Major chord

Connecting communities and supporting growth

The Ordsall Chord links up three Manchester city centre stations for the first time, reducing congestion and allowing hundreds more trains to run each day.

It is only a 300m stretch of new track, but the Ordsall Chord in Greater Manchester is a piece of engineering that will boost the local and regional economy with knock-on effects felt across a much wider area. The viaduct carrying the track connects existing lines through Manchester’s Victoria, Oxford Road and Piccadilly stations. Accommodating it involved realigning existing track, widening viaducts and laying new track, building new bridges, removing redundant structures and restoring heritage features. All civil engineering works were designed by the Aecom Mott MacDonald joint-venture. New public spaces have also been created to allow the area’s rich industrial history to be fully appreciated.

The Ordsall Chord connects to the existing Chat Moss viaduct to the north and Castlefield viaduct to the south; both had to be widened. The Chord itself is made up of three components: the River Irwell crossing, a concertinaing architectural feature known as the ‘cascades’, and the Trinity Way bridge. The River Irwell crossing is a network arch bridge with weathering steel arch ribs, while the Trinity Way bridge is a 100m long composite steel deck with weathering steel girders. The cascades, also made from weathering steel, connect these two structures. The surface of these key members develops a protective patina of rust over time, giving the Ordsall Chord its distinctive reddish-brown colour and outlines its fluid curved shape prominently on the landscape.

The UK’s first network arch bridge

The River Irwell crossing is the first network arch bridge in the UK. The structure consists of a tied arch with a network of hangers. This arrangement creates an incredibly robust and efficient structure – essential for supporting heavy railway loads.

“A network arch uses a lattice of hangers to provide significantly greater stiffness and robustness than a conventional bowstring arch bridge, restraining in-plane buckling of the arch ribs more efficiently,” explains Aecom-Mott MacDonald JV engineering manager Brian Duguid. The structural form also makes a significantly shallower arch rise achievable, which is in keeping with the scheme’s original architectural concept.

Spanning 89m across the river, the arch is made up of two ribs that incline inwards 6° to the vertical. The bridge’s composite steel and concrete deck is suspended via two networks of steel hangers. In elevation, the ribs taper so that their cross-sectional height decreases from 2.5m to 0.7m along their length. The cascades accommodate the 300mm height difference between the 2.5m tall rib section and 2.8m tall Trinity Way bridge girder. Brian continues: “The most critical design cases arise during construction, when the lattice is incomplete, or if the hangers are accidently damaged or need replacing. Since the network arch is asymmetric, a very large number of critical scenarios can develop, which had to be assessed individually.”

Building the Ordsall Chord was extremely complex, adds Mike Pedley, senior construction manager with contractor Skanska-Bam Nuttall JV. “There was virtually no element of repeatability across the different parts of the project. Twenty-five precast concrete arches were built on the project to support the expanded viaducts. Each one is different and the network arch is totally unique. Managing that variety was inevitably challenging.”

The team received the prestigious 2018 Royal Academy of Engineering major project award in recognition of the collaboration and engineering skill needed to deliver this project.

Legacy and vision

Ordsall Chord’s success owes a great deal to the client and project team’s collaborative approach to procurement and project delivery.

Network Rail and its contractors came together as part of an alliance, forming a single team that shared the project’s risks, rewards and resources. Key roles were filled by ‘best-for-project’ personnel, regardless of their employer. Design team members took on technical approval roles for Network Rail. Staff from steel fabricator Severfield were embedded in the design team, for example, to help prepare the BIM model and steelwork drawings. A willingness to operate in a different way led directly to major design innovations, such as producing a digital 3D model of the footbridge over the River Irwell with no need for additional 2D drawings.

River Irwell footbridge was designed in 3D (Credit: Matthew Nichol)

Building information modelling

The Ordsall Chord was delivered to BIM Level 2, which demands a high level of inter-disciplinary co-ordination. Design and construction data was passed on digitally to the client for future asset management. The 3D model of the network arch bridge was produced by Severfield’s modellers, who were co-located in the offices of the Aecom-Mott MacDonald JV. The model remained under the ownership of the JV and gave Severfield an intimate understanding of design development so that steelwork detailing developed from the start. The model was also used to co-ordinate reinforcement design and detailing.

“The model and drawings were approved by all parties, but were ‘owned’ by the engineer, who certified the design,” says Brian Duguid. “This is different from the traditional process, where engineers usually prepare their own model and drawings.

BIM allowed all aspects of the structure to be scrutinised

“On the Ordsall Chord, the fabricator acted as an intelligent CAD resource, applying expertise in 3D modelling, minimising the need for later rework. It reduced both the design programme and the time taken to prepare for fabrication.” The BIM model included all major structures as well as foundations and existing underground services.

River Irwell footbridge – 3D design

Pedestrian access over the river is via a new 50m long steel footbridge. The team decided not to generate conventional 2D steelwork drawings, using only a 3D model to deliver the structure. This saved greatly on drawing production time and total design cost.

“Everything built on this footbridge including the nuts, bolts, stiffeners and welding, was modelled,” explains Brian.

“Two-dimensional steelwork drawings were completely unnecessary because the designer, fabricator and future asset owner could interrogate the model. We worked very hard to make that possible."

Viaduct widening

BIM enabled the interaction of new and old structures to be carefully considered from the outset. The irregular geometry of the Middlewood and Castlefield viaducts – both 19th century brick arch structures – was digitally surveyed and the data used to plot their precise shape in a computer model.

This was used to develop the geometry for the new reinforced concrete arch extensions, used to widen the viaduct. Behaviour of the original viaduct could not be predicted through conventional analysis, so cameras and deflection gauges were used to monitor their actual movement under rail traffic loading. The results revealed that each arch behaved differently due to variations in ground conditions and the presence of defects in the brickwork. Deflections of between 1mm and 5mm were recorded in multiple directions and highlighted the difficulty of connecting the new structure with the old.

Concrete extensions are not structurally connected to the viaduct (Credit: Matthew Nichol)

“Where the magnitude of movement was clearly due to major defects, the arches were repaired or strengthened,” says Brian. “To accommodate some degree of movement, we decided to keep the existing viaduct and extensions independent and to link them only via a slab element within the track formation for waterproofing purposes.”

Construction of these concrete extensions first involved installing 900mm diameter reinforced concrete piles, followed by construction of in-situ reinforced concrete pile caps and piers. These supported profiled precast concrete ‘skewbacks’ to match the existing viaduct. The skewbacks also support precast concrete arch barrels and spandrel walls, which were infilled with mass concrete. All this work was carried out while lines remained operational. Further work was carried out during a blockade when trains did not run. This included improvements to waterproofing and drainage before final installation of the track bed.

Network Rail project director Mike Heywood believes that the Ordsall Chord brings together three important features: respect for the area’s heritage, a key piece of rail infrastructure and the prospect of regeneration.

“With many of the people working on the project being local to Manchester, the team had a strong connection with the project and its aims. Having brilliant people and brilliant teams undoubtedly helped make it a success.

“You can have the best ideas in the world, but if you can’t find a way to design and construct them, then that vision will never become reality. On the Ordsall Chord, we trusted each other’s knowledge to make this unique project successful.”

Game-changing civil engineering

The Ordsall Chord is part of Manchester’s rich industrial heritage. When it was unveiled to the public in 2017, comparisons were made between it and George Stephenson’s Liverpool to Manchester railway, which crosses the site. Both are considered triumphs of engineering that also support economic growth.

Many of the railway structures that the Ordsall Chord interacts with are listed for historic and architectural significance and part of the site is in a conservation area, so construction had to comply with strict heritage requirements.

“Conservation-led repairs were based on a thorough understanding of the historic fabric of these structures,” says project heritage architect Bernadette Bone. “This ranged from the need to match the type of mortar used to repair brickwork to the choice of materials and their forms to fit harmoniously with existing structures.”

(Credit: Matthew Nichol)

A masonry bridge built for Stephenson’s railway in 1830 has been restored as part of the project. Drainage was improved to prevent further damage to the structure, which carries Grade I listed building status, the highest heritage protection designation in the UK.

A steel girder bridge, which had been crudely joined to Stephenson’s bridge in the late 19th century, was dismantled to allow full restoration of its northern elevation. A ‘ghost’ pier from this girder bridge remains as part of the final scheme.

Bernadette adds: “Stephenson’s bridge, the Ordsall Chord and the River Irwell footbridge connect a pair of piazzas either side of the river. These spaces link to new regeneration sites, which have been carefully articulated so that they respond not only to their immediate context, but also enrich the experience of historic transport structures in the area.”

Key benefits of the Ordsall Chord

• New direct links to Manchester Airport from across the north of England
• Congestion at Manchester Piccadilly station reduced by 25%, with some services being re-routed through Victoria station
• Increased capacity
• Improved connectivity
• More employment
• Better links between new businesses and residential developments

Collaboration delivers innovation

With no precedent for the Ordsall Chord’s asymmetric network arch bridge, the team worked hard to design a structure that was efficient and practical to fabricate and construct.

Rationalising the design of the network arch bridge required Aecom-Mott MacDonald to work closely with Network Rail, Skanska-Bam Nuttall, Severfield and architect BDP. Temporary construction loads and permanent in-service forces in the structure had to be understood in meticulous detail to progress the design to fabrication and construction. Specialist bridge architect Knight Architects was employed by the engineering design team to provide additional advice. Input from Severfield was invaluable in developing a single computer model that provided clarity to the nuances of the structure from the start.

This undulating feature connects the River Irwell Crossing and Trinity Way bridge (Credit: Matthew Nichol)

Constraints

The network arch bridge’s asymmetry is due to its arch ribs tapering along its length. The complexity this introduced to the design was exacerbated by the need for an unsymmetrical construction sequence. Piers of an existing highway bridge provided temporary supports in the river but were positioned skew in relation to the network arch bridge. Site constraints added further challenges.

The alignment of the existing railway meant that the network arch bridge could not span perpendicular to the river banks and would need to be built over the new footbridge, while two 7m wide channels below remained navigable. This limited options for temporary support and erection.

Prefabrication was preferred wherever possible to minimise activity on site, but this had to be balanced with the constraints of transporting heavy or long members. Another consideration was the effect of stresses locked into the structure during installation and hanger stressing. Throughout design development, detailing had to support the architectural concept of the Chord’s trademark ‘ribbon’ of weathering steel. A shallow rise for the network arch was also essential.

Achieving both these aspects was challenging: if the network arch rise was too high, the sag curve of the cascade would compromise the ribbon effect; too shallow and its structural efficiency would suffer.

There were also additional constraints placed on the ribs due to Network Rail’s desire to make them easy to inspect and maintain. Their preference was for all steel members to be formed from open sections. Aecom-Mott MacDonald explored a number of options to satisfy all these requirements.

Designed to attract attention

“Our studies revealed that it would not be feasible for the arch members to be formed from an open section,” explains Mott MacDonald principal engineer Rusi Rusev. “This was due to the significant amount of additional stiffening required, that would be both costly to fabricate and to maintain.” Design effort, instead, was invested in improving a closed section shape for the arch ribs.

Preliminary planning drawings suggested a trapezoidal cross-section, but it was decided that its flat elevation would lack the definition needed to achieve a fluid, ribbon-like aesthetic.

Working closely with the architectural teams, the cross-section shape was modified to a hexagon, generating a ‘crease’ line, which would draw the eye of an observer along the structure.

“The design of the hexagonal shape was the result of combining aesthetics, structural performance and fabrication needs,” says Rusi. “The fold in the web plates [creating the crease line] was used as a stiffening edge to improve the structural efficiency of the section.

“By analysing different geometries, we produced a design that avoided warping. This had the effect of simplifying fabrication and reducing costs. This crease line is continued in the shape of the stiffeners on the Trinity Way bridge and cascades, meeting the architect’s brief for visual continuity.”

Getting this crease line in the correct position, as well as developing an overall geometrical ‘rule’ for the hexagonal cross-section, was the next challenge.

The magic formula

After assessing many different geometrical forms, a good balance was achieved by positioning the crease line at 30% of the overall height of the section and fixing the web at an angle of 7.2° and 22.0°. The overall width of section varies from 1200m to 1750mm, corresponding with an overall height ranging from 700mm to 2500mm. Varying the geometry of the cross-section creates the desired tapering effect.

Tie beam conundrum

The network arch’s deck steelwork was designed to meet Network Rail’s preference for open sections. It is painted a subdued grey colour, so that the reddish-brown weathering steel ribs are more prominent. The tie beam is designed in the context of the inclined arches where its web lies in the arch plane, with the top flange perpendicular to it.

The bottom flange is horizontal to facilitate simpler erection and deck connection. The resulting section has the appearance of a lop-sided I-beam, requiring complex structural analysis to understand the stresses in it.

Each hanger network is made up of an inner and outer set of hangers. Outer hangers are anchored directly into the web, while inner hangers are anchored on the inside face of the tie beam. The tie beam was preset and precambered along its length to compensate for movement during construction and ensure that its final position and geometry was correct.

The asymmetry of the network arch, its complex geometry and the need to maintain stability during erection greatly influenced the tie beam’s design.

Economies of steel

Contractor Skanska-Bam Nuttall split the Ordsall Chord into packages, enabling conservative estimates for the amount of steel to be calculated early on and purchased at the best possible price. Severfield worked closely with the design team to simplify the geometry of the cascades. These 16m long fabricated beams with welded stiffeners were designed in 3D.

“Working together, the weight of the cascades was reduced by 25%,” explains Severfield senior project manager Jarrod Hulme. “Initially, the design included a twisted form, but this would have been expensive to fabricate. Together, we revised the stiffening features. Welded onto the cascade’s beam, these take on a strong architectural role, with their spacing and shape varying as the beam curves and tapers. This gives the Ordsall Chord its amorphous quality.”

The weight of the cascades reduced by 25% during design development

Elsewhere, instead of increasing the thickness of the tie beam’s web from 40mm to 75mm to accommodate the clevis plate in each hanger’s anchor, it was designed with a cut-out. This allowed the thicker clevis plate to be welded to the tie beam. Not only did this save on the amount of steel on the project, it also provided another level of interest in the structure. “We worked as a truly integrated team to arrive at this solution,” adds Jarrod.

Innovative installation solutions

Spherical bearings fitted in the hanger anchorages provided extra tolerance for erection and hanger installation. Approximately 1.5° tolerance is provided by bearings in each anchor. This absorbs up to 0.5° installation tolerance and 1° in fabrication and erection.

Mott MacDonald design team leader Tim Abbott explains that these bearings also provided more flexibility for the final construction sequence, which required the network arch bridge to be built piecemeal in its final location, rather than assembled nearby as a complete structure and slid into place.

“We had to build the bridge in parts because of site and programme constraints. This had enormous implications for the design of the structure, hanger stressing sequence and arch behaviour,” he says.

Temporary piers in the River Irwell, which had been used during demolition of a former bridge, provided support for the bridge deck during construction. The piecemeal erection of the deck, together with the skew temporary supports, led to a large number of construction scenarios being analysed to ensure safety. Weight and lifting restrictions meant that the twin arches had to be lifted from third-points instead of from their ends, as would be more common. Temporary cables restrained the arch ends until the arches were fully connected.

“The in-service design of this bridge was challenging, but not nearly as complex as the construction-phase analysis,” comments Rusi Rusev. Changes to the erection sequence were checked on a versatile analytical computer model and on detailed local models at critical locations during development stage.

“The success of this project is very much a product of the effective and open collaboration between the designers, erector and fabricator. The use of a global BIM model facilitated that collaboration."

Bridge building

On a world’s first project, there is always the possibility of encountering the unforeseen. Hanger stressing, although carried out systematically from the centre and moving out to the ends, was a particularly nail-biting experience.

“With temporary supports located asymmetrically, deck behaviour was difficult to predict during construction. The deck was designed to lift off during hanger installation,” says Mike Pedley, senior construction manager at Skanska-Bam Nuttall.Each arch rib was fabricated in six sections, and welded together in the horizontal position on low level supports on the banks of the river.

Temporary cables supported the arch before hangers were installed (Credit: Matthew Nichol)

With K-bracing, temporary struts and other bracing installed, the ribs were then rotated to near-vertical and lifted into position using two of the largest crawler cranes in Europe. “We chose to do it this way because we wanted to avoid the risks of working at height – it was tricky enough welding it on land,” recalls Mike. “This was safer, reduced temporary works over the river and resulted in a higher quality product. It also saved between three and four weeks on the programme.” The alternative was to build the deck across the river first (on temporary piers) and then erect falsework on top to support construction of each arch member.

The risk of working at height coupled with it taking longer to access, ruled this option out. After the network arch had been lifted into position, hangers were installed and stressed before and after the deck was concreted. New track and other rail features were then installed.

Trains are now using the Ordsall Chord, enabling more rail journeys to be made across the city. The project is already benefiting businesses and the travelling public across the north of England and beyond. This engineering masterpiece will continue to have a positive effect on this area for generations to come.

Levels of detail

BDP head of transport Peter Jenkins conceived the curved form of the Ordsall Chord, although the final design was the result of a collaborative process. “The design changed over the course of the project and together we added value, but the silhouette of the design was always honoured,” he says. The architect adds, “I think the design is better for it. There is a richness which has been added through the detail – you can enjoy it from a distance and even more as you get closer.”

This is evident in the tie beam and clevis plate connection and on the cascades. He explains that the varying spacing of fins on the cascades creates an extremely tactile form. “Looking at it you’re compelled to run your fingers over the fins,” says Peter. “That quality is more common in building design than civil engineering, which makes it all the more special on this project."