Parametric Structural Design

The Digital Workflow of Parametric
Structural Design
Developing Grid Shells in a Nordic Climate

Åshild Huseby and Marie Eliassen
Master Thesis 2018
Civil and Environmental Engineering
Norwegian University of Science and Technology
Department of Structural Engineering

Unusual shapes are becoming more and more prevalent in the cityscape, however they often rely on large and complex configurations of material. New and empowered digital tools – parametric modelling software – allow for continuous structural optimisation parallel with the design process.

This enhances the synergy between architects and engineers and complements the search for alternative solutions with a better use of resources. Shell structures provide the opportunity of combining artistic shapes and low use of material due to their efficient load bearing capacity.

The efficiency of the shell is dependent on its shape and gets challenged when exerted to external asymmetric loading pushing it out of its ideal shape. Until now, previous research papers and articles have provided information about form finding of shell structures exposed to symmetric pressure while omitting asymmetric loading. This leaves an important question of how to cope with such load situations?

In this thesis by Åshild Huseby and Marie Eliassen , the objective thus became to come up with a parametric design procedure for a timber grid shell where the structure was robust enough to handle the asymmetric pressure from climate induced loads. The grid shell in question is to be a cabin built by the students at the Norwegian University of Science and Technology (NTNU) for the use of NTNU students in the Norwegian mountains.

Through the studies in this thesis it was found that Eurocode (EC) provides limited information about design loads for shell structures, and that it can only be adopted to a certain extent. By comparing the EC loads to a load configuration-algorithm built in Grasshopper, it was discovered how EC did in fact not give the worst-case load scenarios.

However, the load distributions obtained from the algorithm showed some contradictions with regards to the guidelines in EC and were thus not accounted for in the design. It was illustrated how the shape is crucial for the efficiency and structural behaviour of the shell, and how double curvature gives a more robust structure which is less prone to deflect.

Optimisation of the cabin’s shape was executed through minimising displacements from external loads. Initially the shape showed substantially larger displacements for the Drifted (asymmetric) snow load case from EC compared to the Undrifted (symmetric) case, and the optimised shape displayed an increase in the initial double curvature.

The optimisation approach led to an improved structural performance and a smaller gap between the structural response for the Undrifted and Drifted load cases.

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