Spanning over one of the busiest suburban road junctions in Dublin, the spectacular cable stayed Taney bridge in Dundrum has brought together engineers from Dublin, Northern Ireland and the UK - and a returned emigre from China - to create a structure which is destined to become a city landmark. The bridge forms one of the key links in the city's first light rail project (see box).
While most engineers enjoy starting with a blank paper, for the engineers at Taney, more than a few lines were already filled in which were to drive the choice of structural form.
Most restrictive was the parallel construction of a longawaited new bypass of the busy Dundrum bottleneck. Currently the whole area has the appearance of a construction battlefield as road traffic snakes through, cheek by jowl with construction machinery.
'Traffic had to be kept moving at all times because it is a major new junction being built. That limited where we could put in new or temporary supports, ' says project consultant Roughan & O'Donovan managing director Joe O'Donovan.
The railway alignment follows the route of the disused Harcourt Street to Bray line, filling in a few more lines on the blank sheet. 'We were also therefore constrained with the level of the railway. Additionally there are significant services crossing the junction, particularly a fibreoptic telecoms cable which could not be moved because of the high cost. This dictated the location of the piers, ' says O'Donovan.
With a deck thickness limited to around 1.3m and piers limited to either side of the wide junction, a cable stayed bridge emerged as the only contender.
Concrete was chosen as the material. 'In Ireland, we tend to go for concrete. With around 95% of our bridges in concrete, there is no real experience of maintenance of steel bridges by local authorities.' says O'Donovan.
The final design was for an asymmetric single pylon bridge, with a main span of 108.5m, a backspan of 21.5m and two approach spans of 18m and 14m.
A 50m high A-frame or inverted Y - 'like a tuning fork' as Roughan O'Donovan chief bridge engineer Keith Wilson describes it - forms the pylon for the structure, one of the tallest around.
Given the height, getting the aesthetic right first time was important as any delays would have affected delivery of the whole of Luas Line B.
While the eagerness of residents for the new rail system might have helped ease approval, the spectacular form displayed on artists' drawings at public consultations proved popular. 'People see it as a positive addition to the landscape, ' says O'Donovan. 'Cable stays seem to have an appeal to most people, compared with an ordinary steel composite bridge, for example, which punters don't seem to like.'
The orignal design envisaged a prestressed deck in both precast and insitu concrete. But up north, Newry-based contractor Graham eyed the job keenly. An earlier success with pioneering technology in Belfast gave it a head start on other tenderers, and was to reap cost rewards for Luas.
'We had worked on the Crossharbour Bridge in Belfast in the early 1990s which had a glued segmental deck construction. We could see that similar methods of construction could be used here, ' says Graham contract manager PJ McCaffery.
Graham brought in some big guns to help. Consultant Robert Benaim was engaged to look at the design and to capitalise on Graham's glued segmental expertise. 'At the tender stage, we looked to innovation to help us win the job and came up with a scheme with Benaim, who we had worked with before, ' says McCaffery.
The plan for the deck was to erect precast concrete shell segments, with the 3.5m long and generally 13.1m wide trough units acting as a shutter, which were then filled with insitu concrete, forming the soffit.
Sections are match-cast off site - one next to the last. 'You get a perfect fit on site as the geometry of the sections is tightly controlled. It allows you carry on casting while you erect the deck, ' says McCaffery.
Stays are installed as the deck proceeds, eliminating the need for temporary supports interfering with the road underneath.
Cable loads are adjusted to ensure the correct deck level.
Saving on shuttering and concreting has reduced the labour required.
'Labour is very expensive in Dublin and this has helped drive down the cost, with significant savings, ' says McCaffery. Prestressing will be by a combination of bars and prestressing strands.
Like all successful ideas, it seems simple in hindsight, but the savings were enough to beat off the competition - no mean task for a prestige job probably passed each day by many an executive of rival Dublin-based construction firms.
Precast units are cast in C65 concrete, with C40 or C55 for the insitu concrete, all containing 40% ground granulated blastfurnace slag (GGBFS). 'This gives low heat generation with the thick sections and very high cement contents we have here, and there are other advantages. If you can cure slag concrete properly, it has long term durability benefits. And GGBFS reduces the risk of cracking over ordinary Portland cement, ' says O'Donovan.
Other advantages are peculiar to Ireland.
'The colour of cements manufactured in Ireland is very dark - GGBFS helps give it a lighter colour, ' he adds.
Insitu concrete is used for the pylon, with temporary works specially designed by consultant Tony Gee & Partners and supplied and erected by Fisher Engineering of Eniskillen.
'It is essentially a steel A-frame which supports the inclined shuttering for the pylon very rigidly, rather than a conventional climbing shutter, ' says Rail Procurement Agency engineer's representative Brian Bromwich.
McCaffery says the temporary works also allow for faster progress and a safer working environment at height, as well as easier setting out.
Each of the 52 stays (13 either side of both spans) designed and manufactured by VSL contain between 19 and 37 galvanised strands, and are fixed by gavanised steel guides into a massive reinforced concrete anchor block on the back span.
Strands are protected by HDPE sheathing with a helical rib on the outside formed by a tiny protrusion. 'The ribs stop wind and rain induced vibration, and are standard, ' says Wilson. But no chances are being taken in preventing a dreaded wobble in the stays should the elements decide to pluck them like a set of harp strings and a rolling train amplify the vibration.
'We've made provision to allow the addition of friction dampers in case there's excessive vibration, ' says O'Donovan.
Extensive foundation works have been constructed by subcontractors Bachy and Rilmount. The huge 8m by 23m by 24m concrete anchor block which holds the stays at the end of the smaller span acts as a 10,000t counterweight to the cable forces but the net difference leaves little net bearing pressure on the ground, despite its huge bulk.
All major structures such as the pylon and supports are piled. The 900mm diameter bored cast-insitu piles are up to 22m long, and are rock socketed into the Dublin granite. Working load pile capacity is up to 3,200kN. 'The piles are big by our standards.
The overlying ground consists of gravels and weak material and for the loads we're taking we need to get into rock, ' says Wilson.
The designers were presented with a new challenge during the course of their work - in the shape of a Dublin metro with larger trains. The bridge had to be beefed up to take this in the event of the scheme becoming reality. Other complexities included provision for insulation from stray DC currents which could corrode reinforcement and for protection against lightning. 'The provisions tend to be conflicting which made it difficult, ' Wilson notes.
Remarkably for a job of its complexity, the 56 week contract due to be completed at the end of October is valued at a modest Euro 8M (£4.9M).