Parametric Design of Grid Shell Structures

Abstract

The characteristics of the joints play a considerable role in the stability of grid shells. Therefore, the connections are usually assumed to be rigid during the design phase. However, considering the semi-rigid behaviour of connections in the design could be beneficial. This leads to two major challenges. (1) The application of semi-rigid joints increases the indeterminacy of the structure. And (2) the current connection design strategy is not well-equipped for the integration of semi-rigid connections in the design. The following research questions is formulated: How can a semi-rigid approach to steel connection design and considering the semi-rigidity of the joints, be combined in a parametric design strategy for grid shells?

To answer this, three objectives have been formulated. Objective 1 focusses on the connection design, creating a design method for connection based on a pre-determined stiffness. Objective 2 focusses on the influence of joint stiffness on the structural behaviour of grid shells. Objective 3 is to design a grid shell, applying the results from objectives 1 and 2. Finally, the method is applied to a case study.

Results from objective 1 show that the load ratio can significantly influence the stiffness of connections. Also, design parameters, such as plate thickness and bolt spacing, can be adjusted to achieve different stiffness values. Combining these findings, a design space is generated to enable stiffness based connection design. Results from objective 2 show that the axial stiffness and the out-of-plane bending stiffness of the joints are relevant for the stability of the shell. Depending on the boundary conditions, shape and size of the shell, also in-plane stiffness parameters are relevant. For objective 3, a design workflow is proposed. A design space for the connections is combined with joint stiffness optimisation, resulting in the design of a grid shell with reversible connections. The application is checked with a case study of the C30 Shell. Complexities with increased size of the shell were managed by segmentation of the shell and clustering of the nodes. Resulting in a structure with 58% reversible joints.

The following conclusion is drawn: A semi-rigid approach to connection design and the inclusion of semi-rigidity of the joints in the structural design of a grid shell can be combined in the design of a grid shell. This can be achieved by defining a relation between the connection design and the joint stiffness design. This way, a design space can be created that links the connection design to pre-determined stiffness requirements and load ratios in the structural design. Which allows for efficient design iterations and eliminates guesswork in the design of both the connection and the joint stiffness distribution of the shell.

For effective application of this method it is important to be aware that the initial design largely determines the efficiency of the end result. The effectiveness of the stiffness optimisation, the segmentation of the shell, and the clustering of the joints all impact the result of the design significantly. Future research could be directed towards a better understanding of these aspects.