e-mosty September 2017: Mersey Gateway Bridge. Arenales Bridge. Mersey Gateway Bridge. Arenales Bridge. BLWT. - Page 11

3.3.3 Main bridge deck The main bridge deck is made from reinforced and post-tensioned concrete. It comprises 154 segments, each segment is around 33m wide and 6m long. Each segment was made in the same way – reinforcing steel bars are placed into the mould and about 130m 3 of concrete was then poured inside. The deck was generally cast in-situ in 6m long segments using a form traveller. For segments around the pylons, the form traveller was suspended from the pylon. The “C” shaped form traveller provided improved access to the top of the segment during construction in comparison to a conventional form traveller. Figure 10: Form Traveller casting standard segments The deck was built in three separate sections using a balanced cantilever method outwards from each of the three pylons at a rate of around one six metre section per week. This allowed the bridge deck to “grow” from either side of the pylons until it met the connecting bridge deck and the structure was complete. The largest number of deck segments - 33 - was cast from the south pylon leading to the south elevated approach viaduct and 31 were cast from the South pylon in the other direction towards the Central pylon. The North pylon had 29 deck segments cast on one side and 27 on the other, while 17 were cast from either side of the Central pylon. Another pair of machines was launched from the North pylon and then the third pair started from the Central pylon. From the third segment onwards, the connection anchor boxes called ‘delta frames’ were installed for the steel stay cables, which in turn were then attached to the upper pylon. The form travellers powered by a hydraulic system moved forward on a set of rails to the next position and the process was repeated. When the main bridge deck was complete, the form travellers were dismantled and recycled. Internal longitudinal post tensioning was provided in the top flange for the initial cantilever construction before the stressing of the first cable stays. These tendons were anchored on the end face and did not encroach on the internal formwork. Thereafter, longitudinal post tensioning typically consisted of 27 strand tendons anchored at the junction between either the top or bottom flange and the web and then deviated in plan. The tendons were installed and stressed after completion of the key segments between cantilevers. At the North and South pylons the deck is supported on pot bearings in the permanent condition. During construction a temporary longitudinal restraint and moment connection was provided between the deck and pylon via a system of concrete pads and additional temporary prestressing that clamps the deck to the pylon hammerheads. At the Central pylon, a monolithic connection was provided for both permanent and temporary condition. To support the deck during construction and to ensure stability of the balanced cantilever, two temporary piers were built under each of the main span cantilevers, approx. 72m from the pylon. Three pairs of form travellers operated to build the main bridge deck with each pair operating as a unit. The 270 tonne machines acted as movable concrete moulds. The works started with two travellers working in tandem. They were assembled at the South pylon before being lifted to their starting position around 25m above the riverbed. Construction teams then cast a pier table – a rectangular shaped platform – around the bridge pylon before preparing to start work on the main bridge deck. 3/2017 Figure 11: Form traveller machines positioned on each side of the hammerhead