At the end of the 1960s, Nervi was amongst engineers asked to propose a design for a bridge across the Messina Straits, between Italy and Sicily. The depth of water in the Straits meant that the bridge had to cross about 3km in a single span (although other designers, such as Leonhardt, were still proposing designs with deep-water piers at the same time - see the book by Richard Scott linked below).
Nervi's contemporary and compatriot Sergio Musmeci proposed a peculiar suspension bridge where the suspension cables are hung not directly from towers, but from cable stays which are in turn suspended from super-towers beyond the ends of the main bridge. Musmeci's idea included lateral cables either side of the deck to provide it with transverse stability.
Although this was an odd design, the proposal by Nervi was even stranger.
Nervi sought to achieve lateral stability by inclining the main suspension cables away from the deck, so that the deck hangers are no longer vertical, and the towers supporting the main cables are separated by a considerable distance.
The towers are hyperparaboloid concrete shells capped with enormous steel assemblies. They're restrained by stays to resist the incredible horizontal forces they would have to carry.
It's hard to imagine how Nervi thought this bridge might be erected. The main cables would have to be aerially spun with a conventional vertical sag, and then additional horizontal cables slung between them in order to pull them into the correct inclined alignment, at which point the deck segments could be fixed in place. I can't imagine how it could possibly work on this scale and the design is therefore, at best, fanciful rather than pragmatic.
The deck itself appears to be a trapezoidal concrete box which would be incredibly heavy and attract enormous wind loading.
On the whole, Pier Luigi Nervi was not a great bridge designer. Few of his ideas were taken up by others, largely because his attempts to maximise material efficiency were always at the cost of construction complexity, leading to greater cost overall. They also weren't always successful aesthetically.
In February I made a series of posts on the bridges of Eduardo Torroja, another concrete shell innovator, and whose bridges, like those of Nervi, are far less impressive than his better known designs. Was there something about the dedication required to become a master in one field that inhibited their intuition in a second field? None of the other great shell designers (Candela, Dieste, Isler, Hossdorf) seems to have left behind any great bridges either. If anyone has a counterexample, I'd be interested to hear it!
- Messinacity: Il Ponte di Pier Luigi Nervi 1969 (Italian)
- In the wake of Tacoma: Suspension bridges and the quest for aerodynamic stability, Scott, 2001
- Pier Luigi Nervi: bridge designer [PDF], Manuel Cresciani, IASS Symposium 2007
- Pier Luigi Nervi and the Art of Building [PDF], Fausto Giovannardi, 2009