Bemused Dubliners slow down as they drive along the quays to look at the giant creature-like structure which has almost filled the River Liffey just upstream from O'Connell Bridge in the city centre.
'Don't tell me they're drilling for oil now. Or maybe they're building a hotel in the Liffey, ' quips a passer by as he surveys the skyline bristling with tower cranes.
Surprisingly few seem to realise that what is under way is construction of the Liffey's newest span, the Blackhall Place Bridge. Designed by worldfamous architect engineer Santiago Calatrava, it could well become Dublin's most notable modern landmark .
The bridge is a collaboration stretching from Dublin to Belfast to Zurich. The IR£5M (£4M) contract is being carried out by Irishenco, owned by UK contractor Mowlem. Calatrava has joined with Dublin consultant Roughan & O'Donovan (ROD) on the design, while fabrication of the structure will take place at Harland & Wolff's Belfast shipyard.
It could not be a greater contrast to the Liffey's other crossings. The single 40m span bridge consists of two tied steel arches, curved in plan and inclined outwards, from which the deck is supported on steel hangers.
Four lanes of traffic will be carried by the bridge whose deck grows from 13m in width at midspan to 18m at the ends. Outside the arches, a pedestrian access and meeting or viewing area cantilevers to the side. The composite steel deck consists of cross-girders spanning transversely between longitudinal box girders which form the ties to the arches, while the cross girders cantilevering from the side of the bridge form the walkways. These will be paved with granite flags, while translucent glass at the walkway edges will give a view to the river below.
The white steel structure will be lit to provide a spectacular new addition to the river and Dublin's skyline.
Structural design work has been carried out jointly by Calatrava's team in Zurich with ROD in Dublin. 'The appearance is entirely within Calatrava's province, but we have also assisted with aspects of the detailed design. It has been a unique project for us, working with Calatrava and working in steel, as less than 2% of bridges in Ireland are in steel, ' says ROD senior bridge engineer Nigel O'Neill.
While continental engineers in designer suits may muse over the finer points of detail, the dirty work of building the bridge is already under way in the Liffey. The site must be one of the most tightly constrained in Dublin, flanked by the quays which act as vital traffic arteries carrying some of the city's heaviest traffic. Road closures during the day are not an option, and the narrow streets have no space to give up.
The engine of Irishenco's site is the giant Mowjack jack-up barge and crane brought in from the UK. Nestling in the river, the crane lifts material, brought to site during temporary lane closures at night, onto two floating barges moored in the river. The waterway is tidal and rarely navigable at the site.
'It's a very tight site. In fact at the moment we've no site, as we've decided to get on with building the cofferdams to form the bridge abutments before assembling the site offices, ' says Irishenco project engineer John Bennett. Site operations are carried out from the Mowjack, with visitors and workers delicately ferried between the Liffey and dry land in a crane skip.
Work has begun on two cofferdams beside each quay within which the piles for the bridge abutments will be bored by Sam Foundations. The bridge will be supported on four bearings, each resting on a concrete block cast up from a pile cap. This block will be granite clad to match the existing quay wall, but both block and pile cap will be separated from the old wall by a joint filler, to allow the bridge behave independently of the quay wall. 'The bridge however has the appearance of being supported on the quay wall, but you could take it (the wall) down and the bridge would not be affected.
The pile group below is eccentrically loaded by the bridge, ' says O'Neill.
The eccentric loading means some piles are in tension, hence each 900mm diameter pile will have a 3m rock socket at the toe.
'We will be driving to limestone rock, which slopes considerably on the site - it's 10m down on the south side and 15m down on the north. The ground consists of around 1m-1.5m of rubble and silt, then compact gravel over limestone, ' says Bennett.
The site lies in one of the oldest parts of Dublin, near to the first settlement on the Liffey.
Archaeological investigations by divers have uncovered an old quay wall and some leather and crockery artifacts, and liaison will continue during excavation of the pilecaps. Some of the finds were relatively recent. 'We were surprised with one find when the divers found a revolver, ' says Bennett, hastily adding that it was long past working order with no clue as to its origin.
Erection of the bridge will be carried out from a temporary platform to be designed by contractor's consultant Carroll & Brown and consisting of tubular piles and beams with jacks and a platform underneath for access.
'We have had to leave two channels free for the university boat race which influenced the span of the beams and has been a difficult constraint, ' says Bennett.
The erection sequence envisages bringing both box girders and attached cross girders in two sections to site where they will be welded, followed by the inner cross-girder sections running under the carriageway. The carriageway girder along the spine of the bridge deck will then be bolted to the cross girders.
The arches will also come in two and be welded to the box girders at the ends, with the cables then fixed in position and tensioned.
Vehicles will run on a blacktop surfaced precast concrete slab when the bridge opens in the spring.