Fears that erecting two cable stay structures side by side at a new Manchester business park would be overkill have resulted in the decision to build a bridge that has few equivalents.
Designed to carry the twin track Metrolink extension over four tracks of Network Rail's Manchester to RochdaleTransPennine route, the structure is a 'finback bridge'. Structural engineer Gifford director Ian Hunt can think of only one other UK example, and he concedes it wins no prizes for beauty.
Hunt points to the spine running end to end down the reinforced concrete structure's centre line. 'A finback is a basically an inverted box girder deck. The main structure is above the running surface rather than below - in section it's like an inverted T.
'There's very rarely any need to build a bridge this way, and the fin running down the middle of the carriageway is visually intrusive.' Development of the Central Park business estate on the site of a former psychiatric hospital in east Manchester involves the construction of a new station and of a bridge across mainline rail track for the Metrolink tram system, which is being pushed out from Manchester Victoria to Oldham. It also requires the construction of new road access via a jacked tunnel box under the railway. Total infrastructure works cost is £25M.
The station was nominated as Central Park's landmark structure. Gifford, with architect Aukett Associates, came up with an attention grabbing copper-clad elliptical canopy suspended by a spray of cable stays fanning out from a single inclined mast. They also produced a handsome, single pylon cable stay design for the bridge.
'But it was felt that the bridge was in competition with the station. We were asked to tone it down and come up with something less dramatic, ' Hunt says.
The structure now under construction and due for completion this summer will be decidedly understated, fulfilling the client's brief to the letter.
Most people will not give it a second glance. But to the structural connoisseur the finback bridge will be an intriguing oddity.
Why the unorthodox design was pursued rather than a conventional box girder has been dictated entirely by site constraints, notes project manager Steve Jones.
Joint venture design and build contractor Balfour BeattyBilfinger & Berger was handed the challenge of threading the Metrolink extension into the site along a disused rail line which joined the Trans-Pennine route from the east. The twin light rail tracks had to pass under an existing road bridge, entering the site at ground level. But the bridge is only 50m from the point at which the Metrolink tracks must cross the Trans-Pennine route, forcing the tram lines to climb to the necessary clearance height in a staggeringly short distance.
'Network Rail needs 4.8m clearance - it wants to be able to electrify the route in future - and there's not far to get up that high, ' notes Jones. 'The original proposal was to go beneath [the Trans-Pennine line] through tunnel.' Alterations to Thorpe Road Bridge so that the Metrolink track could start climbing sooner were also considered, but the cost and disruption involved in both options made them unattractive.
'We were able to go over the rail lines by keeping the bridge deck as shallow as possible. By adopting a finback design we were able to get deck thickness down to 600mm, ' Hunt states.
The cramped site has required the finback bridge to be skewed obliquely across the mainline tracks. Its main span will be 80m, with an overall length of 120m.
On a traditional structure, using a span to deck depth ratio of 1:20, 'you'd be looking at a deck at least 4m deep', notes Jones with pride. 'This is very rare.' Restricting deck depth has enabled Gifford to perform the equivalent of a high jump with a two pace run-up, gaining the necessary altitude with a 5% gradient approach ramp. Trams can manage a maximum 6%.
As well as resisting bending, the fin must act in torsion as, to marry up with the Metrolink route alignment east and west of the Trans-Pennine line, the bridge is also curved in plan. The deck cantilevers 10.25m either side of the central fin, meaning that selfweight alone puts the structure under considerable eccentric loading. Add to this the dynamic loading of trams - 5t/m, some 9m off the centre line - 'and it's a huge torsional weight', Hunt says.
To cope with the loads the fin is fairly hefty at 2.25m wide by 4.5m high at each end, swelling to 7m high at the pier. Wall thickness changes asymmetrically along the length of the fin, ranging from 250mm to 1m. Deck and box are being constructed in C50/60 concrete.
Within the box are 18 posttensioned prestressing tendons.
Their arrangement is unusual for a prestressed concrete structure, in that they behave in a similar way to stays on a cable stay bridge.
Specialist ubcontractor Freyssinet has worked with Gifford on designing the cable tendon arrangement - they run from anchorages just above deck level over the fin's internal pier, tracing wide inverted Vs in elevation. In plan they deflect on integral reinforced concrete deviators at 5m centres as the bridge curves. Freyssinet has also specified cable numbers, dimensions and strengths. Each will be composed of 37, 15mm diameter strands, and will be tensioned to 75% of its 1,000t maximum capacity.
Where the finback shows a clear advantage over a cable stay design is in the weather protection afforded to the cables, cocooned within the box, and in the fact that they are tensioned during prestressing of the concrete structure to a level that far exceeds that imposed by live loading.
They will never be subject to the fatigue of stress-relaxationstress cycles, helping reduce maintenance demands over the course of the structure's 120 year design life.
Balfour eatty-Bilfinger & Berger has already erected shuttering for the bridge to the north of the Trans-Pennine line and started concrete pouring.
The contractor is building the structure off line to avoid working over Network Rail's tracks, operating just within a 5m exclusion zone. The joint venture plans to rotate the completed structure into place during its one and only planned rail possession in July.
With the swing in mind, the bridge has been designed with a downstand pier sitting on a hefty 360mm diameter pin bearing, allowing the necessary 21.3° of rotation. It will be equipped with PTFE coated pot bearings at either end, allowing the nose and tail to slip into place, says Jones.
Because of its one thirdtwo thirds proportions, the relatively heavy nose will be supported during the rotation on a temporary, 1.7m deep, steel box girder. With the rotation complete, the nose will sit down onto its bearings under its own weight. The tail end will be post-tensioned down to a heavily ballasted west abutment.