Fig. 3: The Ordsall Chord ribbon
entity, had to be designed to respect the significant
heritage of the site. Following extensive public
consultations, the aesthetic requirement for a slim
and elegant but emblematic structural line has led to
the choice of the architectural signature of the Ordsall
Chord, a ribbon in weathering steel, connecting and
unifying visually the adjoining structures, as shown in
Figure 3.
The requirement for visual continuity between the
network arch and adjacent parts of the viaduct influenced the shape of the structure. It should be noted
that in plan, the viaduct is not on a straight line but is
slightly curved at this location. A series of parametric
studies using BIM t echnologies were performed, in
order to establish a harmonic relationship between
the curved and the straight lines of the ribbon in both
plan and elevation. This had a significant influence on
the height, shape and inclination of the arch. Too tall
an arch would benefit its stiffness but affect the
harmony with the adjacent straight line. Too shallow,
it would benefit the aforementioned harmony but
would not suit a railway bridge, whose stiffness is
profoundly important for the safe operation of the
railway itself.
Network Rail, as the Promoter, sought a structure,
which was robust and straightforward to maintain,
but these aspirations had to be balanced with the
need for an architecturally very high quality structure
at a visually sensitive location. Figure 3 also illustrates
a typical arch rib together with the main girder of
Trinity Way Bridge and the link structure, and this is
the result of the aforementioned parametric studies.
It is worth noting that the behaviour of the structure
with open and box sections was also studied. Open
sections offer easy access for inspection compared to
boxes. However, in order to achieve the targeted
appearance a great amount of additional stiffening
was required. This additional stiffening would increase
the inspection and maintenance effort significantly
and deemed this solution impractical. Finally, the
project team decided that the main arch and bracings
would comprise box sections, with the remainder of
the steel structure being fabricated from open steel
sections.
The choice of steel type was also important, as it
affects both the aesthetics and the maintenance
requirements. Weathering steel was chosen for the
main arches and bracings and painted steel for the
longitudinal ties and transverse girders. The choice of
weathering steel, generally, minimises the whole life
maintenance costs. However, a separate study was
performed to ascertain, how any graffiti could be
4/2016