successfully removed from the weathering steel
without damage to the stabilising patina.
The rise to span ratio was set at 0.1525, which is at
the lower range for a network arch. In order to
enhance the appearance of the arches a 6 degree
inward inclination was prescribed at planning stage.
The solution was a good compromise between
aesthetic and maintenance requirements and the
collaboration between the Planning Authorities, the
Architect, the Promoter, the Contractor, the Bridge
Designer and the Steelwork Fabricator was
instrumental in achieving the final structural
configuration.
3.
The Erection Sequence
It was recognised from the very early stages of the
project that the design of this structure was heavily
dependent on the construction methodology to be
adopted. The methodology was heavily influenced by
the requirement to have a navigable channel in the
river for the duration of the construction.
Furthermore, in order to reduce the impact of the
temporary works on the construction programme, it
was decided to use the same temporary works to
facilitate the demolition of Prince’s Bridge. This
effectively led to a “piecemeal” erection approach
and necessitated in depth discussions between the
Steelwork Fabricator, the Bridge Designer, and the
Contractor, in order to determine the exact
methodology, from which the design of the structure
evolved.
The tie beams will be erected first in sections followed
by the transverse girders to form a ladder deck. The
alignment of the bridge with respect to the river
means that the deck will be installed on skewed
temporary towers in the river.
When the tie beam sections are installed, they will
deflect and twist as they are asymmetric and the
centre of gravity and the shear centre are not
coincident. In order to understand the behaviour of
the asymmetric tie beam sections better, the deck
erection was re-analysed using a shell-element based
model for the ladder deck.
Such an analysis was required to evaluate the stability
of the tie beam sections in the temporary case since
the cross section does not have an axis of symmetry.
It was also required, in order to understand the buildup of rotations about the longitudinal axis of the ties.
The open section has relatively low torsional stiffness,
which meant that the twist generated during erection
may lead to problems during the installation of the
hangers. This risk was mitigated by incorporating
transverse props and ties that assist in the installation
Fig. 4: Construction sequence
4/2016