photo credits: J. Wayman Williams  





Grötzingen Open Air Theater
photo credits: Frank Doring

How does Isler create the models for his hanging membranes reversed?

Heimberg Tennis Center
photo credits: J. Wayman Williams



“One does not actually create the form; one lets it become, as it has to according to its own law.”

Heinz Isler (born 1926) graduated from the Eidgenössische Technische Hochschule (Federal Institute of Technology) in 1950 with a degree in civil engineering, continuing on as a teaching assistant for Lardy from 1951–53. He later defined the invaluable lessons Lardy ingrained in his engineering students in this way: “a) that we have a sense for esthetics b) that we have the right to use it c) that we are allowed to mention our opinion d) and that we can find and express it in our projects.” For Isler, this training proved far more lasting than any on engineering theories.

Isler’s greatest contributions to the history of structural art were in the area of concrete shell constructions. After experimenting with pneumatic forms to create shell shapes, he discovered what he came to believe was the best method for creating these forms, the hanging-membrane reversed. His inspiration came on a building site in 1955. He saw wet burlap draped over a mesh of steel bars in such a way that within one square opening the cloth hung in a domelike shape solely of its own weight. From this observation he concluded that the cloth carried itself in pure tension, so that when reversed it could be the form for a concrete shell under pure compression. This last observation was critical because while concrete does not function well in tension—cracks will develop—it functions beautifully in compression.

Although Isler began to experiment with hanging-membranes reversed as early as 1957, this idea became a dramatic reality only in 1968 and led to a series of masterpieces, including the Grötzingen outdoor theater and at the Heimberg Indoor Tennis Center). He developed a meticulous approach to their construction. First, he would find a form through the use of models. Next, he would test the forms by the use of other models. Finally he would have the structures built according to a carefully designed construction procedure.

A further consideration for these buildings is thermal gradient. A concrete roof, even in compression, can develop cracks if it is subject to disparate temperatures across its surface. Isler had solved this problem earlier in his career while working with pneumatic forms. He used a fiberboard as the construction formwork for the finished shells, which was then left in place anchored to the concrete. The fiberboard acted as insulation so that the concrete exposed to the elements would have no thermal gradient—i.e., the inside concrete surface temperature would be nearly the same as that outdoors.

© 2003 The Princeton University Art Museum