Largest iris-style retractable roof

The Mercedes-Benz stadium in Atlanta, Georgia, USA – home to the Atlanta Falcons – boasts the world’s largest circular retractable roof, covering an area of 5.8 hectares (14.5 acres), with an oval opening measuring 104.35 m (342 ft 4 in) in diameter at its widest. Designed by architects HOK and engineers Buro Happold, the stadium roof consists of eight intersecting "petals" – each 67 m (219 ft 9 in) long, 23 m (75 ft 5 in) wide and weighing 453.5 tonnes (500 tons) – with the whole supporting system using 19,050 tonnes (21,000 tons) of steel and 16 electric motors.

The eight wedge-like sections of the roof open and close using an action similar to that of a camera's aperture. In a demonstration to the press on 25 July 2018, the stadium roof opened in 8 minutes 10 seconds, and closed again in just over 7 minutes. The official cost of the stadium was $1.6-billion (£1.19-billion); it seats 75,000 spectators, expandable to 80,000 for marquee events.