From aesthetic considerations it was essential that the
new bridge should be a good neighbour to the
existing two bridges and not visually dominate them.
In the earlier feasibility study the new bridge was
envisaged to be a 3-tower cable stay bridge with the
central tower conveniently sited on Beamer Rock,
which was a sensible solution. As the extent of overlapping is increased, the
structure becomes stiffer, and the bending moment in
the tower reduces. Once the overlapping zone is
approximately 25% of the span length, deck
deflections are equivalent to a single main span
bridge, and the peak tower bending moments reduce
to about two-thirds.
The instability of the central tower of a cable stay
bridge is well known structural phenomenon and in
the earlier feasibility study this was resolved with the
use of pyramid type similar to the towers of the multi-
span cable stayed Rio-Antirion Bridge in Greece. The crossed cable solution was taken forward for
development of the Specimen Design which formed
the basis of the tender design. There are approach
bridges on either side of the main cable stayed bridge
and the deck is continuous over its total length of
2638m between abutments, Figure 49.
However the pyramid type towers would have visually
over-whelmed the existing two bridges and would
have had a massive footprint on Beamer Rock, Figure
47.
Figure 47: Pyramid towers in the feasibility study
In order to resolve the instability issue and have slim
towers the cable fans were extended beyond the mid-
span so that adjacent cable fans overlap which in
effect means crossed cables at mid-span, Figure 48.
Figure 49: Preliminary General Arrangement
The Specimen Design had two variants for the deck of
the main span as steel-concrete composite section
and steel orthotropic section with a central array of
stay cables. The approach viaducts also had two
variants for the deck as steel-concrete composite
section and prestressed concrete section, Figure 50.
The towers were slim mono-pole towers and
approach viaduct had V-piers, Figures 51 and 52.
Figure 48: Crossing Cables – effect on deflections
Figure 50: Approach viaduct deck variants
1/2018