A design tool for timber gridshells

Abstract

In the last few years the timber gridshell has gained popularity. Recently two gridshells were constructed: the Weald and Downland gridshell in 2002 and the Savill Garden gridshell in 2006. These structures are examples from which the benefits of timber gridshells become apparent. A gridshell can display elegance and style, with its slender ribs curved into shape. It is also a sustainable structure, as the use of material is small, due to the shell behaviour. Also the timber can source from sustainable resources. Despite the advantages of the timber gridshell and the interest in sustainable engineering and free form architecture, the gridshell is not used very often. What can be seen as an important reason for this is the fact that the design process for a free form gridshell is rather complicated. An iterative design process is used to determine the grid geometry, which is only known to a few people. Main goal of this Master’s thesis is a study into the application of a design tool based on the geometrical properties of the grid of a grid shell, i.e. equal distance between the nodes on the quadrangle grid. The gridshell structure is a structure built with long slender laths. The laths are positioned in a flat quadrangle mat with one or more layers in two directions. This mat is then pushed and pulled into the desired shape by bending the laths and deforming the quadrangle meshes into rhombic shapes. When the desired shape is achieved, the laths are attached to edge supports and the structure is stiffened by diagonal bracing or applying a continuous layer on top of the laths. Timber is outstandingly suitable for this kind of building method. It is lightweight compared to its strength and can be bent and twisted relatively easy. Timber has always been used in structures by mankind. It was not until the twentieth century for timber to be used in large scale shell structures. In 1975 the first large scale timber gridshell was finished. This structure is the Multihalle in Mannheim. The structure can be seen as true pioneers work. The geometry of the structure was determined by physical form finding and it was constructed by pushing up the flat mat of laths by aid of scaffolding towers and fork lifts. More recently the Weald and Downland Gridshell and the Savill Garden gridshell were constructed. The former was constructed by lowering down the flat mat into shape on a special movable scaffolding. The latter was constructed by simply laying out the grid on a pre-shaped formwork. The gridshells were designed by aid of a computer form finding technique. The gridshell design tool has been set up to generate a gridshell grid on an arbitrary surface. The method used to generate the gridshell geometry uses two spheres to determine the intersection points of the gridshell laths. If the two spheres are positioned in such a way that their midpoints are located on the surface and that the two spheres are intersecting, there will be two intersection points between the two spheres and the surface. Together with the sphere midpoints, these four points form a mesh in the gridshell grid. A script has been created to execute this determination of points in a sequence which locates all possible intersection points on the surface. The sequence starts from start-off sections, interpolated on the surface. This implies that the correctness of the grid is dependent on the correctness of these start-off sections. Although the results of the grid generation tool look promising, further testing is advised to prove this. The design tool has been set up having the possibility to check the curve angles of the generated grid. From these angles the bending stresses can be calculated and checked if the bending or torsion stress criteria are exceeded. If this is the case, the checked element is given a colour. The generated structure can be checked visually for stress levels exceeding the stress criteria.