novel structural solution, the challenge of the materialization as well as of the fabrication
techniques of the solid glass block wall were assigned to the Delft University of Technology and in specific
to researchers Ir. Faidra Oikonomopoulou and Ir. Telesilla Bristogianni under the supervision of Prof. Rob
Nijsse and Dr. ir. Fred Veer.
In principle, a glass masonry wall of the abovementioned dimensions is plausible thanks to the high
compressive strength of glass and the 3-dimensional nature of the masonry wall that makes the
construction buckling-resistant. Indeed, just one glass brick can carry the complete dead load of the façade!
The lateral stability of the glass façade was further enhanced through its geometry. Four buttresses,
formed towards the interior of the façade by interlocking bricks, result in a continuous relief of increased
rigidity. To obtain an all glass structural system a colourless adhesive should be applied for bonding the
bricks. The lack of standardized strength data and building guidelines for such an application of structural
adhesives necessitated the research and testing on various different adhesives at the Glass and
Transparency Lab at TU Delft, in order to find one that would fulfil both the visual and structural
prerequisites. Actually the mechanical properties of the adhesive are equally critical to the ones of the glass
blocks for the developed system, as it is their interaction as one structural unit that defines the structural
capacity and properties of the glass masonry. The most favourable structural performance is when the
adhesive and the glass blocks fully cooperate, allowing the masonry wall to function as one rigid unit
against loading, resulting in a homogeneous load distribution.
Most adhesive candidates were discarded already in the early stages of research due to their coloration,
insufficient shear strength or too high flexibility. Eventually, visual prototypes and structural tests directed
the research to a photo-curing family of clear adhesives especially designed for glass to glass bonding, with
high shear stiffness and good long and short term compressive behaviour. Visually, the adhesive has a
similar refraction index to glass and does not discolour when exposed to sunlight. Another important feature
is its photo-catalytic curing which allows for fast construction: it takes less than a minute for the adhesive to
cure under UV-radiation, acquire its full structural capacity and become moisture- and water- resistant.
Series of 4-point bending tests on 1.2 m long glass beam specimens, each comprising 3 arrays of glass
bricks, proved the monolithic behaviour of the system when bonded by this adhesive. All specimens acted
as one single rigid unit under loading by failing with a straight, clear cut in the middle.
However, the structural tests also revealed that the chosen adhesive reaches its optimum (and desired)
bond strength when applied in a layer of a mere 0.3 mm thickness. The low viscosity and effectively zero
ideal thickness of the adhesive together with the inelastic nature of glass generated several implications
concerning its homogeneous application that resulted in exceptionally strict allowable tolerances regarding
not only the brick’s dimensions but also the overall façade. Not only the size of each brick unit, but even
each layer of the construction of the glass wall had to be confined within a tight dimensional precision of a
quarter of a millimeter. This demand of an unprecedented high level in accuracy and transparency in
construction introduced various challenges during the engineering and construction of the Crystal Houses
façade, calling for innovative solutions.
The required ±0.25 mm tolerance necessitated the post-processing of the manually cast glass units: during
the cooling of the molten glass, natural, inevitable shrinkage occurs to all surfaces of the glass component.
After the annealing of the bricks, this shrinkage was removed from the bonding surfaces by a CNC
polishing machine to provide glass elements of the desired precision.
A completely transparent wall bears yet another challenge: Any small defect in the adhesive layer, even an
air bubble three meters above the ground, is entirely visible. To minimize inconsistencies in the glue layer,
a special bonding technique for the uniform distribution of the glue was developed, utilizing specially
designed PURE® moulds for controlling the flow, spread and amount of the adhesive.

4. More information
Faidra Oikonomopoulou F.Oikonomopoulou@tudelft.nl
Telesilla Bristogianni T.Bristogianni@tudelft.nl