27 April 2012
27 April 2012
Amsterdam’s Stedelijk Museum of Modern Art has recently undergone modernisation to include a new entrance and exhibition space that has been created using composites, in the shape of a bright white “bathtub”, 100 meters long and floating serenely above a paved square.
Teijin explain how architects, engineers and experts in both aramid and composites had to push the boundaries of their fields because the bathtub could only be made out of a pioneering new composite material, with Teijin Aramid’s Twaron para-aramid fibre as the key ingredient.
In 2004, five Dutch architectural firms were invited by the City of Amsterdam to submit their proposals for the renovation and extension of the Stedelijk Museum of Modern Art in Amsterdam. The winners were Amsterdam-based firm Benthem Crouwel (BCA), whose design stood out both visually and for its technical ambition.
The original building was designed in the highly decorative Neo-Renaissance style that was so popular at the time. According to Teijin, the City of Amsterdam wanted to restore this old building to its former glory, while building a new extension and entrance right alongside it.
Whereas the old building has straight lines and corners, Benthem Crouwel decided that the new one should have smooth, curved lines. And as the old is highly ornamented, the new should be simple. In addition, the rough brickwork of the old building would contrast with the extension’s perfectly smooth surfaces. The two buildings would, however, be linked together by the use of the colour white, which had long been used inside the exhibition rooms of the old building.
Teijin explain that by combining these elements, Benthem Crouwel designed a large white “bathtub” structure, which appears to float freely in the air alongside the old building, above a spacious new entrance hall. It houses various exhibition spaces, lecture rooms and other spaces, and additional exhibition areas have been created underground. They say that an important part of the architects’ vision was that the surface of the bathtub – the façade – should be seamless. However, the only way to construct such a large surface is to construct it out of a number of smaller panels. And panels made from conventional construction materials need a little space – in the form of dilation seams – to expand and contract as the temperature rises or falls. They said the dilation seams (which, depending on the material, might be anything up to 20mm wide) would totally spoil the flawless look the architects were aiming for and so the architects needed to find a material with absolutely minimal thermal expansion.
According to Teijin, the architects were eventually referred to the Dutch engineering firm, Solico, which has extensive experience of designing lightweight and strong products for the construction and defence industries. They explain that Solico also has expert knowledge and insight into the behavioural properties of many different materials, such as aluminium, glass and composites.
Solico was first asked to carry out a feasibility study to see whether there were any materials that would make it possible to create the super-flat, super-smooth finish the architects wanted. As well as minimal thermal expansion, Solico also needed to test for rigidity – or, to put it the other way round, for the potential of the material to buckle or warp, particularly in windy weather. On such a large, high-gloss surface, any distortion would be immediately visible – ruining the overall effect. Plus, the façade needed to withstand the potentially large differences in temperature between the inside and the outside of the bathtub.
The architects had supplied Solico with a 3D digital model of the outermost surface of the façade. Isolating certain sections, Solico added in the dimensions and properties of the different elements and materials, and included static sections to indicate where the façade would be fixed to its supporting steel structure. Using data from the Dutch building codes on wind loading and temperature ranges, Solico could then use this model to simulate how particular composite materials would withstand the various stresses and strains of high winds, high summer temperatures and freezing winter frosts. To really make certain of the wind-loading data, a physical model of part of the façade was built – including the static mounting points – for testing in a wind tunnel.
For the wind testing, Solico only tolerated a deformation level of up to 0.3% of a panel’s total dimensions. And its tolerance for thermal expansion was even lower. Even on a hot summer’s day, the panels could only expand by 0.1% of their total length. According to the building codes, the range of temperatures that the facade will be exposed to is -25ºC to +35ºC.
Teijin say that both glass and metal were out, due to their thermal expansion properties, therefore the only option was composites, and fiberglass was not suitable as its thermal expansion was still too high. Solico’s feasibility study showed that the optimal solution to meet all requirements was a composite sandwich construction. In this construction, the outer skins consist of composite laminates, reinforced with carbon and aramid fibres.
Teijin say the basic principle is that, whereas the resin expands as the temperature raises, both Twaron aramid and Tenax carbon fibres actually contract, due to their negative longitudinal thermal expansion coefficient. The result is a composite with a minimal thermal expansion. Teijin Aramid’s fibre research laboratory measured its thermal expansion on actual samples of the proposed composite. The experiments were in line with the calculations made by Solico and confirmed that the Twaron-and-Tenax-reinforced composite identified by Solico could indeed turn the architect’s vision into reality.
The first step was to find out whether the sandwich construction would perform as well in real life as it had in Solico’s feasibility study. So Holland Composites were chosen to construct a prototype – a set of 6 panels, bound together, sanded and coated just as if they formed an actual part of the façade. Using this test panel, Holland Composites and Solico were able to confirm that the Twaron-reinforced composite could enable a flat, smooth appearance. Combined with its minimal degree of thermal expansion, which gave the composite material the green light, this meant that Holland Composites could begin construction.
Although the largest panel would be 15 meters high and 3.5 meters wide, Teijin explain that it wasn’t their size that caused the biggest challenge during production. It was the fact that they had to be perfectly flat.
So Holland Composites had to design a huge table – larger than the largest 15m by 3.5m panel – that they could use as the base for injection moulding the laminates for the composite panels. Once this table was constructed and in place at their factory in Lelystad, Holland Composites could start the actual panel production. The panels are produced by vacuum injection moulding. The first step was to lay down the unidirectional fabrics of Twaron para-aramid and Tenax carbon fibre on top of a film on the surface of the giant table. Next, a layer of PIR foam is put in place, and the sandwich is completed with a second set of layers of Twaron and Tenax fabrics. Plastic film is then wrapped around the sandwich to make the mould completely air-tight. A vacuum is created and tubes leading from vats of vinylester resin are attached. Once the valves in the tubes are opened, the vacuum pulls the resin out of the vat and into each and every nook and cranny inside the mould. Once the resin has hardened, the impregnated fabrics and foam are transformed into a tough, durable sandwich construction with one perfectly flat surface.
The panels were transported to site and once in place, they needed to be bound, sanded together and finally coated with a slick layer of glossy white paint. Teijin explain that the architect’s vision of a super-flat façade depended on how the panels were actually mounted. Each one had to be fixed to the steel skeleton with extreme precision. If just one was only a single millimetre too far forward, or too far back, they say it would distort the shape of the entire 100m-long, 25m-wide façade. And with 185 panels to mount, there were nearly 1,800 mounting points that needed to be positioned with sub-millimetre accuracy. The main contractor and the construction company that built the underlying steel structure had done an excellent job and met the guaranteed accuracy of placing the mounting hooks within 3mm of the designated position. However, to achieve the desired level of smoothness for the façade, an even higher level of accuracy was required.
In order to solve this dilemma, Holland Composites first needed to know exactly which mounting points were out of alignment. They say a company specialising in precision assessment was brought on board. It used lasers to draw up a map of the exact position of each of the mounting points – which revealed which ones needed to be adjusted, and by how much. The next step was to “move” the mounting points either forward or backward. To do this, Holland Composites had designed a set of six plastic caps that could fit over the mounting points already welded to the steel structure. The six caps were shaped so that they could effectively shift the position of the mounting points of the panels by one, two, or three millimetres, either forward or backwards. Next, a team of construction workers scaled the steel skeleton; each equipped with a bag of plastic caps and a map of the mounting blocks’ locations, and started putting the caps in place.
In order to protect their outer surfaces, the panels had to be handled with extreme care. Holland explain that the team commissioned a unique attachment for a forklift, which could carefully lift each panel using vacuum clamps – just as if it was a pane of glass. This attachment could be used to position the panel with millimetre precision. The panels each had up to 15 hooks fixed to its inner surface. And all of those hooks needed to be placed over the plastic-capped mounting points at exactly the same instant. The only solution was to use a remote control to twist and tilt the panel until it was in the exact position, before gently lowering it into place. The use of this high-precision technique was a first for the building industry.
After the panels had been mounted, they needed to be bonded into one single surface. Holland Composites had designed the panels so that, when the panels were placed directly next to each other, there was a 2-inch gap between the outer skins – Twaron-reinforced resin composite laminates – of each panel. Holland Composites then glued a strip of the aramid-reinforced laminate into this gap, directly onto the exposed PIR foam beneath, and bonded it to the laminate skins on either side. They say this “bonding laminate” formed a strengthening bridge between the panels, ensuring the entire façade behaved as a single unit.
The entire 100m-by-25m façade had to be painted in one go. This was done using a four-phase painting method, which Teijin say enabled the painters to start and finish the painting process per phase in one go. At one end of the façade, they installed paint sprayers at three levels – one above the other – raising them off the ground on moveable aerial platforms. These “platforms on wheels” then drove carefully along, in perfect synch with each other, with their onboard sprayers coating the façade as they went along.
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