Theatre of Folded Dreams

IBOIS, the Laboratory of Timber Construction, within Lausanne's technical EPFL university, is one of middle Europe's major timber research centres, with a focus on shell structures, digital fabrication and new timber materials. In 2008 IBOIS completed its first full building, Chapel de Loup, which highlighted a glue-free folded plate geometry within its structure.

Now, the research centre has completed a second, more ambitious, similarly glue-free, double layered folded plate project, the Vidy Theatre pavilion, on the shores of Lake Geneva, close to Lausanne.

This in-depth research paper by Christopher Robeller, Julien Gamerro, Pierre-Olivier Coanon and Prof Yves Weinand documents the form generation, design development, and construction challenges of the Vidy Theatre from inception to completion.

This paper is part of Fourth Door Research's guest research section

1  Background to the project

Folded plate structures combine different load-bearing functions in their form. Due to the rigid combination of several oblique surfaces along their edges, the elements function simultaneously as a plate, slab and frame (1 for notes see end of paper) The resulting stiffness makes it possible to span larger distances without support. The majority of such constructions were realised with concrete in the 1960s and 1970s, the plates being folded in only one direction, described at the time as 'prismatic folding.’

The construction of such a fold with wood material, which in addition to the excellent ratio of weight and strength also allows for sustainable constructions, was only possible with the introduction of cross-laminated timber plates (CLT) and structural laminated veneer lumber (LVL). A first example of such a construction, inspired by Japanese origami paper folds, is found in the Chapel of St-Loup. (2)

The first prototypes with plywood plates were fabricated and assembled with the help of custom made templates. (2) An experimental folded plate barrel vault made of a total of 144 elements could be built using analogue manufacturing technology in the form of a circular saw that could be tilted for angular cuts. However, as a result of manufacturing and joining with templates and guides, it was necessary to work with repetitive plate geometries, in order to minimise the required templates and guides. The prototype was built using 8 geometrically different elements, from which only singly-curved folded surface structures could be realised.

1.2  Integral Attachment of Timber Plates

More complex, doubly-corrugated, folded surface structures, consisting of a large number of differently shaped components, have recently been achieved using integral attachment techniques (3)(4)(5). This oldest principle of joining technology uses the form of components to transfer forces between them. A transfer of traditional wood panel connections from the carpentry sector was demonstrated in a research project. The integral dovetail tines served not only to transmit the forces, but also critically to assist joining a fast and precise assembly of a large number of differently shaped components. Dovetail joints belong to the group of the so-called prismatic, single-degree of freedom connections. The shape of such connectors allows only one insertion direction, enabling the embedding of only one possible, correct positioning of the components in the final construction in the prefabricated connection. Thus, a complex, doubly-curved overall shape of the load-bearing structure was made possible by means of the joining technique. The deflections on this doubly-curved folded plate structure were up to 40% less than in a comparable singly curved variant (3).

A further development where the number of differently shaped edge connections were again significantly increased, was with two-layered wood panel folding structures (DLFP). A novel connection technology with double through-tenon connectors allows for a complete integral connection of both layers. For example, all four panels can be joined directly along folded edges, the double tenons also serving as spacers between the two layers and absorbing shear forces (4) (5).

1.3 The Vidy Theatre

The Vidy Theatre, Lausanne, is an open meeting space for everybody dedicated to contemporary creativity. The theatre, located in a building designed and built by architect Max Bill for the National Exhibition in 1964, stands on the edge of Lake Geneva, in the heart of French-speaking Switzerland.

The theatre consists of three permanent buildings and a temporary space housed in a tent set up in the adjoining park. To replace the tent a permanent – though dismountable - thermally insulated pavilion was developed in a partnership between the Timber Construction Laboratory IBOIS, EPFL and the Bureau d'Études Weinand, who were the responsible architects and engineers.

The construction of the Vidy Theatre pavilion allows for the implementation of a novel type of load-bearing structure, exclusively made of wood panels. The doubly-corrugated, antiprismatic folded plate structure achieves its mechanical performance through the rigidity of the joints. All plates are joined by innovative wood-wood connections. The project’s research aim was to further develop and implement the knowledge gained through recent research at the EPFL IBOIS laboratory. It is the first time these experimental wood-wood connections have been applied to a building of this scale. The wood-timber connector’s are unusual as they are an integral part of the panels. This construction therefore requires customised prefabrication: connectors and panels are factory cut together in a single operation. Once assembled, these wood panels ensure the building structure solely, minimising the need for metal connectors.

2 Construction System

The construction system for the Vidy Theatre represents a consistent development of the previous prototypes. The two-layer construction explicitly uses the integral connection technology to join particularly thin plate cross sections, allowing the structure to span a distance of 16 to 20 meters without supports, with a plate thickness of only 45mm. The distance between the two layers is 300 millimeters from the upper side of the outer plate-layer to the lower side (simultaneously the visible side) of the inner plate layer.

The hollow interspace with a depth of 210 millimeters is also used for insulation, making the double-ply construction alongside its static properties, significantly more effective than single-layer construction with thicker plates. While requiring simpler geometry and joining, single layer construction needs more complex solid-state insulation, and can only be realised at greater cost.

The load-bearing timber construction of the theatre is constructed in 11 building segments between the 12 main axes. Each segment between two axes is prefabricated in 3 parts; two wall elements and one roof element. On site, the wall segments are connected to the respective neighboring wall segment, the roof segment being placed on top of them afterwards. In the two-layer joining technique used, there are four different steps, each with different tenon shapes, depending on the respective position of the folding edge in the construction. A general distinction is made between two situations in which either one segment is connected to one adjacent segment (figure 1, steps 1 and 2), or the case where one segment is simultaneously connected to two neighbouring segments (figure 1, step 3 and 4). In the first two cases the tenons are oriented at right angles to the edge, while for the next two cases the tenons are rotated within the plate plane, ensuring the insertion direction of all the tenons of the plate is parallel.

2.1 Structural Analysis of the plate joints

The global mechanical behavior of the Vidy Theatre is complex, particularly regarding the wood materials anisotropic characteristics and the folded shape. A number of experimental tests were therefore carried out to examine the strength of the compounds, within various parameters, to help determine the most suitable wood-based panels.

In previous studies on the strength of integral tine and tenon joints, only Laminated Veneer Lumber panels (LVL) were used, given their very homogeneous structure due to the many cross-linked layers, each only 3 mm thick. With the Vidy Theatre project, CLT panels were also considered for integral connections for the first time. One reason for this was a special sustainability factor that regional Swiss wood could be used. A special feature of the Swiss panels, (examined in the next section), is the additional lateral gluing of the board layers. This results in more homogeneous behavior, which is very important for the integral connections. The research focused on the behavior of the double-tenon and single-tenon joints under bending stress. This stress is decisive in folded wooden structures. Figure 2 shows a simplified, single-layered finite element model with the help of which the greatest expected bending moments were determined.

2.2 Comparison of different plate types

The first series of tests were carried out through a single-layer tenon-hole connection between two plates at a 90° angle. The tenon width here was 150 millimetres, and the fibre direction of the plate cover layer followed the grain of the tenon as it would do in the live project. In order to compare two different wood-based panels, two different CLT panels were tested (with side-glued boards) and a LVL panel (see table below). In addition, four series of different geometries and properties were tested for a total of 12 specimens per panel type, which together assisted investigating the influence of the tenon rotation along the folding edge. This is important because the folding angles in the theatre roof are different and require such rotation θ of the tenons. There is no difference in the bending stiffness between a rotation of θ = 0 ° and a rotation of θ = 15°.

The influence of an additional screw connection was also investigated. The test results were more homogeneous with the screw connection, since an ideal initial position of the plates is ensured. On average, this resulted in a higher bending stiffness of approximately 10%. Figure 3 shows the bending test setup for the single-layer joint. A 20 kN cylinder, pressing on a steel lever arm, transmits the rotary motion to the horizontal plate with the tenon hole, while the vertical plate with the tenon is fixed rigidly with 4 bolts. The lever arm ensures that the force input to the horizontal plate is always perpendicular. The angle of rotation and the forces were detected by 2 inclination captors and 4 force measuring cells. During these tests, the rotation was limited to 30◦. A joint with larger strain is structurally useless and also beyond the limits of current European standards.

The failure of the test specimens under the bending load always occurred at two fracture points. These research experiments with single-layered connections with different types of plates have shown that, for the specific integral tenon joints used in this project, CLT panels showed better mechanical properties under bending than veneer plywood boards. The 45mm thick plywood board with 5-ply structure was the best solution for several reasons:

-       Higher stiffness compared to 40 mm BSP
-       Higher yield moment
-       Allows larger rotation in elastic part compared to 40 mm CLT
-       Ultimate moment almost double than that of 40 mm CLT
-       Homogeneous behavior even with oblique tenons (examined at 0 and 15 °)

2.3 Double-layered experimental testing

Following the investigation of the single-layer tenon joints and the selection of the 45mm CLT panel as the most suitable material, bending tests were tested with the actual two-layer structure. Here, the upper layer of the test sample was connected to a single tenon while the two lower layers penetrate each other with a double tenon prior to being connected to the upper layer. The experimentally examined configuration of the tenons shown in Figure 4 is a special case in which one of the four connecting lines is not integrally joined. This special case occurs with the in-situ connections where the prefabricated roof part is placed on the prefabricated wall segments. Here, all four connection lines must be joined in a concurrently executed step. In the illustration, the connection points of this connection line are shown in red. This particular compound was chosen because it obtained the highest bending moments in the finite element model.

Tests showed a considerable increase in the mechanical efficiency due to the two-layer design. In principle, the connection behaves as a single-layered variant: in order to compensate the bending moment, two opposing forces act, the intensity of which is proportional to the lever arm. With a single-layer connection, this is equal to the plate thickness of 45 mm. With the two-layer plate, the value increases to approx. 250mm. As a result, the forces against the single-layer variant are reduced to one-fifth.

3 The automatic generation of plate and joint geometry

The generation of all plate components is done automatically using a CAD plug-in that was custom built for the Vidy Theatre project, developed with the software development kit (SDK) Rhino Commons and the programming language C#. The Grasshopper software is used as a user interface, in which the input parameters of the design can be edited and modified, including a real time preview of the 3D components.

Outputs include a 3D preview of the plate geometry including all joints, as well as a visualsation of the insertion directions for each regular joint edge in the polygon mesh. Once the parameter is adjusted, the geometry can be output into the CAD program through three switches for different representations.

3.1 Relevance of fold angles in the structure

One of the most critical parameters for the fabrication is the cutting angles, at which the tool must be inclined to produce the plate components. Such orthogonal folds are also structurally beneficial. However, it is not possible or feasible to achieve the overall geometry without a deviation beta from these orthogonal angles

On the CNC machine used for the cutting of the plates at the plate manufacturer and wood processing facilities, the maximum tool alignment angle was beta=60 degrees. This allows for a maximum fold angle of 150 degrees. Taking into account the tool holder profile, in this case a slim thermo-shrink chuck, the required clamping length of the shank-type milling cutter can be calculated. This protrusion is to be kept as short as possible, because it makes the cutting prone to vibrations which reduce cutting speed and cut quality. Larger protrusions therefore require larger milling cutter diameters, however this increase in diameter would have a negative effect on the notches, which are required for the cutting of the concave corners in the plate contours (see section 4).

3.2 Optimisation of fold angles and edge lengths

The Vidy Theatre’s basic polygon mesh was first designed to suit the architectural constraints and requirements of the building. The almost rectangular cross section in the XZ plane allows for an efficient use of the interior spaces, but it also creates a challenge for the folding system. The solution was to combine a prismatic fold for the wall elements with an antiprismatic fold for the roof elements. The joints between these two systems have the most obtuse fold angles in the building, which increase from the center of the structure to the front and end symmetrically.

Following the initial design of the polygon mesh, the CAD plug-in allowed to display all fold angles, as shown in the color coded Cad model in Figure 5. The mesh was subsequently modified for a more efficient fabrication, optimising the maximum and average values for the length of edges (resp. max plate sizes for transport and machining), as well as the fold angles.

3.3 Output of the plates for fabrication

Figure 6 shows the plates from one of the 11 axis segments of the building prepared for the production data. Each axis segment consists of three prefabricated components: two wall elements, shown in red color in Figure 7 and one roof element, shown in blue color. The wall elements are attached to the base and the adjacent axis segment on site, after which the roofing element consisting of a total of 20 prefabricated plate components is placed on the wall elements. This in-situ tenon connection is also designed as a double tenon connection, but with some adjustments for easier on-site attachment. Normally the lower plate in the double-layer connection is inserted along a vector on its own plane (regular insertion), while the upper plate is inserted along a vector that lies on the plane of its counterpart (inverted assembly). This creates an interlocking assembly, where the two parts of a neighboring segment cannot be removed along the same direction, as their insertion directions are at too large an angle away from each other. It is only possible to disassemble one plate after another.

For the in-situ joints in between the prefabricated wall and roof segments, this assembly principle could not be applied, since the two plates of the wall cannot be inserted separately, because the entire wall segments were prefabricated including insulation material and external ventilated cladding. The same applies to the roof prefab elements, where the upper and lower plate have already been connected and cannot be inserted separately. As a consequence, the in-situ joints were inserted in one step, where the entire roof element is lowered onto the wall tenons, along the World Z axis.

4 Fabrication

Due to the 114 different folding angles in the basic folding form of the theatre various slanting cuts are required for the fabrication of the components. The parts were therefore manufactured with a 5-axis CNC NC-PMT/190-TUCU/ISO40 machining centre (anno 2000) from the manufacturer CMS, which was available at the facilities of the timber plate manufacturer Schilliger Holz AG. The setup is illustrated in figure 8.

4.1 Cutting of concave corners

Due to many concave corner points in the polygons, for example between the through tenons or at the slot cut-outs within the plates, inclined cutting with a shank-type finger cutter was required. The necessary 5-axis CNC machining, which is required at approximately 500 different component edges, with thousands of differently inclined tenon geometries, could not be efficiently generated with standard CAM software solutions for regular timber construction tasks. We instead used a custom developed DLFP-Fabrication CAD plug-in for the automated ISO G-code generation of integrally inserted wood-based panels. The underlying algorithm has already been used in previous projects and has been further improved and adapted for the Vidy Theatre [6]. One of the necessary adaptations was a special post-processor for the CMS machine of the industry partner company. Using the CAD plug-in, various special details of integral connections can be created automatically. These include, for example, notch cuts in the concave corners, which are necessary for the insertion of the sharp corners on the inserted tenons.

4.2 Development of a custom interface

The CAD plug-in, programed in Microsoft Visual C#, was implemented in the visual programing environment grasshopper, which is part of the Rhinoceros 3D CAD software. This interface allowed converting the plate contour data out of the 2D plate matrix, automatically into ISO G-code that can be sent to the machining centers OSAI Series 10 control system, simply by a hatch selection. The custom-developed plug-in required inputs for the general cutting parameters, as well as a list of polyline contour curves, defining the plate shapes. Within this list, the program automatically finds top and bottom contours that belong together, defining an outside contour or inside cutting contour (tenon slot). The differentiation between outside and inside contours was achieved through the different orientation of the curves, inside contours were clockwise, while outside contours were counter-clockwise. This plug-in was split into five main components:

-       drilling of temporary fixation points

-       5-axis cutting shank-type cutters (fig 9)

-       5-axis cutting with saw blades
 
This special “app” that was supplied to the manufacturer, providing an alternative to the generation of CNC G-Code files directly by the planning team.  The chosen CAD plug-in solution however allowed for much improved flexibility at the manufacturer. The head CNC technician at the factory was provided with project specific tools, which allowed him to generate the required G-Code by himself, rather than providing pre-generated G-code files. This allowed for the nesting of the final plate geometries onto raw plate work pieces by the company.

4.3 Simulation for customised 5-axis cutting

Another important aspect of this chosen strategy was the possibility to integrate customized functions for the tool path display and a cutting simulation component (Figure 9 above)

This simulation tool displays the machine motions during the cutting, which is particularly important with the large amount of cuts at different angles. It was also impossible to check this code manually, because the CNC files for the fabrication of individual plates were more than 10.000 lines long. The factory’s CNC technician could use this function for checking every CNC program for possible errors, before sending it to the machine. A slider in the visual programing environment allowed the technician to scroll through the program line by line, from the beginning to the end of the G-code.

4.4 Perpendicular cutting

As previously explained, for the Vidy Theatre project concave corners in the plate contours require cutting with a cylindrical shank-type cutter, set to a diameter of d=12mm. This relatively small diameter required three cuts along the contour, splitting the total depth of the cut into smaller parts, which reduces tool vibrations.

A particularly critical parameter in this context is the tool alignment angle beta during the cutting. In many situations, the cutting with an inclined tool introduces a pull on the work piece, which can cause strong vibrations. The first counter measure to this problem was an improved clamping strategy. While the thicker CLT plates, usually cut at this factory at a 90° angle, are not clamped at all, the plates for the theatre project were clamped with an average 10-20 wood screws onto the machine table. The holes for this clamping technique were also handled by the CAD plug-in. A second measure to avoid vibrations was to avoid tool inclinations along the axis of motion of the current tool path. Considering a lower and upper polyline defining the plate contours, this is simply done by choosing a tool orientation axis, which is the shortest path between the two rails. However, this is not always possible. Figure 10 shows how the fabrication tool deals with this problem. The tool moves along this contour in a counter-clockwise orientation (climb cutting) from the right side of the image to the left. It first encounters a concave corner, where the tool must be inclined.

With the next segment, the end corner is convex. The tool path must therefore start with the inclined orientation from the previous cut, but it can retreat to a safety plane 20mm above the plate shortly after, and return on a short path line between the two contours. At the end of the line, an extension of the tool path is calculated to fully separate the pieces with the required length. The following line segments in the picture follow the same logic. Whenever the tool inclination along the cutting line can be avoided, the cuts are extended instead. Concave corners however are preserved and tangential notches are added.

4.5 Hybrid cutting technology strategy

The previously explained principle is further improved through a hybrid cutting strategy for line segments with a concave start point and a concave end point, using a saw blade for certain cuts automatically. Figure 7 shows that the plate contours of the Vidy Theatre contain various concave corners, which require the cutting with a shank-type cutter. However, for longer straight lines in the cutting paths, cutting with a saw blade improves the efficiency greatly. Through integrating a threshold value, which indicates the segment length, using a saw blade improves the overall efficiency. This value was finally set to 500 mm, which proved to be the length under which the continuous cutting with the shank type cutter would be faster. All longer cuts which qualify for a saw blade cut are skipped in the initial cutting procedure with the shank type cutter, which will instead pull off and start a rapid (ISO G0) motion above the end point of the line, where it returns to the next cut.

The workflow of the G-Code production was as follows: First, the plate contour polygons from the 2D plate matrix on the world XY plane were nested onto the raw CLT work pieces, with the aim of reducing waste material as much as possible. In the next step, fixation points for the clamping of the plates were added, each with a safety distance away from the tool cutting paths. In order to transfer these points onto the raw plate of the CNC machine, a drilling program was generated in order to mark the fixation points, by the CAD plug-in.

4.6 Variation in plate thicknesses

The 5-layered CLT plates that were used for this project were not sanded down to a precise thickness after the lamination procedure, resulting thickness variations of 45mm – 48mm, usual in timber plates. The thickness was ensured to be not less than 45mm, and a complementary strategy was chosen to ensure that the through-tenon joints fit precisely into their neighboring plates.

This was achieved through an additional “planning” program for each plate, where rectangular pockets were milled along all tenons and double-tenons. The depth of these pockets was set to exactly 45mm above the CNC tables zero plane. This ensured that the plate thickness of the final plates is exactly 45mm in these critical areas, where the tenon size needed to be equivalent to the slot size in the roof counterparts, once the parts are jointed.

5 Prefabrication assembly

5.1 Supports for the insertion pre-positioning

A key advantage of the integrally attached, double-layered folded plate construction system is that it allows for the construction of wide spanning shell structures without the need for a full-size mold or elaborate support structure, as is necessary in concrete shells or other types of timber shell structures. Instead, the integral connectors act as assembly guides, defining a unique insertion direction for each of the plates. This not only allows for a much reduced construction cost, time and manual labor, but it also allows for the construction of large amounts of individually shaped plates. At the Vidy Theatre double-curvatures in the roof structures become possible, providing a shell behavior rather than the cylindrical singly curved vaults that were used in previous prototypes of antiprismatic folded surface structures. However, in the full-scale prefabrication assembly, a certain, minimal support structure was required for the construction of the roof elements in three consecutive steps, for the insertion of the centre plates (A), intermediate plates (B), and outer plates (C). This procedure requires four supports in the form of vertically, in the YZ plane positioned planar “M-shaped” plates, with the approximate negative shape of the intermediate roof plates (B). Figure 12 shows this setup, which was designed to be re-used for the eleven roof elements of the theatre.

In order to define the right angles for the intermediate plates, which is individual for each roof segment, the short connecting slants between the M-shaped plates and the ground supports were fabricated with different lengths for each roof segment. The exchange of these slants allowed the rest of the supports to be re-used.

In figure 1, Step 2 it was already shown that the intermediate segments must be connected first. These elements must therefore be placed on the support structure, in a position that allows for the insertion of the remaining plates. A precise placement is required especially for the central elements, as these connect to the intermediate elements on both sides simultaneously. For the final, “locking” plates in each roof segment, the system was designed to have the insertion direction from above for all top layer plates, which simplifies the assembly. In a final step, the thermal insulation material isofloc was injected into the hollow spaces of the prefab segments. In order to monitor the effectiveness of the vapor barrier and water infiltration, an electronic monitoring system was installed in between the layers, on the lower ends of all roof segments on both sides.

5.2 Insertion-optimized chamfered joints

After the assembly of the first building axis, which was used as a test element, though also in the final building, the procedure was optimized with chamfered through-tenon joints, where the tips of the tenons are 15mm less wide on two thirds of their protrusion length. Similarly, the sides of the double-through-tenons decrease in width by 15mm on each side, on two thirds of the intermediate space between the two layers of timber plates (see figure 13). This chamfering allowed for a quicker positioning and insertion of the plates. The implementation of this chamfer details at a late stage of the project was easy to implement, due to the algorithmic generation of the plate geometries.

5.3 Measurement of deflections on roof segment 1

Before the final production phase, the roof segment of the first building section was produced for testing purposes, along with two shortened versions of the wall segments. All connections, fabrication and assembly steps were carried out as in the final building. After the assembly (figure 14), the additional weights were added to the ground plane to prevent any motions. On the roof, a load of 2x 10kN was introduced, as illustrated in figure 26. Afterwards, a vertical deflection of 12-14mm was measured at the reference points illustrated in the figure. Also, a horizontal deflection at the supports of 24 mm was measured. After the unloading, the vertical deflection was reduced by 6-7mm, while the horizontal deflection remained at 24mm

6.0 On-site Assembly

6.1 Transportation to the site

The prefabricated wall and roof parts were transported from the assembly in the factory, to the building site on special trucks. This was necessary due to the size of the parts. While the width of the elements was fixed at 2.6 meters, the length increases towards the center of the building up to 21 meters. In the reverse direction, the total height of the elements increases towards the front and back of the building, up to a maximum of 3.85 meters.

The Figure 16 shows the on-site connection details of the prefabricated wall elements. On the ground plane, half-lap jointed, pre-cut slants were mounted on the floor slab. This concrete plate was specifically cast along the folded shape of the walls. Figure 15 also shows the ventilated exterior façade, which was pre-mounted on the wall elements in the factory, before the parts were transported to the site.

In between the eleven segments with a width of each 2.6 meters, the prefabricated building components meet on ten vertical connection planes in the XZ plane. The elements are connected with wood screws diagonally inserted from the outside. After the connection of the wall elements with the ground supports, and with one another, the roof elements are inserted using a mobile crane. As previously described, the in-situ connections of walls and roof are subject to the highest bending moments in the structure, hence these connections were designed as through-tenon joints, even though the assembly had to be carried out on site. The insertion vector of these joints was vertically along the world Z axis, which allowed for a simple and rapid assembly. Like the connections in the entire structure, the joints were produced without gaps.

7 Conclusion

The Vidy Theatre Lausanne project shows how automated production technology, which is already widely used in timber construction, can be used for new solutions in joining technology – and ultimately lead to new structural typologies. The novel integral connections in this structure allowed not only the transfer of forces between the components, but they also served as integrated locator and positioning aids. During prefabrication, a unique correct alignment of the components can be embedded into the shape of the connections. This allows simple, rapid and precise joining, even with 308 different plate shapes, 456 different joint angles and over 3.000 automatically generated and fabricated tenons and slots. For the Vidy Theatre Lausanne, the connections are of particular importance.

The two-layer design offers great advantages both statically and also regarding prefabrication. The system allowed for the integration of inexpensive flock-insulation between the layers, which had great advantages over solid thermal insulation on such a complex shaped folded plate roof structure.

The project demonstrates the successful realisation of the previously proposed double-layered folded plate construction system with integral double-through tenon joints [6]. This system does not only use the connections to join the plates within the upper layer and within the lower layer, but it also connects the two layers with each other with the double-tenons, which also act as spacers to define the correct distance between the plates.

The automatically generated and fabricated integral joining technology was made possible through the development and application of project-specific CAD plug-ins, which have allowed for a new type of digital workflow, drawing the fabrication technology and its capabilities closer to the architectural planning process. While the algorithms for the generation of the 3d geometry were only used by the planning team, the fabrication algorithms were packaged into a custom made software tool for the CNC technicians at the wood processing factory. While this workflow integrates the constraints and logic of the construction system, it remains transparent and leaves room for modifications until the final production.

This computation-enabled flexibility has proven to be crucial in the design, optimization and calibration of the plate geometries. The basic parameters, such as the folding form, plate thickness and offset between the plate layers, remained variables throughout the project, which could be optimised following test results, calculations and observations.

Acknowledgements

The construction of the pavilion for the theater of Lausanne vidy was supported by the “Aktionsplan Holz” initiative of the Swiss federal environmental office BAFU.
The authors would like to thank all of the project partners and supporters, the Theatre of Lausanne vidy, Atelier Cube Architects, the City of Lausanne, Arianit Shevelli, David Riggenbach, Project Manager for Blumer-Lehmann Holzbau AG, CH-Erlenhof, Renggli AG, CH-Schötz, Schilliger Holz AG, CH-Küsnacht, Balteschwiler AG, CH-Laufenburg.

Fourth Door/Unstructured Acknowledgments

This is an edited version of a paper by Yves Weinand, and his I-BOIS team, originally published in the Journal of the International Association for Shell and Spatial Structures: J. IASS
The authors can be contacted by email - Christopher Robeller christopher.robeller@epfl.ch, Julien Gamerro julien.gamerro@epfl.ch, Pierre-Olivier Coanon pierre-olivier.coanon@epfl.ch, and Prof Yves Weinand yves.weinand@epfl.ch

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