For nearly a century architects have been dreaming of a material as strong as steel and as transparent as glass. We have now learned so much about glass that it is increasingly used as a sort of transparent concrete.
TU Delft has several world-renowned experts on glazing among its teaching staff. Professor Mick Eekhout (Architecture) and his company, Octatube. construct glazed roofs all over the world. Closer to home, the South and East glazed atriums in the university’s BK City are his creation. Working to a tight schedule, Prof. Eekhout and his team expanded the existing main building with two enormous cubes, largely constructed in glass. For London’s Victoria and Albert Museum, he constructed a spectacular glazed roof over a building in the museum’s courtyard. Twisting, double-glazed panels rest on sloping laminated glass bearers up to 11 metres long.
Alongside his positions in the faculties of Architecture and Civil Engineering and Geosciences, Professor Rob Nijsse also works with consultancy firm ABT, for whom he created the wavy glass windows of Rem Koolhaas’ building for the Casa da Musica in Porto, Portugal. This year he surpassed himself with even larger wavy windows at Antwerp’s Museum aan de Stroom (architect Willem Jan Neutelings). As well as being visually attractive, these wavy windows are stronger than flat glazing, says Prof. Nijsse.
Calculations show that the fluted glass panels barely sway under the effect of the wind, keeping the forces on the securing points to a minimum. Looking at these structures, it is easy to forget what a fragile material glass is, and how unexpectedly it can behave. But then Prof. Nijsse recalls the glass shelters at Nijmegen’s bus station: a glass sheet was placed carefully on its glass bearer, and crack! Well, such things happen. Another time it was the beam that gave way. “That’s a strange sight,” says the glazing expert from behind his frameless spectacles. Prof. Nijsse subsequently discovered that the holes in the beam were a little ragged. Peak stresses are generated at those points, and any crack can be propagated through the material at lightening speed. The Octatube factory also has its collection of splintered glass panels. Recently, because crows picked up stones from an adjacent roof and took great pleasure in dropping them on Eekhout’s glass roof. Once he realised what was happening, he had the gravel stabilised with tar. Problem solved! Eekhout puts things in perspective: “It’s bound to break at some point, but what matters is that the structure must never fall to the ground.”
Find Zappi
According to Prof. Eekhout, architects’ fascination for glass dates to the early days of modernism, just after the First World War. People wanted to escape pain, poverty and suffering, and consequently placed their hopes in transparent, business-like designs and architecture, represented in Germany by the Bauhaus movement and in the Netherlands by De Stijl. Back in 1919, German architect Mies van der Rohe designed a transparent skyscraper with glazed walls for Berlin. Forced to flee from Hitler, he later built the skyscraper in Chicago. The Van Nelle factory in Rotterdam dates from the same period, and expresses the same yearning for clarity and transparency. The hope was that transparent buildings would lead to a transparent society. This was before the bankers and insurance companies developed their preference for mirrored glass.
In his inaugural lecture, when appointed a professorship in 1992, Prof. Eekhout called for a quest for an imaginary building material called “Zappi”, which would be as transparent as glass and as strong as steel. When Dr Fred Veer came to work with him in 1995, his task was: “Find Zappi!”
During his quest, Veer has tested the strength of hundreds of glass samples. There can be few people who have made as many splinters as he has. He found that glass with a breaking stress of around 30 MPa (N/mm2) is not only 20 times weaker than steel, but also that chance plays a major role. “Glass almost always fails in places where the edge has been damaged during manufacture,” wrote Veer in an article summarising ten years of research. Furthermore, glass failure is immediately catastrophic: as soon as the breaking strain is exceeded, the material loses all integrity. Only very recently has it become possible to calculate the failure of glazed structures. Professor Jan Rots (faculties of Architecture and CEG) explains that the finite element method normally used to calculate forces in structures fails with glass after the first crack. The results that come out of the computer are then nonsensical. Prof. Rots devised a method to restart the calculation after each crack. He calls this ‘Sequential Elastic Calculation’. The results here tally with the reality in the case of extremely brittle materials like glass.
Veer has come up with a practical method of inhibiting the spread of cracks. He constructed panels and beams from smaller stacked elements. This can be achieved using lightsensitive resins, with adhesive PVB (polyvinylbuteral) films, or with the spectacularly strong Sentryglass (see the demos on Youtube). He used this method to develop an eight-metre long hollow beam, intended for use as an aquarium. Artist Stefan Gross thought it would be great to use a completely transparent aquarium as a bridge below the light source in the roof of the library. Veer designed, built and tested the aquarium, but was refused permission to install it. Prof. Eekhout felt the design was irresponsible and even dangerous.
Veer perfected the concept of glazed loadbearing structures with steel reinforcement. Last year, Dr Christian Louter obtained his doctorate cum laude with his thesis, “Fragile yet ductile”. The basic idea was simple: if the glass fails, steel must take up the stresses. For this, Louter constructed a variety of multi-layered glass beams with an internal laminated steel strip or wire along the underside. He then loaded the beam well beyond the breaking stress of glass. A beam 1.5 metres long and 28 centimetres high was loaded with 1200 kilogrammes. The glass creaked and cracked in various places, the beam bent several centimetres, but it didn’t fail. Lauter concluded that the construction was safe, provided the stress remains below 15 N/mm2. “We haven’t discovered transparent steel,” notes Veer, “but glass can be used like reinforced concrete, and it is considerably stronger than concrete.”
So while we haven’t found Zappi yet, we can do far more now with glass than seemed possible 20 years ago. The constructive use of glass means that ever-larger surface areas of structural glazing are possible with increasingly less steel support. The glass ribs behind the 14-metre high glass façade of the Apple Shop in Boston and the Apple Cube in New York (both designed by James O’Callaghan) are good examples. We’ve also got better at avoiding pitfalls. Veer has already stated that cracks often start at points of minor damage. Prof. Nijsse therefore takes no risks, and has his wavy panes fitted with a steel profile fixed with elastic acrylate while still in the factory.
Prof. Eekhout sees it as essential that design and execution should occur within the same company. This avoids sloppiness and misunderstandings. He sometimes worries about unforeseen hazards, and thinks of his experimental projects as swords of Damocles, dangling above his head. “Experimentation during a project is risky, but it’s essential for innovation,” he says Prof. Nijsse emphasises the need for further research, for example into glazed columns and the use of adhesive fixings for glass in place of steel connections. “We are now at the front of the pack,” he observes. “But the others are catching up.” China is already producing wavy glass. “We need to keep pushing the limits if we are to retain our position.” Prof. Nijsse is aiming to identify three or four externally financed PhD students to continue the research: “Otherwise, we’ll have to close down our lab.” (JW)
