Post-tensioning System Stabilizes first Movable Stadium Roof in Lille

Context

The flexible use of the stadium is ensured by a special lifting platform at the northern end of the pitch underneath which there is an event arena with additional galleries for nearly 30,000 visitors.

The movable roof of the stadium is carried by two main lattice girders that are connected to two transverse girders consisting of steel lattice formwork. Additional movable transverse girders are placed on the main girders, permitting the opening and closing of both halves of the roof in a mere 30 minutes.

The two longitudinal main girders have a span of approx. 205m and a height of 16m. The bottom chords of these steel lattice girders contain five tendons each in order to increase their load carrying capacity and limit deflection. These tendons are located between the bottom A chord plates and anchored at the joints. Two tendons with 55 strands each are installed along the entire length of the bottom chords, and three tendons with 37 strands each strengthen the middle section of the bottom chord.

Solution

Of particular note are the two ends of the girders, where the bottom chord is deviated upwards at the last joint, and the tendons are deflected by approx. 15°. For this purpose, DYWIDAG developed a special deviation saddle in which the 55 PE coated strands are guided individually. The deviation saddle consists of an exterior galvanized steel tube containing individual embedded PE tubes. The cavity between the steel tube and the PE tubes is filled with ultra high performance grout. The prefabricated deviation saddle is placed in an additional curved steel tube that is welded to the steel girder.

The principle of individual strand deviation makes it possible to replace both individual strands or the complete deviation saddle. Similar deviation saddles have already been used for stay cable or extradosed bridges.

However, in these cases, a load change may not result in a relative displacement of the strands in relation to the saddle. In the case of the Lille stadium, such a relative displacement with minimum friction is explicitly required. The displacement results from the change of forces in the tendon due to the flexible opening of the roof.

In order to determine friction and to prove the durability of the strand PE sheathing as well as the load-bearing capacity of the deviation saddles, a comprehensive testing program with up to 10,000 load cycles was successfully carried out in DYWIDAG’s test laboratory in Unterschleissheim. The tests were performed in close coordination with the owner and supervised by the Universität der Bundeswehr München.

HDPE-coated, galvanized, waxed and PE sheathed 0.62" strands with a nominal tensile strength of 1860N/mm² were used for this major project. DYWIDAG supplied 8 deviation saddles for 55 strands and a total of 20 Type DG-P 37 and DG-P 55 DYNA Grip Anchorages that are usually used for stay cable or extradosed bridges.

All of the tendons are equipped with DYWIDAG DYNA Force Sensors that make it possible to continuously monitor the strand forces. For this purpose, a total of 38 DYWIDAG DYNA Force Sensors were installed at defined strands near the anchorages. These sensors are connected to several multiplexers and a readout unit in the roof command room of the stadium.

DYWIDAG has been able to successfully prove the versatility of the DYNA Grip system in this major project. The Lille stadium shows that tendons can not only be used in classical prestressed concrete construction, but also for pure steel structures.

Completion of the stadium is planned for July 2012.