The perfect fit for a glass dome roof

Entering the expansie dome of the AFAS Experience Centre, it is hard not to be impressed. With a diameter of 42 metres and a height of over 24 metres, this majestic structure covers a theatre that can accomodate up to 850 people. The space between the theatre hall and the glass dome allows for a foyer with a capacity of over 625 people. Including the two floors around the theater, the total capacity exceeds 1000 people.

Engineering and building

Octatube was commissioned by Just Architects and client AFAS Software to engineer and build the glass dome roof with horizontal lines and triangular glass panels, undertaking the complete technical development of the design. This presented an intreguing structural challenge.  

1,000 glass panels neatly divided

The primary question during the design phase was how to create a structure where the glass panels are evenly distributed over the dome, without any noticeable deviations in the pattern. We began with the largest possible shape: a triangular glass panel measuring 3.2 metres at the base of the structure. As we progressed towards the top, the panels gradually became smaller, forming the dome with a total of 11 horizontal rings. Between rings 9 and 10, the number of glass panels per ring decreased from 48 to 24, preventing the triangular panels from becoming too sharp and difficult to produce. The steel rings are interconnected with tubes that follow the distribution of the glass panels, creating a steel construction with a regular pattern of triangles.
 

Technical specifications of glass panels

The dome is covered with over 1,000 glass panels, consisting of an inner laminated pane of 2x8 mm, a 16 mm argon-filled spacer, and an outer pane of approximately 10 mm thick single safety glass. The first ring is positioned below ground level and stops just above the water surrounding the dome. The glass in this bottom ring features a perforated blue screen print to reduce reflections from the water. From the seventh ring onwards, the glass is provided with a dense screen print in the same blue colour, serving as sun shading. On the inside, these glass panels are covered with acoustic panels.

Perfect fit of tubes and specially designed nodes

We constructed the dome using hollow steel tubes that follow the division of the glass panels, creating a steel structure with a consistent pattern of triangles. The tubes are connected by beautifully designed nodes, each connecting six tubes. At the centre of the node, we designed a short circular tube perpendicular to the node to which the tubes are connected. Integrated into the centrepiece is a light spot that can be individually rotated in every direction, giving the dome a striking appearance in the evening. All the wiring and controllers are placed within the tubes.

Precision engineering and quality control

Despite the beautifully designed nodes, there was no certainty that everything would fit together perfectly. Therefore, we created mock-ups, performed tests, and had samples made of the cutting. Restoring deviations would have taken a lot of extra time, so we paid extra attention to the cutting.

Closing the dome

Closing the dome at the top was an exciting moment. The final ring, including six mounted tubes, was placed as one piece into the dome. There was no certainty that the 3 by 3 metre prefabricated element would fit. A dome only works well structurally once it is finished. When there is still a gap at the top, the dome could start leaning inwards due to its own weight. We were quite anxious that it would not fit, but fortunately, the element fitted perfectly

On-site assembly

Flexibility and easy handling were our starting points. We chose not to use large prefab frames but to work with separate tubes and nodes that we assembled on site. While large elements require careful planning, a bigger team, and a large crane, smaller ones provided easier handling and flexibility. We could fully assemble the gigantic tubular steel construction on site with only three people: one person operating the crane and the other two mounting elements in place. The dome was built up like an igloo, in five days per ring.

Custom glass supports 

Architecturally essential was the visual disconnection of the glass skin from the main steel structure. Therefore, we opted for bespoke cast elements. We designed cast steel, dark blue, powder-coated supports to attach the glass panels to the steel structure, making it seem as though the glass is loosely folded over the steel structure.

A space of 24 mm is required between the glass panels to attach them blindly to the blue supports. The sealing gaskets and spacers closely match the colour of the supports. The feet flow over into the sealing gaskets, and because everything has the same colour, it visually ‘disappears’. From the outside, the glass connection is not visible, creating a visually clean dome.

In addition to the supports, we also custom-made the sealing gaskets for an optimal closure of the glass panels.

Dome cut-out and edge beam design

Although the dome appears to be completely spherical from the outside, there is actually a bite out of it. A sphere is inherently a strong structural shape, but by taking a bite out of it, the structure drastically loses stiffness. Since the dome was not allowed to pull or push against the surrounding structures, we designed a 500 mm diameter edge beam around the connection to the theatre tower to redirect the forces from the dome to the floors. This edge beam, and therefore the rest of the dome, is structurally disconnected from the theatre tower. The 1 metre wide dilatation created at the connection is filled with an insulated zinc gutter.

Load transfer and spherical bearing support system

The dome is connected to the theatre tower at three points of support. Normally, the structure is strong enough to sustain itself, but this point of the structure could theoretically be covered with a lot of snow. The supportive points are necessary and provide a vertical force transfer. For this reason, the wall thickness of the steel structure around the edge beam is increased from 6 to 12 mm. The three support points are designed as spherical plain bearings. The two outer supports can move freely horizontally. The middle support, on the other hand, is horizontally restricted in the plane of the tower. It is the fixed point from which the dome can deform due to loads, thermal expansion, and shrinkage. Like the two outer spherical plain bearings, all supports of the steel structure on ground floor level can move perpendicular to the dome centre. To prevent damage from the expansion forces on the floor, the dome is structurally disconnected from the floor. An exception is the large edge beam, which is firmly anchored to the floor and, together with the middle support, creates a fixed framework for the dome.

Architect Steef van der Veldt: 
“First of all, I would like to compliment the people of Octatube on the pleasant and professional collaboration we had with them. It may sound logical, but they understand the language of an architect and how to solve something technically and aesthetically in a responsible manner.  For example, the acoustic grid ceilings in the dome and the slender steel structure with integrated lighting. As a result, the experience inside the dome has become pleasant and particularly attractive. Seen from the outside, it really is a glass “sphere”  in which the closed top - we called it the “keppeltje” (kipah) – is indistinguishable. All in all it is high class performance where the end result fully meets our expectations.”