Heinz Isler

"...I do not say any form which you construct this way is a good form, or must lead to a good solution; but there are forms which can lead to good solutions, and of course that is only the first link in a whole chain of investigations, and the other links in the investigation, model tests, measuring of the first structure, or a model test in scale 1:1 as we have it out here, these are of primary importance. So the engineer[‘s] problem is remaining all the same, but it is the first link, here, the shaping which has been lacking up to now, and this method can lead to a very nice solution."

"[Architects and engineers] must be willing to subordinate themselves to the emerging logic of the shell’s form as it evolves through experimentation."

"Among others there are three methods for shaping shells: the freely shaped hill, the membrane under pressure and the hanging cloth reversed."

"[T]heoretical considerations and derivations … are always based on severe simplifications of the assumptions."

"Heinz Isler’s innovative methods for determining the shape of reinforced concrete shells first became widely known through his presentation of a paper entitled ‘New Shapes for Shells’... At that time reinforced concrete shells were a very popular form of construction worldwide. However, their forms were almost entirely those which could be described easily by geometrical and mathematical formulae e.g. barrel vaults, spherical domes, conoids, and constructed using relatively simple formwork as in the case of the straight boards used for hyperbolic paraboloid surfaces. ...[A] rounded mound of soil encircled by a trench and captioned “Form for a shell in concrete”; a stepped swimming pool of organic shape dug into the ground, labelled “Plastic shell as swimming pool”; and some rectangular ‘bubble’ shell roofs... illustrate the two of the three form-finding methods..."

"The majority of Isler's shells are specifically designed to remain in compression under all load conditions, but in the case of the hyperbolic paraboloid [Heilig Geist Kirche (Holy Spirit Church), ] both tension and compression stresses normally occur. To overcome the adverse effects of the tension stresses Isler applied a pre-stress through the shell surface by post-tensioning using eight cables between the wall tops... This he simulated in the workshop model... as usual for Isler's experimental technique, the roof loads were applied by means of small wooden discs distributed evenly over the surface to simulate uniformly distributed load. In turn these were connected to a hanging network of timber spreader bars and strings in such a way that the distributed load could be applied with a single weight or hydraulic jack. The pre-stressing force was applied by small hydraulic jacks. This arrangement allowed vertical, horizontal and pre-stressing loads to be applied in appropriate combinations, simulating the shell self-weight, full snow load, partial snow, etc. Uplift loads induced by the wind were assessed by appropriate factoring and reversing the stresses (derived from electrical strain gauges attached to the model surface) resulting from downward loads."

"Heinz Isler's involvement in the development of the design for the "Steinkirche" ... could not be more different from that of the church at Lommiswil. ...[A] competition... [was] won by Werner Schmidt... Isler was consulted as an expert in shell design and construction. To understand the structural behaviour... and, in particular, the effect of these glazed openings, Isler returned to his tried and tested methods using physical models. ...To transfer forces across the openings in the shells, the sides of the cut were to be linked by steel bars, whilst the glazing was installed in the plane of the cut surfaces of the removed wedges. ...Isler made plaster casts and Styrofoam models... From these he cast thin latex rubber shells complete with the cut-out openings and with flexible cords linking the two sides... These rods were disposed in a diagonal "zig-zag" configuration similar to that of the lacing on a traditional leather football or shoelaces... When the rubber surface was loaded it was easy to see how the flexible cords behaved. ...compression near the top of the ovoid forms and in tension towards the base. In the final scheme the forces are carried by hightensile bars... To enable the same anchorage detail to be used in all locations, Isler developed a "brush anchor"... Heinz Isler's... method for determining the shape of his shells was to accurately measure a plaster cast of the form."

"Heinz Isler strongly believes that the work of the architect and the engineer cannot be separated in shell design because of its inherent integration of aesthetics and statics. [An] ideal architect-engineer relationship is epitomized by the collaboration of Heinz Isler and Michael Balz... Isler taught Balz various form-finding methods, and consequently the architect began to design his own creative shells by exploring hanging membranes and inflated forms. Although Isler and Balz only worked on a few projects together in Germany, the resulting shells (including the , the Stetten Theater, and the Balz House) are some of the most elegant works that Isler produced in his career."

"Isler’s form-finding method of the reversed hanging cloth was discovered serendipitously in the summer of 1955. On a building site he saw a piece of wet burlap draped over a mesh of steel bars. He noticed that within one square opening, the burlap hung in a domelike shape under its own weight. Isler concluded that the cloth carried itself in pure tension, so that when it was reversed it would become a form in pure compression. ...the three-dimensional version of Hooke’s discovery; a piece of cloth that is hung from several fixed points will create an ideal form that is completely in tension. If the shape is “frozen” and flipped, the resulting shell should be in complete compression, which is convenient for concrete structures since concrete performs well in compression but poorly in tension. The main difference between the work of Gaudí and Isler is that the Spanish architect found his form through a network of two-dimensional catenary shapes while the Swiss engineer only used one hanging element (a piece of fabric) to determine the ideal form of his structure."

"Isler’s methods were completely based on physical modeling and experiments. In fact, he rarely used general mathematical theories when designing his shells."

"is a reinforced concrete shell with the shell thickness varying from 9 cm to 12 cm [3.5 to 4.7 in.]... Since the structure is in compression, instability is an issue, particularly at the edges and supports of the shell. Isler does not use any stiffening edge beams in the Naturtheater Grötzingen; instead, he upturns the edges (which corresponds to the overhanging fabric in the hanging model) so that the double curvature required for increased stiffness is achieved without any increase in shell thickness... However, to reduce the stresses in the supports, Isler simply tapered the legs so that they were thicker than the rest of the shell."