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