Walking into the giant glass dome of the AFAS Experience Center, it is hard not to be impressed. With a diameter of 42m and a height of over 24m the majestic structure covers a theatre that can house up to 850 people. Between the theatre hall and the glass dome enough space is left to fit in a foyer with a capacity of over 625 people. If you count the two floors around the theatre, the capacity increases to over 1000 people.
Cover image: Photo Fred Oosterhuis
And to think that the theatre is part of a much bigger complex that includes an office building, a two-story underground parking garage, conference rooms, studio’s, a gym, an atrium, auditorium and a restaurant. The AFAS Experience Center is, just as the dome, impressively big.
Octatube was contracted by Just Architects and client AFAS Software to engineer and build the dome with horizontal lines and triangular glass panels, taking up the full technical development of the design. An interesting challenge.
Editors note: This is a revised translation of an article previously published in Bouwwereld Nederland. Author: Daniël van Capelleveen
Several studies showed that a system using hollow steel tubes was the best way to construct the dome. But how to design a structure in which the glass panels are neatly divided over the dome, without disturbing deviations in the pattern? “This was the most important question in the design phase,” says Arjan Klem, senior engineer at Octatube and responsible for the 3D-modeling and engineering. Joey Janssen, structural engineer at Octatube involved in the project, adds: “We started with the largest possible shape: a triangular glass panel of 3.2 meter at the base of the structure. Due to limits in glass productions we could not go any larger. Progressing to the top the panels gradually become smaller, building up the dome with 11 horizontal rings in total.” The steel rings are connected with tubes that follow the division of the glass panels, creating a steel structure with a consistent pattern of triangles. Between ring 9 and 10, the number of glass panels used per ring goes from 48 to 24, preventing the triangular panels becoming too sharp and therefore much more difficult to produce.
The dome is covered with more than 1000 glass panels, built up out of an inner laminated pane of 2×8 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 right above the water surrounding the dome. The glass in this bottom ring has 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 color. This print serves as sun shading. On the inside, these glass panels are covered with acoustic panels.
Assembling an giant dome of tubes and glass with just three people
A lot of time and energy is invested in the preparation phase of the project, in which all the major decisions were taken. Special attention was paid to devising an assembly method and the logistics behind it. “The choice was: are we going to work with separate tubes and nodes and assemble these on-site or are we going to prefabricate large frames in the factory? We went for the first option,” says Klem. “Even though the assembly itself is not necessarily easy, the concept is relatively simple. There is lots of repetition, every node per ring is the same and besides, working with smaller elements provides easier handling and creates flexibility. A large component needs to be planned: multiple people have to collaborate, a larger crane is required, transportation is more difficult. In short, smaller elements on-site means less effort.” As a result the full assembly on site could take place with just three people: one person operating the crane, lifting the 50-100 kg elements in place and the other two mounting the elements in place. In this way the full dome was built up like a kind of igloo, in 5 days per ring.
Nodes and tubes come together perfectly
Another essential part of the design was the engineering of the nodes. At each node, six tubes come together. “It is very tricky to cut six tubes at such a precise angle that they come together in a node perfectly,” Janssen says. The centerpiece is a short circular tube perpendicular to the node to which the six tubes are connected. But even with this beautifully designed node, there is no certainty that it all will fit together. That is why Octatube made mock-ups, performed tests and had samples made of the cutting. “This way we could test the tolerances of the 3D laser-cutting of the tubes. Normally, tubes are never perfectly round but always slightly oval. And the cutting introduces heat which may result in deformations. A deviation of 5 mm can easily occur, which makes it much harder to weld properly. Restoring these deviations would take up a lot of extra time. We therefore paid extra attention to the cutting.”
Octatube carried out a number of tests in collaboration with the steel cutter to determine the correct settings of the cutting machine, up to a maximum tolerance of 1 mm. There is a 6 mm tolerance between the node and each tube for finetuning during assembly and for thermal expansion. “But the beauty of a dome is that it expands equally in all directions. The structure does not interfere with itself,” says Janssen.
Integrated in the centerpiece is a light spot that can be rotated individually in every direction, giving the dome a striking appearance in the evening. All the wiring and controllers are placed within the tubes. A damper, hidden from view, in each connection point of the tubes allows an installer to access the wiring if necessary. The dampers were also used during assembly to connect the tubes to the nodes.
Little dark blue feet
Custom elements were designed to mount the glass onto the steel structure. The cast steel, dark blue, powder coated supports visually disconnect the glass skin from the main steel structure. A special connection, as there are no standard solutions for this. Janssen explains: “Usually you see a continuous profile, or a welded connection on the structure to support the glass. But because this element was architecturally essential, we decided to go for bespoke cast elements. These can be both structurally optimized and visually designed. This connections make it seem as the glass is loosely folded over the steel structure.”
A space of 24 mm is required between the glass panels in order to attach them blindly to the blue supports. De sealing gaskets and spacers closely match the color of the supports. “A deliberate choice,” Klem explains. “The feet flow over into the sealing gaskets and because everything has the same color it will visually ‘disappear’. From the outside the glass connection is not visible, creating a visually clean dome.”
In addition to the supports, the sealing gaskets have also been custom made for an optimal closure of the glass panels. “And with over 3500 identical supporting points and 3,5 km of gasket in total, it quickly pays off to have such customized solutions specifically for this purpose,” Klem explains.
Challenging bite out of the dome
Although the dome appears to be completely spherical from the outside, it has been partially cut off where it connects to the adjacent theatre tower: there is a bite out of the dome. This presented the greatest structural challenge. Janssen: “A sphere is by nature a strong structural shape, but if you take a bite out of it, the structure drastically loses stiffness.” Therefore Octatube designed a 500 mm diameter edge beam around the connection to the theatre tower (compared to the 170mm diameter of standard tubes), in order to redirect the forces from the dome to the floors. The redistribution of those forces to the floor is necessary because the theatre tower was not calculated for such forces. In other words, the dome was not allowed to pull or push against the surrounding structure. The edge beam and therefore the rest of the dome is structurally disconnected from the adjacent building. The 1m wide dilatation created at the connection is filled with an insulated zinc gutter.
Nevertheless, 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 as well, from 6 to 12 mm.
Spherical plain bearings
The three support points are designed as spherical plain bearings, similar to the ones used at viaducts. 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 center. “If we had made a restricted connection here, the floor would not be able to deal with this outward load. Especially because the floor is cantilevering out of the building,” Janssen explains. To prevent damage from the expansion forces on the floor, the dome is structurally disconnected from the floor. An exception forms the large edge beam, which is firmly anchored to the floor and, together with the middle support, creates a fixed framework for the dome.
By far most of the time was spent engineering the edge connections. “I estimate about 80% of our time.” Klem says. “That is because the edge has many unique components that all had to be designed individually. Not only the diameter and thickness of the steel tubes, but the specific forces in the individual connections are different as well. The rest of the dome is actually a matter of repetition.”
A literal crown on the work
The time and effort put into the preparation, logistics, tolerances and dimensions paid off during the realization of this grand piece of challenging architecture. It all resulted in an incredibly smooth building process on site, everything fitted together perfectly. Main contractor Dura Vermeer was highly impressed. “Chief executor Gert Nummerdor called us ‘the watchmakers among steel contractors’”, Janssen says.
Closing the dome at the top was an exciting moment for all involved. As a final keystone, the final ring, including six mounted tubes, was placed as one piece into the dome. There was no certainty that the 3 by 3m prefabricated element would fit. Janssen: “A dome only works well structurally once its finished. When there is still a gap at the top, the dome could start leaning inwards as a result of its own weight. We were quite anxious that it would not fit. But fortunately the element fitted perfectly!”
Additional note from the 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.”
Location: Inspiratielaan, Leusden
Client: Afas Software
Architect: Just Architects, Steef van der Veldt
Main contractor: Dura Vermeer Hengelo BV
Engineering dome: Octatube
Main structural engineer: Pieters Bouwtechniek
Installation technician: Homij Building period: start mounting anchors + first edge beam April 2019 Completed March 2020
This article was originally published in IGS Magazines Winter 2020 Issue: Read the full Magazine here for more thought-leadership from those spearheading the industry
Arjan Klem is a senior engineer at Octatube, a Dutch based Design and Build company specializing in bespoke building structures with an emphasis on advanced applications of glass and steel. As a kid he always wanted to become an inventor. Where other kids were aspiring to become an astronaut or a fireman, he spent most of histime playing with Lego or other technical toys. As a grown up, graduated in 2010 from the Technical University in Delft in Architectural Engineering, he can say that he followed his childhood dream and actually became an inventor, working on beautiful and unique architectural projects. Arjan started working in Octatube at the start of 2012 as a junior engineer where his first project was a full glass elevator of the Mauritshuis Museum in The Hague. After that he has done many projects for Octatube, both in the Netherlands and abroad. Some examples are the Victoria&Albert Museum Exhibition road entrance London, The Sammy Ofer Center in London many other projects, including the glass dome for the AFAS Experience Center.
Joey Janssen is a structural engineer at Octatube, he graduated from the chair of Innovative Structural Design at the University of Technology in Eindhoven in 2016. Working at Octatube with its motto ‘Realizing Challenging Architecture’ is a very good fit. The bespoke and complex designs Octatube works on, demand a high level of innovative and solution-oriented thinking. Being part of a Design and Build company means being involved from the architectural design until the last finishing touch on-site. This makes the puzzle of the building process more complex, but also every part of the process equally important. As a structural engineer, Joey applies parametric design and digital innovation to keep control over structural verification. Focus and expertise on structural analysis of both the global structural and connection detail therefore is required. The AFAS Experience Center Dome is a fine example where this all came together nicely.