Stadium Roofs Offer Much More Than Shelter
Olympic feat
To anyone who followed press coverage of the preparations for the 2004 Olympics held in Athens, the facilities and infrastructure required for those games also seemed destined for late delivery. Although Athens won the Olympic bid in 1997, legal and bureaucratic obstacles delayed the start of construction on many projects until 2002. But in spite of the slow start, builders and organizers did make it to the finish line in time, with the bridgelike roof of the main stadium providing the games' most recognizable feature (see Building Types Study, page 232).
To shelter the seating bowl of this 1979, 75,000-seat open-air stadium, Santiago Calatrava, FAIA, designed a roof composed of a pair of striking steel suspension structures, which Calatrava calls "leaves," spanning 997 feet. The polycarbonate-clad fixed covers join at a single point at each end of the field. Each leaf is composed of a 236-foot-tall and 10-foot-6-inch-diameter steel arch joined by cables to a lower arched tube almost 12 feet in diameter. This lower tube counters, through torsion, out-of-balance loads from the curving roof.
Bolted to the torque tube are a series of transverse truss ribs, 16 feet 6 inches on center. The length of the ribs increases from midspan outward, with an average length of 165 feet. The structure touches the ground at only four points-at massive steel "shoes," more than 21 feet tall and 36 feet long, where the upper arch and torque tube merge.
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To finish the main stadium's complex structure on an ambitious timeline of about 18 months, Greek contractor Aktor built the roof in halves directly to the east and west of the stadium site, rather than assembling the roof in its final position over the seating bowl. The process allowed renovation of the existing stadium to move forward unimpeded by falsework and competing construction crews.
The steel erector assembled the top arch and torque tube sections, supporting them on temporary towers. After the upper and lower arched tubes were welded together at their ends, the ribs were bolted to the torque tube, and the cables partially stressed. Crews then used horizontal jacks to slide each assembly on stainless-steel tracks more than 200 feet over the stadium bowl. The last half was slid into place on June 4, 2004, leaving only several weeks for the roof's polycarbonate installation, final cable stressing, and other preparations before the August 13 opening ceremonies.
Construction crews followed a similar sequence for a Calatrava-designed covering for an existing velodrome that is part of the same sports complex. The 4,000-ton, metal-clad roof, consisting of a pair of double inclined arches joined by cables and supporting transverse ribs, was built in one piece on a site adjacent to the cycling track and then moved to its final position using jacks.