Structural Systems Homework 3: Build a shell structure using load-bearing formwork principles
Summary
I built a shell by loading a form in tension, then applying paper mache to the form. The thickness of the completed shell was about 3mm, and the narrowest width was about 500mm. I affixed the shell to a base to more closely represent a real structure. The shell weighed 1.43lb, and supported 250lb.
Description
I chose the top boundary conditions for my shell formwork based on the requirement to load the shell until failure. I wanted to be able to load the structure with known weights in a systematic way, so that I could find the strength to weight ratio for the structure. To this end I weighted the formwork with a flat piece of wood in the centre, with a weight on top of that, so that I could reuse the same piece of wood on the top as the loading platform, to avoid loading the shell with point loads which would penetrate the shell. My shell is thus designed primarily for a top load, for example a platform where people could gather after theatre, rather than the self-weight of the structure.
Figure 1: the shell under construction |
Figure 2: the finished shell before painting |
I chose the lower boundary conditions of the shell to be pleasing aesthetically, as a square, but with curved sides similar to the Kresge auditorium at MIT. The shell is shallow - this is a theatre for somewhere like Houston, where land is less expensive. The space could be used very effectively, for example the space towards the edges would be ticket booths, changing rooms, storage, etc, while the space inside would feel wide and open, accommodate seating and stage.
I used the fabric from an old sweatshirt as the form, and applied glue to stiffen it. However from my experience with similar glues, I did not expect the resulting form to be stiff enough to maintain the essential inward curve that applying the weight caused for the shell, so I applied most of the paper mache while the form was loaded, because the paper should have been quite stiff enough to maintain the shape. Then after a day when that was dry, I removed the weight and applied the remaining paper mache. I used about 35 sheets of 'Tech' tabloid-sized newspaper in the shell, with ‘Elmers’ PVA glue as the matrix.
Figure 3: close up of the inward curve caused by loading the form in tension |
Figure 4: the shell ready to be loaded |
After consulting with the Teaching Assistant, I decided to fix the edges to a wooden base, as this would be more representative of a real structure, and I thought it would give the structure more strength, as the edges would be restrained from splaying outward.
The shell took more load than I expected, so it became difficult to balance the weights (note the small weights ready next to the shell before loading, and the large weights it finished with). The shell took 250lb in weight, but the height of the load was such that I had to reload in a more stable fashion. The shell then took 240lb without any motion, and then failed when I added another 25lb weight. I am pleased that it took near to the highest load twice, as this indicates that the structure was not significantly damaged by loading to 250lb the first time. The shell weighed 1.43lb, so it had a strength to weight ratio of 174.
Figure 5: the shell almost fully loaded, 225lb |
Figure 6: the shell after loading with 265lb |
Figure 7: the shell as failed |
Figure 8: the shell as failed |
Figure 9: close-up of the failed region |
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