In the early hours of the morning of June 22, 2019, a sudden and violent thunderstorm dumped hailstones, the size of a tennis ball, with unprecedented violence, at a speed of 150 km/hour, on the roofing structure of the square next to the Unipol Tower at 6 Via Larga in Bologna.
The metal carpentry lattice structure, supported only on three points, which covered the square for almost 4,000 square meters, withstood the hurricane's stresses without any problems, but the roofing mantle consisting of "air-inflated cushions" made of translucent ETFE material, in the shape of rhombuses with sides of about eight meters, hit by the violence of the ice projectiles were torn at the top and filled with water.
The pillows, emptied of water, were demolished, and from that date on the square remained the metal structure without a covering.
The violence of the incident and the well-founded fear that it could happen again required a long time for analysis, laboratory tests for new materials to be adopted, and search for an appropriate solution from a formal and performance point of view.
The material chosen was a PVC-coated reinforced polyester, which when subjected to equivalent stresses with projectiles at speeds up to 200 km/hour, passed the laboratory tests with ease.
The sheet was shaped for each of the 247 rhomboidal fields and stretched with a structural bow on the diagonal, creating on the extrados a rich cusped pattern with one part lit by the sun and one in shade, and, on the inside, a pleasing play of light.
Finally, in 2022 the new roof was being started and completed.
Photos by Vincenzo Ruocco