For this master design project Thijs de Goede and I designed a roof structure to cover an existing courtyard with a span of 40 by 40 meter. The final design is a pneumatic cushion combined with a steel frame to avoid large horizontal pre-stress forces on the existing building (a worst case scenario).
The flexible skin is an ethylene tetrafluoroethylene (ETFE) layer strengthened with a cable net. The tensile strength of the ETFE is relatively low, therefore, adding a cable net is necessary for constructing large span structures. Three different form finding methods were analyzed for defining the geometry of the cable net:
- Force density (force divided by length)
- Soap film property (constant pre-stress)
- Manipulation of a deformed shape (‘hanging model’)
The soap film model appeared to be best applicable, because it resulted in the most constant cable forces and horizontal reaction forces. For the compression element, that has an arch shape, a more equal divided load is favorable.
Subsequently different cable net geometries were analyzed. The cable net geometries varied in center-to-center distance and direction of the individual cables. The models were loaded with different load cases, and compared with relevant design codes. The diagonal cable net with a grid size of 3550 mm fits best for the given case of the fictional 40 by 40 meter courtyard. This cable net loads the compression element inside the cushion with a more constant load than an orthogonal cable net. For an orthogonal cable net the arched shaped steel frame inside the cushion will be loaded more a-symmetrically, which is not desirable for such structures.
The steel frame inside the cushion consist of four circular shaped arches. This results in a compression force in the arch with edge beams that are loaded in tension. By connecting the four arches with bar at every corner of the courtyard a closed system is created (a belt that is holding the four arches together). The shape of this steel frame led to a minimal visual coverage of the courtyard.
This project got granted with the first prize – Oasys Project of the Year 2016 – in the category ‘Academic Structural Project’. The award winning paper can be found here. The project can also be found as a case study on the website of Oasys Software. Special thanks to Lennert Hupkes for making the render of our design and to our supervisors Patrick Teuffel and Arjan Habraken.