Form finding structures: representation methods from analog to digital

In this paper we present a comparison between different representation methods for form finding structures. The aim is to test the tools used in the present to represent and to generate shapes built in the past. We intend for form finding structures all the shapes that are characterized by minimizing the use of materials, these shapes are the result of an optimization process where the structure itself defines its own shape based on its figure of equilibrium under applied loads. From hanging cables used by Gaudì to inverted hanging membranes built by Heins Isler, the only way to define the optimal form, before the computer age, has been the systematic use of physical models. The Isler's method was based on physical models that he measured to produce resin models on which he could be structural tests. This method, that looks very simple, wasn't copied because you need a very accurate survey of inverted hanging membranes to make a good test models and you have to choose between many possible alternative solutions. We think that one of the main reasons of poor diffusion of this method has been the lack of tools for accurate survey and representation to simplify the management of form finding process. We have investigated two case studies: the catenary cable structure of David S. Ingalls Hockey Rink, at Yale University, designed by Eero Saarinen in 1958 and the inverse hanging membrane of the Naturtheater Grötzingen designed by Heinz Isler and Michael Balz, in 1976. First of all we have built the physical models using the same process of designers and then we have built the same shapes using new tools, the parametric modelling plug-in Grasshopper that allows a generative form-finding process in a digital environment. Our goal is to show how the new 3D modeling tools are changing the relationship between architecture and engineers especially when the "structure" is the "form", such as in the case of so called form finding structures.