The roof was an afterthought that became the building. Behnisch & Partner had modelled the stadium, arena and swimming hall side by side when a colleague, Cord Wehrse, having read a newspaper piece on Frei Otto's tent at Expo 67 in Montreal, proposed draping one canopy over all three; with Carlo Weber and Heinz Isler the competition model was finished using wooden rods and parts of a lady's stocking. Otto joined as development consultant, Fritz Leonhardt and Wolfhardt Andrä as engineers, Fritz Auer as planning lead. Otto worked by trial and error on ever larger physical models while Andrä and Leonhardt computed other parts. The canopy covers 74,800 square metres in all, 34,550 of them over the stadium, carried on two 70-metre masts and six smaller ones. Acrylic was chosen over concrete or timber because an opaque roof would have thrown shadows and ruined television pictures. The roof was completed on 21 April 1972 under the engineer Jörg Schlaich.
The lightness was a political programme, and it had a cost. German Wikipedia states that the tent roof spans the stadium, the indoor arena, the swimming hall and the paths between them, and that the stadium is thereby meant to symbolise lightness, transparency and openness. The same encyclopedia records the consequence: on the morning of 5 September 1972 eight members of the Palestinian organisation Black September climbed the fence of the Olympic Village and entered the Israeli team's apartment at Connollystraße 31; they had no difficulty overpowering the athletes because the doors were unlocked and security during the Games had been deliberately kept loose in order to display the change Germany had undergone since the 1936 Berlin Olympics. Eleven members of the Israeli team were taken hostage and killed; a failed rescue attempt at Fürstenfeldbruck airfield left the remaining hostages, five of the attackers and a German policeman dead. The Games were continued after a day of mourning.
Wikidata Q131610 · Commons: File:Blick vom Olympiaberg auf das Olympiastadion.jpg · Photo: Amrei-Marie · Licence: CC BY-SA 4.0