of the transverse girders. Grade 460 high strength
steel was adopted for the clevis plates and hemispherical bearings were also incorporated to enhance
the out-of-plane rotational capacity, thus providing
greater installation tolerance. The installed north bank
arch nodes, the ties and the temporary props are
shown in Figure 5.
At the same time the ladder deck is assembled on the
temporary supports, the arches and bracing will be
assembled on the south river bank. Following
completion of the installation of the ladder deck
steelwork, the arches and the plan bracing will be
installed via a tandem lift by two large cranes from
the river bank. The arches will be offered to the end
nodes in a prescribed sequence. Temporary tie cables
will be used to control the shape of the arches to
facilitate accurate placement. Once the site joints are
welded, the hangers will be installed and stressed in a
prescribed sequence.
The “piecemeal” erection methodology induces
bending moments in the tie beams that are generally
avoided in network arches. In order to minimise the
magnitude of these moments, the stressing of the
hangers will commence as soon as the structural
steelwork is erected, before the installation of the
permanent formwork and deck.
This first phase of hanger stressing will relieve the
temporary works and it will transfer the gravity loads
in the arches and the tie beams. The hanger
installation and stressing pattern is aimed at avoiding
“slack” hangers in working conditions. The stressing
will commence from the centre outwards so that the
temporary towers are relieved after the first few
stressing operations.
Once the first phase of stressing is complete the
concrete deck will be constructed in sections.
Following the initial cure of the deck slab a second
phase of hanger force tuning will be applied so that
the target in-service performance of the hanger
network is achieved.
It is worth noting that the construction sequence was
refined on a number of occasions to suit various site
parameters and this has led to extensive discussions
between the designers, the fabricator and the main
contractor and their cooperation was key in the
successful and practical step by step construction
methodology.
Fig. 5: Current construction stage
4/2016