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A game of two halves

Contractors are launching a massive 104m long warren truss over Nottingham Station as part of the extension of the city’s tram network. Jon Masters kicks off NCE’s bridges special with this report.

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Push over: The 104m warren truss on its way above the main line station

The last of Britain’s Victorian mainline railways to be built was the Great Central, from Sheffield to London Marylebone. The line was built during the last decade of the 19th century and had to be elevated as it passed through the heavily built up cities of Nottingham and Leicester. In Nottingham, large wrought-iron bridges were built to carry the trains.

The Great Central was closed during the mid to late 1960s and the bridges over Nottingham station were demolished in 1983. But the crossing is back, in the form of a 1,100t warren truss being launched in two halves over the station platforms and into the exact position of its wrought-iron predecessor.

A joint venture of Taylor Woodrow and Alstom is carrying out the bridge launch to form new permanent way for an extension of the Nottingham Express Transit (NET) tram system. Two new lines are being built to the south and south west of the city, adding to a city centre loop and a single line north to Hucknall. Initially the new lines will share the same dual track out of the city centre, before separating towards Clifton to the south and Beeston to the southwest.

“The work is very close to the furthest north rail line of the station, which has to be kept operational”

Ron Sexton, Mott MacDonald

Since 2004, the end of the line for the NET has been a Nottingham station tram stop on Trent Street viaduct. This is a masonry arch structure from the old Great Central railway, terminating just north of Station Street, which runs parallel to the north side of Nottingham’s railway station.

Phase Two of the project will extend the Trent Street viaduct with three new bridges to be built back to back. Sandwiching the main station overbridge, will be a bridge of precast prestressed concrete beams spanning across Station Street to the north and a two span twin box girder bridge with a reinforced concrete deck that spans Queen’s Road to the south, joining onto a reinforced earth embankment.

The station overbridge is the main showpiece of the three. The structure’s 104m long warren truss will have two identical spans.

Each welded tubular steel truss is 4m tall at the ends, rising to 6m tall over the centre pier, with the upper chord described by a circular arc. The chords and end diagonals are 711mm diameter circular hollow tubes and the internal diagonals are 508mm diameter tubes.

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The bridge deck incorporates transverse steel girders 419mm deep, composite with a 250mm thick concrete slab.

Once fully launched, these deck will span either side of a central pier, which has been built on the station’s central platform island making use of pier foundations of the Victorian bridge. These are two 3.7m diameter cast iron caissons filled with mass concrete below platform level.

This has been done with 300mm diameter mini-piles driven through the masonry and granite slab of the platform to socket 1.5m deep into the caissons. The 3m long piles are capped with a 12m long ground beam and a 5.6m high reinforced concrete portal frame.

“A very similar arrangement has been built on old remaining pier foundations where the north pier will support the station overbridge and southern end of the Station Street bridge,” says Ray Sexton, Nottingham station structures team leader for contractor’s designer Mott MacDonald.

“The work here is very close to the furthest north rail line of the station which has to be kept operational.”

Loadings on the existing foundations, heritage and aesthetics were key factors in determining the design of the station overbridge, Sexton says.

A cable stayed structure and launch of a box girder bridge were considered, but both would have been heavier and deeper solid structures. They would also have been more expensive, with the options respectively 33% and 8% more costly.

“The first night was spent loading up and checking the jacking equipment then the launch progressed by up to 13m each night”

Martin Carroll, Taylor Woodrow Alstom

“We discussed the bridge design with Nottingham City Council which wanted an attractive aesthetic in keeping with the station’s heritage. We did a lot of visualisation study from platform level and found the contemporary appearance of the warren truss looked most lightweight and less intrusive,” says Sexton.

“We were replacing what was there originally. A lot of importance was attached to that. Parapets were discussed at length because a new station tram stop will extend onto the bridge and it was decided these should not be solid but glazed so passengers can take in their surroundings.”

Sexton says the design team also worked with Taylor Woodrow Alstom and its bridge launch subcontractor Mammoet to come up with a suitable arrangement of temporary supports. Some of these have been built up from foundation pile caps of the Queen’s Road bridge as the contractor’s site for building the station overbridge truss currently sits in its place.

Earlier this month, Taylor Woodrow Alstom carried out the first of its two big bridge launches starting from this site just south of Queen’s Road.

The first half of the truss, 52m long, 14.5m wide and weighing about 400t, was constructed at its launch height 7m above ground level on six temporary steel towers.

Over five consecutive nights the bridge was launched over Queen’s Road, preceded by its 13m long steel launch nose which guides it onto temporary supports.

“The first night was spent loading up and checking the jacking equipment, then the launch progressed by up to 13m each night, not in a continuous movement, but in cycles of 2.6m,” says Taylor Woodrow Alstom project director Martin Carroll. “The bridge moved by a process of jacking, then lifting, resetting the jacks and supports and dropping the truss back down for pushing forward another cycle.”

At the top of each temporary tower is an array of base plate, PTFE bearing plate, hollow jack and three support plinths.

One of the three supports - a saddle shaped cradle on the PTFE bearing plate - takes the weight of the bridge while it is jacked forward. The other two jacking supports lift the structure while the cradle is moved back for the next cycle.

The bridge was launched without most of the concrete for its composite deck. “We had weight issues that prevented us from launching the bridge with the deck already poured,” says Carroll. The concrete trackbed and pedestrian footways will be completed once both halves of the bridge are in place, although the rearmost quarter of the deck was poured prior to the launch to counter-balance the launch nose, Carroll says.

With the first 52m of the truss now launched over Queen’s Road, the second half can be built from the launch site. In April the whole 104m long truss is due to be pushed into place over the Nottingham Station platforms and then lowered onto its permanent supports.

 

Net phase two

Taylor Woodrow Alstom is building the £530M Phase Two extension of the NET tram system for Tramlink Nottingham, which holds Nottingham City Council’s PPP tram concession.

This involves Vinci Construction and Alstom with transport system operators Keolis and Wellglade, transportation investor Meridiam Infrastructure and investment fund Infravia.

Other significant structures to be constructed for NET’s Lines Two and Three include a 61m long bow string arch bridge over the A52 Clifton Boulevard, a 108m long reinforced concrete viaduct over the River Leen and reconstruction of the Wilford Toll Bridge over the River Trent.

This will involve constructing a new stronger and wider superstructure in two halves on existing portal supports while keeping the existing bridge open to pedestrians.

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