The immense size of the project and its particular technical complexity were the causes for long delays; the work was finally completed in time and the new captivating stadium was officially reopened on July 30, 2004 (O.A.K.A. 2005).
The Athens Olympic stadium roof in the form of a double bowstring tied arch of 80 m high is considered one of the most ingenious modern architectural and engineering projects. This impressive construction of steel and glass is today one of the city's landmarks. Along with its spectacular and pioneering architectural design, the suspended arched roof construction and erection has been a challenge to engineers and an excellent example of integration of construction engineering and hydraulic expertise (Siriani & Di Silverio 2006).
The roof structure is in two halves, each comprising a top arch of a diameter of 3.25 m and a lower torsion tube of a diameter of 3.6 m extending for 304 m over the stadium. The wall thicknesses of the tubes are up to 95 mm. All four tubes merge at their ends and are based on four support steel shoes (19 m x 4 m x 4 m) made from 100 mm thick steel plates. The tubes support wire cables that hold polycarbonate panels of a weigh of 17,000 ton in total. 220 girders cantilever out of the torsion tube supporting the panels, thus forming the roof deck. Finally, the panels are covered by a special coating to reflect 60% of the sunlight. Apart from providing shadow, protecting athletes and spectators against the hot Greek summer sun, the roof served as a carrier for telecommunications and security systems during the Games (Siriani & Di Silverio 2006). The layout of Calatrava's roof can be seen in figure 1.
Fig.1. The Athens Olympic stadium layout and dimensions of the roof (redrawn from Siriani & Di Silverio 2006).
The roof construction project was contracted to the Italian Costruzioni Cimolai Armando Spa of Pordenone. In detail, the stages of the building process were segment fabrication and pre-assembly, segment shipping to construction site, on-site assembly and erection of the two halves of the roof at a distance from the sports arena and finally assembly pulling (sliding) into position. With the exception of the sliding step that was sub-contracted, all remaining stages were undertaken by Cimolai (Siriani & Di Silverio 2006). A number of difficulties arose during the renovations works that were mainly linked to the immense character of the project and the shortage of available time; these issues required technically advanced, ingenious solutions.
All steel fabrication works took place at Cimolai's site in Italy, using a 6000 ton press, one of the largest in Europe. The support shoes and pre-assembled tube segments of up to 15 m in length equipped with cable anchorages, diaphragms and girder connections were constructed there and were subsequently shipped by sea to Athens. Shipping has been a gigantic operation due to the excessive size of the segments of the roof (Siriani & Di Silverio 2006).
A second particularity of the project was that roof assembly and erection could not take place at its final position, so as to allow other renovation work in the stadium to proceed during the assembly process. Thus each of the arch structures was put together separately on each side of the concrete stadium, at 70 m far from either side. Roof