As part of a consortium with Arbonis (VINCI Construction France), which specialises in timber structures, Freyssinet North is involved in work to repair and reinforce the Arena Stade Couvert indoor arena in Liévin (Pas-de-Calais, France).
The consortium's tasks include jacking the structure and repairing the principal rafters that form part of the stadium roof.
Concert and international sporting event venue
Liévin Arena Stade Couvert is a venue that primarily hosts the Liévin IAAF Indoor Meet and other sporting events; it is also used for concerts. It was extended between 2007 and 2009, increasing the seating capacity to 14,000 for concerts and 6,000 for sporting events.
Five transverse and longitudinal bridges are suspended from the structure, for installing the sound and lighting equipment used during events.
The middle section of the structure of Liévin Arena Stade Couvert is made up of six portal frames with a span of 75 metres, each consisting of two glued-laminated timber principal rafters 2.16 m high and 32 cm thick (2 x 16 cm).
On either side of the middle section, the rounded old structure is made up of principal rafters forming the hips.
Microcracks in the principal rafters
The arena has been closed to the public since October 2012, when microcracks were discovered in the glued-laminated timber principal rafters (these elements form the roof of the middle section of the stadium, completed in 2009).
After lengthy surveys and discussion, the project got under way in mid-2015 and the work could be planned. Freyssinet North responded to the invitation to tender in a consortium with timber structure specialist Arbonis (VINCI Construction France).
Repair and reinforcement of the roof structure
The operation was launched in two phases, a firm phase relating to the (new) middle section and two conditional phases relating to the hips (old sections).
The consortium is responsible for:
- installing floor protection throughout the stadium;
- assembling scaffolding to reach the principal rafters and shoring towers to enable the jacking of the principal rafters;
- removing the services (fluids, electricity);
- jacking the structure to restore it to its initial condition;
- repairing the cracks in the glulam principal rafters;
- installing metal tie rods;
- removing the jacks from the structure to activate the tie rods;
- resurfacing the athletics track.
Highly accurate jacking
Twenty-four jacking points were identified with movements ranging from 5 to 15 cm and loads varying from 5 to 30 tonnes. Decking made up of several layers of metal battens and sections was installed on the floor to distribute the loads from the shoring towers. At the top of the 18-m high shoring towers, metal profiles were installed, assembled and welded in order to distribute the forces over all of the tower legs. Mechanically welded components equipped with sliding pads were fitted to the jack heads. End of stroke sensors were installed on the ridges to check that there was no horizontal or vertical movement of the join between the two principal rafters.
The twenty-four points were jacked simultaneously using the SCCM computer aided lifting (CAL) system, assisted by a hydraulic control panel for the six additional channels. The jacking was controlled on the basis of the load in order to relieve the stress in the principal rafters and reverse the sag to enable the installation of the tie rods and studs. The movement and load at each jacking point was monitored. Unlike in conventional jacking, the jacking was stopped when the loads were taken up rather than on the basis of movement.
- Large amounts of protection had to be put in place to prevent any damage to the very new equipment and infrastructure of the stadium.
- To protect the athletics track, which has a low bearing capacity, load-distribution decking made up of several layers of intersecting battens and metal sections had to be installed.
- The geometry and location of the five transverse and longitudinal bridges suspended from the principal rafters were such that the installation of the scaffolding had to be very carefully thought-out to avoid collisions.
- Non-jacked bridge sections had to be detached in order to enable the vertical movement of the jacked bridge sections.
- The jacking of this unusual structure at twenty-four points required many days of consideration and discussion with the main contractor before the jacking procedure was established.
The preparatory phase started in August 2015 and work began in September 2015. Work is scheduled for completion in June 2016.