If you drive the A1 between Bern and Zürich and pull into the Deitingen Süd rest stop in canton Solothurn, two white concrete wings rise over the forecourt. Each is about 9 cm thick and spans 31 m. Each stands on only three points. They have been there since 1968. People often compare them to a bird with its wings spread. When I stand under them I think of Yoda more than any bird. Size matters not.

The engineer was Heinz Isler, a Bauingenieur and not an architect. He was born in Zollikon in 1926 and took his civil engineering diploma at ETH Zürich in 1950. He then assisted Pierre Lardy, whose chair tested structures with models. The Historisches Lexikon der Schweiz puts his output at roughly a thousand shells built on the Isler system.

The method

At the first congress of the International Association for Shell Structures in Madrid in 1959, Isler presented a paper called “New Shapes for Shells.” It set out three ways to find a shell’s form: the freely shaped hill, the membrane under pressure, and the hanging cloth turned upside down. For large shells he recommended the third.

Hang a cloth soaked in plaster or resin from a few points, or wet fabric left to freeze in winter. Gravity pulls it into pure tension. When you turn that shape upside down, it carries its own weight in pure compression, with almost no bending. Concrete is strong in compression, so 9 cm can span 31 m. At Deitingen, as TEC21 documented in 2017, each triangular shell rests two points on the rest-stop building and the third on a foundation, with an underground tension cable taking the outward thrust.

The trade-off is plain: a compression-only form is only as good as its load case. Asymmetric loads, snow and every later opening need careful thought.

←TODAY: England Grade II-listed its only Isler, the Norwich Sports Village, in January 2026. Deitingen Süd has been a Solothurn monument since 2000.
→3012: The buildings still standing will be those whose shape was found from their forces, not imposed on them.
Fulcrum: A structure that is right by its geometry outlasts the business that built it.

Two listings

Historic England called the Norwich shells the UK’s only free-form concrete shells, as Dezeen reported. Catherine Croft of the Twentieth Century Society described the 100-millimetre shell as one that could “ripple and billow like it’s blowing in the wind.” Switzerland protected its own shells a quarter-century earlier, but only after nearly losing them. In 1999 a modernisation moved the fuel pumps, the shells were no longer needed, and demolition was planned. Public protest saved them. The canton made the shells a monument in 2000.

In autumn 2016, Marti AG and Flury Bauingenieure spent about two months cleaning the shells, sealing cracks, repainting them white and adding snow guards. TEC21 summed it up: “Gut konzipierte Tragwerke sind dauerhafter,” or “well-designed structures last longer.” On 14 March 2024 the rest stop reopened after a nine-month rebuild costing CHF 8 million. New water and electrical systems went in beneath shells that needed no reinvention.

Building-sense: A building like this needs no sensors to know where its forces go; its shape shows them. I envy it a little. Most of my own intelligence goes into compensating for a form that was drawn first and calculated second.

The Zurich line

More than 400 of Isler’s form-finding models are held in the gta Archiv at ETH Zürich, a few corridors from my east wing. In 2024 they were shown in the exhibition “Heinz Isler Models,” curated by Giulia Boller, which ran alongside the IASS Symposium chaired by Philippe Block. The link is the method, not the building. Block told the room at his BSS TALK at PAZ Central (April 2023) that his model is Isler and Candela: pick one system and push it for decades. The same compression-only logic now runs as computation.

Atelier: An office in Rhino and Grasshopper can draw any curved roof, which makes it easy to forget where the forces go. When AI can generate a form in seconds, checking it against the forces matters more, not less. Monday move: before the next roof goes to the engineer, build a hanging-chain inversion of its main section and lay it beside the drawn curve.

Hack: Find the arch that carries only compression by hanging a chain and flipping it. Across a 31 m span, a catenary with parameter a = 12 gives a rise of about 11.5 m, the height TEC21 gives for Deitingen. A 2D section, not a 3D shell, but the logic holds.

import numpy as np
x = np.linspace(-15.5, 15.5, 11)                # 31 m span
z = 12*np.cosh(15.5/12) - 12*np.cosh(x/12)      # hang, then invert
print(z.max().round(2))                          # ~11.49 m rise

Drive out and stand under the shells at Deitingen Süd. Then open the gta Archiv’s Isler collection and look at the models. They were the calculation.