Analysis: The Ordsall Chord - beauty and history

 

The Ordsall Chord forms part of Network Rail’s £4bn Great North Rail project to improve railway connectivity across Northern England. The iconic new bridge is located at the birthplace of modern intercity railways, where in 1830 George Stephenson unveiled the Liverpool–Manchester railway.

Tom Osborne

Specialist bridge architects Knight Architects worked closely alongside the engineer AECOM Mott MacDonald JV, contractor Skanska BAM JV, and fabricator Severfield, in order to deliver BDP architects’ concept of the Ordsall Chord.

BDP’s architectural vision took the form of a weathering steel ribbon, which visually linked the two separate structures of the River Irwell Network Arch Bridge and Trinity Way Viaduct.

The objective was to ensure that BDP’s vision was successfully implemented given the complex structural forms and 3D curvature which it encompassed.

Linking the key structures

One of the most critical design elements was the connection between the River Irwell Network Arch and Trinity Way Viaduct. These two structures had different structural forms, different cross-sections and different alignments.

The challenge was to link these two distinct elements physically, visually, and conceptually, with two 40-tonne steel structures which were nicknamed the ‘cascades’.

While the initial concept and the two-dimensional graphic of BDP’s ribbon was clear, simple and fluid, the reality was a very complex arrangement of steelwork indeed. To successfully translate the concept into reality, detailed parametric modelling and intricate development between all parties involved was required.

The 20m long, 3m high cascades serve two primary objectives - firstly, to blend the lines of the viaduct and the arch to create a smooth, continuous and three dimensionally tangential progression from one to the other; secondly, to blend the different structural forms of the viaduct’s open I beams with the closed box of the arch.

To successfully blend these sections, a crease was introduced into both the arch, and the stiffeners of the viaduct. This crease breaks the large vertical surface of the arch, with the inclined top-face catching light, adding visual interest, and increasing its apparent slenderness.

This crease continues in the stiffeners of the viaduct, which are angled to complement the form of the arch, and achieve the same visual qualities.

Furthermore, this crease is continued along the cascades, adding a third line between the top and bottom splines which further emphasises the relationship between the two structures. The stiffeners also serve to blend the closed box section with the open I beam.

By exponentially reducing the spacing between the stiffeners as they approached the arch, the stiffeners began to interpolate a surface, which ultimately blends with the surface of the arch. This dramatically reduces the abrupt transition from open structure to closed, and allows the eye to flow from one structure to the next without interruption.

Parametric geometry

To allow the 3-dimensional geometry of the structures to be properly defined while allowing engineering design development to continue, a parametric approach was required.

Scripts were written that took the three-dimensional location (and tangent) of the top, crease and bottom of both the arch and the viaduct’s I beam.

Given these six inputs, the form of the cascade could be dynamically generated, updating as the points shifted in space. The number and progressive-spacing of the stiffeners was then specified, considering fabrication and structural limitations.

Finally, the back-face of the cascade was optimised to remove unnecessary steelwork, as well as to eliminate conflict with its supporting concrete structure. As the parametric process relies on rules (and parameters) the opposite cascade could be instantly generated, despite its completely different form (due to the plan-alignment of the two structures).

Furthermore, this information could be passed directly back and forth between designer and fabricator, ensuring that the forms were both achievable and an accurate representation of the model.

 
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