Running behind the 1960 Olympic sites is a fairly modest highway overpass that connects the Via Flaminia to arterial roads north and east. It was part of the planning for the Games, and also part of Nervi’s commission. While it’s hardly as eye-catching as the Palazetto, it’s worth a look on its own.
The supports are made of in situ concrete. They’re wide at the base, and narrow at the top, as you’d expect in a viaduct that required some ductility. But it’s how they’re that shape that’s impressive. A constant theme in Nervi’s later work involves concrete supports that change section gradually along their length, in this case a very subtle transition from a broad diamond shape at their base to a square at the top. The resulting form is easy to define with a series of straight ruling lines–connect the corners of the top shape with the midpoints of the base, midpoints of the top to corners of the base, then divide the resulting line segments into equal parts, connect those with straight lines, and you have a series of curves surfaces defined by straight lines.
These shapes were useful to Nervi because they allowed rotational freedom in one direction at the top, and in the opposite direction at the bottom. Here, I suspect they were designed to allow the overpass deck to rotate slightly under differential loading while maintaining a robust, fixed connection to the foundations (happy to be corrected by the commentariat, here…)
Building for work to make these shapes, of course, was something of a trick. But looking closely at the concrete itself provides a clue:
What you’re seeing there are the impressions left by nail heads sticking out of board-formed concrete–a leftover that would have been eliminated by higher standards in more architectural concrete. Here, in a highway overpass, they were apparently acceptable. Each support has three or four of these lines of nail heads, and I think these show that the form work consisted of narrow, thin boards, each of which was slightly twisted between nailing strips to achieve the gently curving surface. The edges of the boards, being straight, followed the ruling lines of the geometric shape, and the nailers were designed to force each board into a very slight twist, one that made up the difference between one ruling line and the next.
To me, this is a perfect example of Nervi’s clever form work detailing–he was consistently able to achieve stunningly complex forms with relatively crude methods. This was in part due to his insistence on maintaining a full concrete laboratory and yard on the outskirts of Rome, where he and his office could experiment with techniques and materials before they went on site. In this case, it would have been important to get the thicknesses of the boards exactly right, for instance–too thin and the hydrostatic pressure of the concrete would have warped the boards between nailers, too thick and the boards would have been too difficult to twist,
There are other, more impressive examples of these sectionally-transforming supports–the Palazzo dello Sport in Rome, also done for the Olympics, and the Palazzo del Lavoro in Turin, done about the same time, have quite different takes on the idea. I have tentative appointments to see both in the next couple of weeks, and. I’m curious to see whether the tell-the-tale nailer details are evident or not…