Although there are not many examples, it is important to present the first project carried out in 1975. By Frei Otto, the Multihalle in Mannheim (9). The structure can be seen as a true pioneering work in the timber structures area. The geometry of the structure was defined by physical form-finding while it was constructed by pushing up the flat grid of laths with aid of scaffolding towers and fork lifts.

Another reference in the timber gridshells field was recently built, in the United Kingdom, as part of a new conservation center building and storage, Weald and Downland, to the Open-Air Museum near Chichester, in Sussex. The building has drawn international interest from architects, engineers and carpenters. Besides Downland, there is a half a dozen examples of the kind. After Mannheim was built, other good examples were designed such as: the Helsinki Zoo viewing platform, by Ville Hara, in 2003; for its image and the integration of this type of structure with a metal structure, the Savill Building, in Windsor Great Park, by Buro Happold and Glenn Howells Architects, a large four-layer timber Gridshell, in 2006; the Courtyard roofing of a rural villa, Italy, by cmmkm - architettura e design, Roberto Ruggiero, Alfonso Petta, Felice Grasso and Fabio Figlia, in 2007; the Masseria Ospitale’s terrace roofing, Italy, by cmmkm - architettura e design with Bernardino D’Amico and Filomena Nigro, in 2010; the Pavilion Japan, the Haesley Nine Bridges Golf Club House, in Korea, and the Centre Pompidou-Metz, in France, 2010, by Shigeru Ban.

Based on the research of the state of knowledge, about the subject in question, a case was found to be a good example to point out due to the proximity in size/proportion and geometry to what was intended, as well as because is well documented.

A recent example, ZA Pavilion depicted in Figure 1, was presented as a temporary cultural venue and was designed during a student workshop in Cluj, Romania. It is a simple solution with a great result. The ZA Pavilion was based on the construction process and it is now possible to find the shape of a timber gridshell by simulating its real construction process. A simple square grid is easy to imagine, but connecting multiple “trunks” with an intricate variety of beams criss-crossed between them requires some serious thinking (14).

This double layered structure is made from Siberian larch laths, with a section of 70mm x 20mm, reaching a height of 4m and spanned 18mx13m. For the construction of the structure, the grid was assembled from 3.5m by 3.5m modules, using a double-splice joint connection with the aid of 2 additional timbers between the laths (Figure 2).

As visible in Figure 1, from the geometry of this pavilion, four arched entrances stand out as areas of highest curvature. In these areas, the structure alters from the remaining grid, being, each layer, made of 2 laths, each one with 10mm in thickness. This solution improves the bending process avoiding breakages.

Finally, for the bracing, a third layer of ribs with the same cross-section of the grid laths was included (14).

In the present case study, the method crossed between theory and practice went through several phases. This was not a linear process but an experimental one, based on a trial and error approach. Starting with some handmade sketches, looking for a simple geometry that was able to demonstrate different possible situations / problems, as it is possible to see in the example below (Figure 3).

The hand drawn sketches, were a very important stage because without too much effort it was possible to try different things, as different propositions, symmetries, details, etc. Also, this allowed to move forward to the next phases with a better idea about what to look for.

In this case, after some geometric experimentation this step brought some clarity about what would be the desired volumetry, as well as warned about some necessary care due to the use of two layers. Some of the care that was necessary to prevent was the necessity to allow the sliding of the pieces between the same lines of the different layers, since the outer lines would always have the greater perimeter.

After defining the overall dimensions, height, area, grid, it was time to try the idea in a model. This model was constructed, at 1:10 scale, as per figure 4 below. This model was not an exact replica of the mesh to be constructed, but rather a mimic of the general concept (15). It was constructed with only one layer and wire-adjustable connections to allow some movement between the lines during their deformation. Its’ objective was to demonstrate the behaviour of the different lines and there is no doubt about the torsion, flexion and possible breakage movements (9). The importance of the first phases was proven by the time it took to build because of the high number of connections. At this stage, it would not be possible to test all the variants studied previously in the drawing phase, due to the use of material, cost and space needed.

The construction of this model allowed to realize that the arcs of the entrances would be the most tensioned, being these the elements that would define the maximum deformation that the material would allow. Furthermore, it was possible to understand that the elements when wet increased their elasticity considerably, facilitating the process of deformation.

It is important to note that current software, has tools that allow to easily design complex structures (16). In this specific case, the software used was Rhinoceros 5 (17) and Grasshopper (18). This conjugation of software consisted in the use of the visual programming of the Grasshopper to conceive the models, and, Rhinoceros to visualize the same ones.

Before starting the modeling, it was necessary to define a strategy because the way it started would condition or facilitate the rest of the project. It was required to foresee some problems and difficulties associated with each methodology and the parameters that would be flexible during the design process.

Thanks to the previous phases, some priority characteristics were determined, that helped to define and choose the best way for the construction of the three-dimensional model. The priorities were: the defined shell-like final image; to obtain the greater number of workable parameters; the quick creation of an automatic model and parameters; give priority to the methods that make less use of drawing in Rhinoceros; and, since it was an academic scope, work in a way that somehow could bring more knowledge to the user experience.

However, Grasshopper did not have the ability to simulate the constructive process of the mesh, creating the necessity of making use of the Kangaroo Live Physics plug-in (19). This computational tool is based on a method called “Dynamic Relaxation” (method that allows to obtain a geometry in which all the forces are in balance) (20). It started with the definition of a base geometry (flat mesh), to which elements were associated with the properties of the material to be used. Then the boundary conditions were imposed and the forces of deformation of the mesh were defined. A simulation was then carried out until the mesh reached equilibrium, thus obtaining the new geometry. The described process, that can be seen in Figure 5, was repeated, changing the applied forces and following three factors that were taken as the most basic level components of the designing tool: material model; approximation of the target shape; equilibrium form finding procedure (13). It was finished when the desired geometry was reached, based on previously imposed requirements: geometry of the final shape; internal forces; support reactions; forces needed to bend the grid into shape; geometry of the grid before it is bent into shape; information on the accuracy of the results; maximum buckling of the elements, etc.

It is important to note that this model was used only as a geometric exploration tool, because it did not reproduce the actual characteristics of the material and the conditions of the constructed gridshell.

Despite the good final result of the model obtained from Kangaroo Live Physics, another model was developed using another Grasshopper’s plug-in, named Karamba. Using the same procedure as per the development of the previous model, the new one gave the opportunity to introduce more variables to the model, obtaining a more realistic final form.

The result of the previous phases was the project of an Elastic timber gridshell with a flat grid with about 80m² open. This flat grid would be tensioned, making it deform until it gained a new shape, as shown in figure 6.

A flat shell with a regular square grid in the two-dimensional planes with the corners cut, using double layer and with two axes of symmetry. There was a line spacing of 0.7m, with 9m lines composed of 3 wooden elements each. This would result in a tri-dimensional structure with bolted connections in 6cm tears holes, so it could move during its construction, as can be seen in Figure 7. With variable heights of 2.1m and 3.4m in the span arches and the centre of the mesh respectively.

To keep the cost controlled and since the wood was offered, the locks were designed with wooden elements that are placed diagonally to the grid (Figure 8).

To begin construction, the pieces were separated and organized by quadrants (see Figure 12a) and then the respective quadrants were assembled, as previously mentioned, at the construction site. The constituent elements of the quadrants were joined by simple connections with M8 screws, measuring 8 and 10 centimetres in length.

Several pieces of aid were placed in the quadrants to the subsequent application of locks, as shown in Figure 12b. It was necessary to place these pieces in this first phase joined to the structure through the existing connections in the quadrants.

To allow the support system designed for the structure, it was necessary to place two elements that “embraced” the elements in the area of future supports. These elements, like the previous ones, are joined to the remaining structure through the existing connections.

An extremely important detail was that the attachment bolts were not tightened properly, thus allowing the structure to move without becoming too rigid during the erection process (see Figure 13a).

In order to finish the first day of the Workshop, the different quadrants were connected as it is visible in Figure 13b, through similar connections to the previous ones. Some pieces were placed all over the grid, with the nail gun, making shims between the two layers, creating more points of contact between them.

Two hours before the beginning of the deformation of the shell, the pieces began to be irrigated the pieces in order to ease their deformation (see Figure 14a). At the same time, several pieces were built with wooden elements to serve as anchors and assist during the process of erecting the mesh. Moments before beginning this process, the grate was transported to the place where its final position would be (see Figure 14b).

The centre of the shell was erected manually, and the struts constructed were placed in the inner zone of the grid, as can be seen in Figure 15a. The next phase was to mount the tension application system on the flat mesh. The system used was a set of 6mm steel cables that would come to be pulled with the aid of two lever winches, one in each direction.

During the application of tension, the supports could move horizontally, the grid was lifted and placed on elements of wood, thus allowing the movement of the supports, presented in Figure 15b.

The application of tension was started through the winch lever; safety cables were placed parallel to the tensioning cables (see Figure 16a). After a first application of tension, sometime was given to the wood so that it could adapt to the imposed deformation.

During this waiting time, the cables were adjusted and the winches lever for a second application of tension. The weights were also repositioned, which were the definitive support of the structure. During the interval in between the application of tension, hammers and pliers were used to adjust the connections and help the structure to accommodate itself to its new shape, as is visible in Figure 16b.

With all these processes completed, a new phase of tension application began. The processes described above were repeated until reaching the desired form for the structure (see Figure 17a).

Once attained the desired geometry (see Figure 17b), it was time to fix the structure. For this, the locks were placed in the previously placed parts, all the connections were tightened, the concrete blocks were placed properly to serve as support and prevent the horizontal displacement of the structure and the lever winches and tensioning cables were removed.