Robert Maillart's Salginatobel Bridge proves that a work of engineering can become a work of art with no ornament added, through the right expression of forces alone. Maillart was not an architect but an engineer; yet his bridge is counted among the most beautiful reinforced-concrete structures of the 20th century.

The bridge crosses a deep Alpine valley with a single reinforced-concrete arch. Maillart's invention is to design the arch not as a solid block but as a hollow box section: the arch is a closed tube of thin concrete slabs above and below joined by side walls. This void minimises material — and so dead weight; concrete is placed only where the force flows, and only as much as needed. The arch carries three hinges — at mid-span and at the two springing points; these hinges release the stresses caused by temperature change and ground settlement, letting the bridge 'breathe' without cracking.

The result is a form in which the thickness of the material thins and thickens according to force, so that the arch draws an elegant profile, slender at the centre and widening at the supports — form is the visible state of structural logic directly. Maillart adds no shape for aesthetic concern; beauty arises of itself from the honest expression of forces. Salginatobel is a masterpiece of the tradition the engineering historian David Billington called 'structural art', and it shows that reinforced concrete can produce not monumental mass but a slender, calculated grace.