Oh, this is fantastic…
I’m going through the various Nervi notes, books, and archival material that I’ve been collecting over the last year or so, and inevitably starting to find connections between projects that I hadn’t noticed before. This, to me, is the best part of a research project, because at some point the themes become pretty clear and for a while it feels like everything has its own momentum.
Take the Ponte del Risorgimento, for example, a bridge in Verona that Nervi designed in 1962. Like many projects constructed after the crest of the miraculous Italian economy of the 1950s, it suffered from the country’s sclerotic bureaucracy and wasn’t built until 1968. It’s never gotten quite the attention that other Nervi projects of the era have, and I have to admit that when I saw it last summer I was a bit underwhelmed. The sides of the bridge undulate so that the structure is widest over the supports, in addition to being deepest. The depth story is pretty intuitive–it’s a doubly supported beam, and this family of structural types has its greatest bending moment over the two supports. (OK, can’t resist–here’s the classic doubly supported beam, by Nervi student, employee, and disciple Myron Goldsmith:
Anyway, you can see the similarities, and at the time I chalked the Ponte up as a good example in concrete that might pair with the UAL hangar as another good example of a statically-shaped doubly-supported beam.
But, I’ve been going through Nervi’s variable section pier designs this weekend, and something clicked. These piers are some of his better known structural elements, about which more later, but for now it’s enough to say that they were made by twisting straight boards to match stacked metal jigs, connecting an ideal shape at one end with a (different) ideal shape at the other, creating a set of ruled surfaces:
That’s one of the classics, from the Palazzo dello Sport. And if you look closely at that pier, or at the image of the Ponte, above, you can see the timber form marks–both geometries were figured out and formed in the same way, by setting out the end sections as well as a set number of transitional shapes between, and then laying straight lines/boards into them.
So why would you go to the trouble to make the deep parts of the bridge bulge out, in addition to bulging down? Down gives you greater section modulus over the support, to counter the greater bending, but what does out get you, besides a little extra lateral stability in a heavy piece of structure that surely doesn’t need it?
Ah, this is a lesson in doing your reading. Mario Sassone writes, in his essay on the bridge in the exhibition catalogue to Pier Luigi Nervi: Architecture as Challenge:
The cross-section of the bridge varies not only in height, but also in form, going from a trapezium shape in the abutment area to an upside down trapezium in the arch area. The width of the compression zone is then always wider than the width of the tension zone, following the alternation of moments that compress the lower fibres in the supports and upper ones in the bay.
Bingo. Nervi could handle the tension aspect of a really big bending moment by adding more steel, which doesn’t do anything to change the shape of the bridge. But to handle compression, he had to add more concrete, and the further that extra material from the center (neutral axis) of the bridge’s section, the better. So the angle of the board forms changes as the need for additional compressive material changes; there’s more on the bottom of the shape over the abutments, because the bridge is getting bent in that direction, but where the moment ‘shifts,’ and causes more compression in the top of the section, toward the center of the span, the boards start to lean the other direction, making the top of the deck wider than the bottom.
Subtle, but genius, and all of a sudden what had been a nifty but less than remarkable bridge becomes part of Nervi’s long-running experiment in ruled surface formwork and an integrated example of not only section modulus at work, but also of the complex moment distribution across the section of a doubly supported beam.
Any morning with one of those moments of clarity is a pretty good morning. Celebrated with a walk down to Forno Campo di Fiore for lunch.