The construction of Phase Two of Nottingham’s tram network has been going on for a while, but much of it has been hidden from view. Now, though, there are some exciting developments that, for the first time, show clearly that the project is well on the way. The most visible of these is an iconic new bridge which has appeared adjacent to Nottingham railway station.
First of all – a bit of background. Councillor Jane Urquhart, portfolio holder for Planning and Transport at Nottingham City Council, has been involved with NET since she was first elected to the Council in 2000. She became closely acquainted with the construction of the first line between Hucknall/M1 junction 26 and Nottingham railway station. This was not without its controversies at the time, but has since become a great success. Jane says that even some of its strongest detractors have come to see that it was actually a good idea after all!
Learning for success
Jane feels that important lessons were learned during the construction of Line One. These included the necessity of getting closely involved with the detail of local issues, the importance of providing easily accessible information in large quantities by diverse means of distribution on a frequent basis, and the need to be prepared to adjust the scheme details and programme when this is required to overcome local issues. There are definitely some pointers here for other controversial projects (HS2 anyone?).
In Jane’s opinion, Line One has been a resounding success. Since its opening, all other local public transport modes have seen increased growth rates in addition to the new traffic on the trams themselves. NET Phase Two will build upon this success, adding key destinations including Queen’s Medical Centre, the University of Nottingham, Clifton, Beeston town centre, Chilwell and M1 junctions 24 and 25 to the tram network.
The city council is part-funding both the new tram lines and the current improvement works at Nottingham railway station from its workplace parking levy. In December 2011, Nottingham City Council awarded the Net Phase Two contract to Tramlink as part of a twenty-five year PFI (private finance initiative) concession. Construction is being delivered by a joint venture between Taylor Woodrow and Alstom. Some 600 people are currently being employed in connection with NET Phase Two, so it is an important source of local employment.
Nottingham City intends to add further transport network improvements in the future. There are ambitions to take forward further tramlines – the original feasibility studies in the 1990s considered six or seven possible lines and there is potential to go ahead with these or shorter extensions to existing lines in the future. In Kimberley there is already a campaign for the existing Line One to be extended there.
The council is interested in the tram/train concept, which is seen as offering possibilities for extending services outside the tram network onto some of the under used (or indeed unused) ‘heavy rail’ lines in and around the City. Also, fully integrated electronic ticketing covering all modes is a firm objective.
Asked about the possibility of an East Midlands Passenger Transport Authority, Councillor Urquhart said that Nottingham City Council is already a partner in a local enterprise partnership with the County Council and their equivalents from Derby and Derbyshire. This is recognised by Government and good relationships are being built. There is seen to be potential for extending this to cover the whole of the East Midlands by agreeing to join up with Leicester City and Leicestershire County councils.
One of the termini of the two new tram lines that make up Phase Two is at Toton, close to the newly- announced HS2 hub. Naturally, Cllr Urquhart, Tramlink chief executive Phil Hewitt, and Martin Carroll, NET Phase Two project director for the Taylor Woodrow Alstom joint venture, are all very enthusiastic about the prospect of connecting this into the NET network. The hub will be within the area of Broxtowe Borough Council, but there is likely to be close co-operation between the two councils to ensure that the relatively short link could be constructed. Indeed, the view appeared to be that the tram might get to the hub site before HS2 itself!
Back to the bridge
Martin and his structures expert Andy Bannier described the important features of the Karlsruhe Friendship Bridge and its construction. The key players in the works are the Joint Venture (JV), together with Cleveland Bridge Engineering and Mammoet. The first half of the bridge recently appeared high above ground level on a site at Crocus Street on the Queen’s Road side of Nottingham Station. Visible over a wide area, it is the most obvious indication of the progress of NET Phase Two. When completed, the bridge will be 104 metres in length and 14.5 metres wide.
Because of the restricted size of the erection site, it was only possible to assemble one half of the Warren Truss structure. This was slid some 50 metres towards its final position in order to free up the site for the erection of the other half of the truss. The two will then be connected before the whole thing is slid the rest of the way to its final resting place.
Two permanent piers to support the bridge have been constructed between Queen’s Road and platform 6 of the Station and between platform 1 and Station Street. A third, central pier is under construction within the listed station buildings on Platforms 4 and 5. This has meant removal of part of the roof, and has been closely supervised by the authorities because of the listed status of the buildings.
New bridge – old alignment
The new bridge will sit on the exact line of its predecessor, the old Great Central Railway bridge, removed in the early 1980s. Two of the foundation caissons of the old bridge are being re-used to provide part of the support to the new structure, although they have had to be strengthened with mini-piles. The remaining foundation loads will be carried by CFA (continuous flight auger) piles.
Like the old Great Central bridge, the new one will be flanked by two smaller bridges, one over Station Street and the other over Queen’s Road. The site hoardings are decorated with aerial photographs of the old structures, an interesting comparison with the new works now taking shape.
The sliding of the first half of the bridge started on the night of Monday 11 February 2013 and continued each night of that week up until completion just over a week later. The bridge finished up spanning Queen’s Road at the end of this phase of the slide. To that effect it had been erected horizontally at a level 7.3 metres above that of the road. When the full bridge reaches its final resting place across the Station, it will be lowered somewhat and will be on a gradient, in order to match up with the existing tram viaduct on the city side and the new works to be built on the other side of the station. In total the bridge will have moved by around 100 metres horizontally when it is finally in place.
The design of the structure has had a significant effect on the sliding design, due to the significant stresses arising from the temporary support conditions involved.
During the second slide it needs to span 52 metres across the station between permanent supports, although temporary trestle piers are being used at intermediate points to reduce the gaps encountered during the first slide across Queen’s Road. The forces generated at the temporary points of support will be very high. The tubes of the trusses are 711mm in diameter and have 40mm wall thickness and, although they were hot rolled for greater strength, they can still only be propped within 1.3 metres either side of each of the truss nodes as propping elsewhere along the bottom boom tubes would overstress the structure.
Steelworks sub-contractor Cleveland Bridge Engineering had a choice of only two rolling mills worldwide that could hot roll steel tubes of the required thickness and diameter, one in Korea and the other in Japan. The latter were successful in winning the order.
The strength issue meant it was not possible to install the 250mm thick reinforced concrete deck nor the tram tracks prior to the sliding operations, since this would have overstressed the bridge during the slide. However, during the second stage slide, significant counterweight will be needed at the rear end of the bridge to balance it as it cantilevers between supports at its front end. This weight will be provided by installing the rear quarter of the deck, meaning that the total weight of the structure being slid will be around 1100 tonnes and be 104 metres long!
The size of the trusses of the bridge obviously precludes bringing them to site in one piece from the fabrication shop. Smaller sections have therefore been brought and joined on site. This means welding connections on site and painting the affected areas on site afterwards. Cleveland Bridge is taking responsibility for all of this, including the painting in the shop and on site.
Lifting and sliding
Sub-contractor Mammoet is in charge of the specialist lifting and sliding operations, and has also supplied some of the specialist craneage required for the site erection works. Taylor Woodrow has coordinated and managed the temporary works.
At each permanent or temporary support pier there eis a PTFE coated bearing plate under each of the two bottom booms of the bridge. While the bridge is being slid, its weight is taken at each of these points by a sliding saddle, shaped to fit the tubular section of the booms. The saddles sit and slide on the baseplates, which are sufficiently long to allow a slide of 2.6 metres to be made.
Once a slide has been completed it is necessary to return the saddles to the start of the baseplates so that a further 2.6 metre slide may begin. To permit this, at each baseplate there are two movable jacking supports. These can be placed anywhere on the baseplate to be clear of the sliding saddle and within the permitted part of the bottom boom. They contain jacks which lift the truss clear of the sliding saddles so that these can be relocated as required. It was anticipated that relocating all of the sliding saddles in this manner would take around an hour. Once this has been completed, the slide can recommence for the next 2.6 metres.
To reduce the loadings on the bridge as it cantilevers between supports 52 metres apart, a 13 metre long launch nose has been added to its front end. This means that the relatively light nose lands on the next pier some 13 metres before the main truss and transfers some of the weight to the pier as it reaches it. Consideration was given to building a nose that would also serve as the permanent structure, or at least a part of it, for the Station Street bridge. Unfortunately this did not prove to be a practical or economic idea.
The completed bridge will carry twin tram tracks with 3 metre wide public walkways on each side of them. It will form a key part of the interchange arrangements between the rail station, the new tram stop between Queen’s Road and platform 6 and the bus stops on Queen’s Road. This iconic structure will highlight the NET system and the progressive ambitions of the City of Nottingham.