Design optimization of gridshells equipped with pre-tensioned rods

Gridshell are fascinating examples of structures able to combine aesthetic qualities and optimal structural performances thanks to a complete merging between "shape" and "structure". To make gridshell structurally efficient is their capacity to cover large spans with light systems, exploiting the inherent strength of a double curvature shell. The design of gridshell is often the result of strategies based on both form-finding and structural optimization techniques, sometimes combined together to obtain efficient structural solutions and architectural conception of space. Pretensioned rods, in some cases incorporated in the structure of gridshell, could particularly influence the structural performance of gridshell in terms of both its global deformability and stress distribution within its members. Consequently, pre-tensioned rods, when present, represent additional structural components to necessarily consider in the design optimization process of gridshell. Aim of the present paper is, first, to analyze the influence of pre-tensioned rods in the design optimization of gridshell and, then, to propose an optimization procedure for gridshell equipped with pre-tensioned rods. To this end, some simple gridshell examples and the case study of the Smithsonian Museum canopy in Washington are analyzed in the paper. The obtained results underline the importance of considering the presence of pre-tensioned rods both in the phase finalized to find the free-form shape and, moreover, in the structural optimization process.