Form-Finding: Nature’s Own Design PrinciplesMar.29.2019
Throughout history, people have sought connections with nature. In particular, designers have observed nature, investigated its materials, and abstracted its forms and qualities.
The Garden of Secrets, a collaborative project between TEALEAVES and UBC Botanical Garden, dives into the realm of plant-inspired biomimicry and biophilic design with the goal of celebrating nature beyond its edible, visual and medicinal properties.
Wanda Lewis, Emeritus Engineering Professor at Warwick University and Author, has dedicated 30 years of research on form-finding, which is reflected in her book ‘Tension Structures: Form and Behaviour’. Her work focuses on the achievement of structural forms by using nature’s own design principle: maximum stability and strength with minimum weight.
TEALEAVES: What is form-finding?
Lewis: Well, I certainly did not understand the concept, when I graduated as a civil engineer in Poland. The best definition I can come up with is that it encompasses methods of shaping structures by means of forces applied to them. Many methods are possible using physical and computational modelling. Why do we bother with form-finding? Because form-found structures exhibit maximum strength/mass ratio, as observed in natural objects; objects that are known to be highly optimised structures.
TEALEAVES: How did you begin your research into understanding form-finding?
Lewis: My journey to understanding form-finding design began with tension structures. Up until then, I believed that we could build any structural shape we like! Then, I learned that it is not possible to impose a geometric form on a tensioned surface – as it will always adopt its own, unique, minimum energy configuration that has to be found through form-finding, I have written books about it. Form-finding conforms to the “form follows force” principle; we start with a set of forces (usually permanent load) to define the form, then analyse structural response to imposed loads; in conventional design, the first step is missing.
TEALEAVES: What methods do you use to experiment with form-finding?
Lewis: It is easiest to explain principles of form-finding using physical modelling. So, we have:
Soap film models: with applications to tension fabric and ‘free form’ enclosures. It is worth noting attractive features of soap film surfaces. They are known as minimal surfaces, and are characterised by (i) constant surface tension), (ii) minimum surface area, (iii) zero mean curvature – every point is a saddle point. Because of these features, it is advantageous to exploit the concept of a ‘minimal surface’ in form-finding design of tension fabric structures.
Inverted hanging models: made of fabric or chains/cables; the principle here is that the structure is initially in tension, but when its shape is ‘frozen’ (in resin, plaster, etc), and turned upside down, it develops pure compression – a desirable structural action for a shell or dome structure. A chain model produces a series of optimal curves known as catenaries, and this relates to the work of the famous architect, Antonio Gaudi.
Inflatable membrane models: with application to pneumatic membrane roofs and domes.
TEALEAVES: How has the soap film analogy been used as inspiration for design?
Lewis: Practical designs realized using the soap film analogy included a garden canopy project of the famous architect Frei Otto from Stuttgart, dating back to 1957. As this was an early application of form-finding design, the structure was built by scaling up the geometry of physical models (made of soap film and fabric).
Another example of soap film design, this time, a semi-rigid, beautiful, ‘free-form’ structure is the ILEK building, formerly IL (Institute for Lightweight Structures), also known at the time as Frei Otto’s institute. The looped window was created by using a thread resting on a soap-film surface and drawing it upwards. The structure, is, in fact a pre-stressed cable net supporting rigid cladding. I had the privilege of working at IL, for short periods between 1989 and 1994, and met Frei Otto and his multi-disciplinary team of architects, biologists, mathematicians and engineers, all working on natural (minimal) forms of structures.
TEALEAVES: Do you have any modern examples of the soap analogy?
LEWIS: The modern example of practical application soap film analogy in design is an umbrella project in Madinah. These unique, hydraulically operated structures (250 of them) , are made of expensive woven Teflon. Each umbrella has 85’ by 85’ span, and in total, they cover an area of 1.6 M sq. ft . Given the amount of R&D and the use of expensive materials, that included gold, these are some of the most expensive structures in the world (working out at $5 m per umbrella). The designers: SL-Rasch from Stuttgart, who are my industrial collaborators, continue with Frei Otto’s vision of focusing on minimal designs.
The design of the umbrellas in Madinah used interactive computational and soap film modelling to arrive at the optimal shape of these structures. Research has shown that stability of a membrane surface modelled as a soap film is very good, because the whole surface utilises its strength in resisting imposed loads.
TEALEAVES: What are the initial steps in the design process for these forms?
LEWIS: In computational modelling of minimal (soap film) surfaces, we start with spanning an initial ‘guessed’ surface between boundaries and prescribe constant surface stress. This gives rise to the out-of-balance forces in the surface and lack of static equilibrium, because we are not starting with the correct surface. We then carry out incremental adjustments of surface geometry to restore equilibrium and continue with iterative calculations until static equilibrium, and the final form is reached.
TEALEAVES: What types of natural forms do we often come across?
LEWIS: Natural forms of rigid structures, such as shells, can be arrived at using an ‘inverted hanging model’ technique. I picked two examples of the work of a Swiss architect- engineer, the late Heinz Isler: Shell roof for the Garden Centre in Camorino completed in 1973, and roof over a gas station in Deitingen dating back to 1968. These shells are extremely thin (around 3’’ thickness) . These shells have withstood the test of time; they keep their integrity, look beautiful, do not crack, or deflect much.
TEALEAVES: Are there any famous examples of this technique being used?
LEWIS: An inverted hanging model technique employing chains was used by Antoni Gaudi of Barcelona. He used a set of chains loaded with little bags of sand as shown, which produced pointed catenary shapes. He photographed the model, turned it upside down to get a form that was to be used for Güel Colony church. Unfortunately, the project was not completed due to Spanish Civil war. We can presume that Gaudi used the same method in designing the famous Sagrada Familia. The elegant looking spires are an optimal form for that particular load case, but clearly, it was a well chosen load, as the structure, again, withstood the test of time.
TEALEAVES: Do you use computational models or physical models when you are illustrating?
LEWIS: I work building computational models, but use physical models for illustration. The basic computational form-finding steps are as follows: (i) the structure is represented by line or surface elements and gravitational loads are applied (ii) this puts the surface into pure tension; (iii) the surface geometry is then adjusted iteratively during computations, until full equilibrium is reached. In the final, inverted form, tension is converted to pure compression – a desired action for a shell or dome structure.
TEALEAVES: Finally, how do you connect form-finding to biomimicry?
LEWIS: Certain features of form-found structures are observed in Nature: for example, trees grow in the direction of constant surface stress; if their branch gets broken off, the healing process forms a minimal area over the ‘wound’. Tiger’s claws, another example, also grow in the direction of constant surface stress.
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