4.
The hanger network
Each network is formed of 46 proprietary tension
assemblies, comprising solid steel bars and cast fork
anchorages. The layout of the hanger network is
generally based on the theoretical concept of
directing the resultant network forces radially with
respect to the arch axis. This is achieved by aligning
the intersections of the hangers radially towards
a common focal point.
The layout of the hanger network was further
developed through a series of studies investigating its
efficiency in terms of maximum and minimum hanger
forces and the resulting bending moment profiles in
the arches and the tie beams.
In order to facilitate the replacement of hangers and
their stressing during construction, each network was
split into two parallel planes. The theoretically derived
layout had to be adjusted to provide sufficient space
for the development of the anchorages and, in the
case of the short hangers, to allow the installation of
the stressing equipment. The resulting layout is shown
on the general arrangement drawing.
5.
Arch geometry
The main arches have a continuously varying cross
section that forms a “crease” line, which is visually
continuous from the tip of the arch to the end of the
approach viaduct. The shape of the hexagonal cross
section for the arch was carefully designed so that
out-of-plane curvature, or “warping”, of the plates is
avoided.
This was achieved by maintaining constant inclination
of the upper and lower plates with respect to the true
vertical plane perpendicular to the axis of the bridge.
The cross section definition is also shown on the
typical section drawing.
The “ribbon” theme for the Chord resulted in a very
deep arch cross section towards the North end of the
bridge. The material distribution along the arch
compensates for the increase of section by reducing
the plate thickness. This reduction was balanced
against the requirement for introduction of
longitudinal stiffeners since the latter in combination
with the hanger anchorages would lead to congestion
within the box.
6.
Deck geometry
The aspiration for ease of maintenance was most
influential in the development of the deck geometry.
4/2016
Although a closed box would have been a more
convenient structural form, an open section geometry
was adopted so that touching distance inspection
could be undertaken for all deck steelwork not
encased in concrete, without the need for confined
space inspections. In order to avoid unnecessary
eccentric effects, the arch and tie axes intersect over
the centreline of the abutment bearings. In order to
improve the load path from the hangers into the tie
beam, the web was aligned with the outer plane of
the hanger network. The bottom flange of the section
was maintained truly horizontal to facilitate site
installation.
The deck was also designed to incorporate the
Promoter’s requirement for a connection that
provides a secondary load path for the vertical loads
and this is formed by bolted shear key end plate
connections for the transverse girders. These were
positioned within the footprint of the tie beam
bottom flange so that the visual aspiration for “clean”
soffit is achieved.
The prominence of the exposed arch surfaces means
that their interruption is undesirable since the visual
continuity will be compromised. With that in mind the
junction between the arch and the tie beam was a
defining interface with respect to the cross section
geometry. This interface is shown for the North End
node in Figure 5.
7.
Conclusions
The River Irwell Bridge is the first railway network arch
bridge in the UK. At the time of writing, the structure
is in the construction phase and is due to be
completed in summer 2017.
Its complex configuration and the construction
sequence adopted, which was tailored to the
particular site constraints, presented a number of
challenges to both the design and construction
phases. However, the collaboration and ingeniuity of
all the professionals involved has been key to the
delivery of this structure.
8.
Acknowledgements
The authors would like to express their gratitude to
the wider team of professionals from Network Rail
(Promoter), Skanska Bam JV (Contractor), BDP
(Architect), Severfield (Steelwork Fabricator) and the
AECOM-Mott MacDonald JV (Designer), who
collaborated closely to deliver this landmark structure.