Hitting the right note: the Ordsall Chord

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In a tandem lift using the UK’s largest crawler crane, contractor Skanska BAM, working for Network Rail, installed a pair of steel arches over the River Irwell to form the UK’s first network arch railway bridge. Set to become an instantly recognisable part of the Greater Manchester skyline, the bridge is the centrepiece of the Ordsall Chord. A key part of the Great North Rail Project, the Government’s £1 billion programme of rail improvements across the North of England, the Ordsall Chord will create a new rail link between Manchester’s main rail stations, Piccadilly, Oxford Road and Victoria, and reduce the need for services to turn back to serve Manchester airport, unlocking vital capacity. It is scheduled to open to traffic in December 2017. Rail Minister Paul Maynard said: “These improvements are at the heart of our plans for the Northern Powerhouse. This is a demonstration of our commitment to deliver change that passengers want, such as increasing direct links between Manchester, Liverpool, Newcastle and other cities, providing more room and faster, more frequent services by 2020.” Ribbon development The new twin-track railway is only approximately 300 metres long. Yet, within this distance, it crosses the River Irwell, the city ring road at Trinity Way, the Manchester, Bolton and Bury canal, the pedestrianised Hampson Street Bridge and local roads – and it ties into two existing rail lines. This is also prime regeneration territory, with developments planned on the Manchester and Salford sides of the river. The surrounding area abounds in historic interest, including the former terminus of the world’s first intercity railway between Liverpool and Manchester, now the Museum of Science and Industry. George Stephenson’s Grade I listed masonry arch bridge lies immediately downstream of the site. Clearly, a special bridge was needed for this special location. WSP|Parsons Brinckerhoff worked with architect BDP to develop the concept design, which was taken to public inquiry in April and May 2014. The numerous site constraints naturally favoured different structural forms at different locations. However, it was essential to find a way to unify the appearance of the new link, respecting the surrounding heritage and the regeneration aspirations. This was achieved through the architectural concept of a ribbon in weathering steel. The ribbon theme links the half-through girders of the Trinity Way crossing, dipping through an emblematic swoosh to spring over the River Irwell in a single span of 89 metres. A network arch was suggested by the WSP|Parsons Brinckerhoff engineers as a possible solution for the river crossing. Unlike more conventional bowstring arches, where the hangers are vertical, in a network arch the hangers are inclined and cross each other at least twice. The inclined hangers act to distribute concentrated loads from the deck to the top boom of the arch. The network arch therefore achieves the stiffness required to meet deformation criteria under rail traffic loading, yet retains a shallow span-to-rise ratio of 15.25 and an overall slim, elegant appearance. This allows for visual continuity to be achieved between the arch and the adjacent structures. Additionally, the architectural considerations have driven some of the more complex aspects of the bridge engineering. The top chord has a hexagonal profile to reduce the visual bulk of the section. The arch girder is asymmetric and tapers from 2.5 metres deep at the north end, adjacent to Trinity Way, to a slender 0.7 metres deep at the south end beside Stephenson’s bridge. The arches are inclined at six degrees from the vertical to enhance the appearance. Construction engineering With the Transport and Works Act Order granted in March 2015, design and planning proceeded in earnest. AECOM Mott MacDonald was appointed by Skanska BAM to undertake the detailed design. Equally important, and critical to the success of the project which was carried out under an alliance arrangement, the steel fabricator Severfield was appointed at an early stage.

Photo: Russell Wykes.

The construction sequence was agreed and refined with input from all these parties. In particular, it was decided to erect the bridge in a piecemeal fashion, rather than slide the completed structure into place. The chosen approach required less temporary works in the river and hence gave considerable programme advantages. The flat sections of the deck were assembled on four temporary supports, with their positions chosen to maintain a pair of seven-metre wide navigable channels in the river. The supports had a dual use, and previously also served to dismantle the former Hampson Street bridge. This truss bridge had to be removed to make space for the new arch. The temporary steel columns were installed through holes made in the Hampson Street bridge deck. The sections of the arches were brought to the compound on the east bank of the river. In a prime example of safety by design, the site welding occurred at ground level with the arches assembled flat. The arches were then rotated to the vertical, pivoting on a 300mm diameter steel pin at each end. Careful setting out allowed the second arch to be swung up in the correct position relative to the first, both at six degrees to the vertical. The permanent K-bracing, to restrain the top chord against buckling, was bolted into place using high strength friction grip bolts. Big lift The assembled arch steelwork weighed 634 tonnes, out of a total bridge tonnage of 1303 tonnes. To lift this out over the river, heavy craneage was required. At the north end, this was no less than the largest crawler crane available in the UK – a Liebherr LR 11350, with a capacity of 1350 tonnes, provided by Weldex. By contrast, the crane at the south end was a LR 1750 with a ‘mere’ 750 tonne capacity. The cranes were booked a year in advance. Even more impressive was the fact that the cranes moved during the process. Having picked up the arches, the cranes tracked forward in tandem, moving approximately 30 metres. Several pauses were made while 580 tones of counterweight was added to stabilise the large crane, and 350 tonnes for the smaller crane. The crane pad comprised a 500mm deep reinforced concrete slab, and a six-metre distance adjacent to the river wall was piled with 600mm diameter CFA piles. The arches had to be landed on, and connected to, the node units which were already installed on the deck. These formed the starting sections of the arch box girders. Detailed surveys had been made of the node steelwork, and the ends of the arch sections had been ground to match the survey. Due to the location of the lifting points close to the quarter points of the arch, the ends of the arch were expected to deflect inwards by approximately 125mm during the lift. The sequence required that the north end of the arch was landed first and a temporary bolted connection made pending the permanent welds. The southern end was then relaxed into place, with final adjustment controlled by steel strands connecting the bottoms of the arches. Hangers The next stage will be installing and stressing the 46 hangers on each side with access from MEWPs running on the deck steelwork. The hangers, ranging from M85 to M100 solid steel bars, will be stressed from the centre outwards and this initial stressing stage will lift the bridge clear of the temporary river supports. Further stressing stages will follow after casting the reinforced concrete deck. Load cells in each hanger will allow live monitoring of stresses. Brian Duguid, engineering manager for AECOM Mott MacDonald, said: “This is a complex, interdependent structure. Similar to a ‘cat’s cradle’, as you adjust the stress in one hanger, it affects the stress in all the other hangers.” The inextricable relationship between the design of the structure and the construction sequence was reflected in the close working relationship between the designer and fabricator. Indeed, Severfield embedded a number of its staff in the AECOM Mott MacDonald offices and the geometric BIM model was produced by the fabricator’s modellers working alongside the design engineers. Equally important for the final design were Network Rail’s requirements for maintenance and robustness. The bridge is designed to permit a pair of hangers to be removed to allow for replacement, without restriction on rail traffic loading. A worst-case accidental damage scenario has also been considered, involving the potential loss of four pairs of hangers in series. The weathering steel finish, as well as providing the architectural theme, means that no maintenance painting is required for the arch steelwork over the railway, therefore minimising whole life cost. Where possible, open sections were used in preference to closed box sections, in order to allow for touching distance inspection of all platework faces. The arch top chord had to remain as a box section to achieve the slender dimensions envisaged at the public inquiry stage. However, the architectural detailing as a hexagonal section provided considerable benefit in stiffening it. The angled joint between plates acts similar to a longitudinal stiffener but simplifies the fabrication details within the box section.

Photo: Russell Wykes.

Pride of Manchester Following the successful lift, Severfield project manager Jarrod Hulme said: “After months of detailed design planning workshops, we successfully completed the tandem lift of the network arches onto the bridge deck, over the River Irwell. I am extremely proud of everyone involved in reaching this milestone. Over the last few weeks the Severfield team has been working incredibly hard to get everything in place for today’s tandem lift and we are delighted with today’s achievement on this fabulous project.” Programme manager Allan Parker from Network Rail agreed: “I have been working on this project from the very beginning and I am extremely proud of every milestone we have achieved. However, the sight of the arches elevated over the River Irwell was very special and will live long in my memory.” Peter Jenkins, BDP transport architect director and project lead architect, recalled the early days of the project: “I still have my original sketch of the bridge concept from when BDP started work on this exciting and challenging project over five years ago. It is therefore hugely satisfying to reach the dramatic moment of the bridge arches being lifted into place over the river.” Brian Duguid of AECOM Mott MacDonald was also happy and relieved: “We have worked on major structures around the world, but this has been as demanding a project as any we have worked on.” These quotes illustrate the universal expression of those involved: pride in this project. It is civil engineering at its finest, an exemplar of alliancing, collaboration, complex technical analysis, BIM techniques and of sheer hard work. But, more than that, this bridge is set to become an urban icon and symbol of the transport-led economic development of the whole northern region. Written by Mungo Stacy, technical director at WSP|Parsons Brinckerhoff