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Reach for the sky

Structures: Dublin Spire

The skyline of Dublin will shortly be dominated by a bold new monument that is bound to cause controversy.

Diarmaid Fleming reports.

Paris's Eiffel Tower and the London Eye both had their critics before becoming symbols of their cities. Dublin will be next when a 120m high stainless steel cone is completed in the heart of the Georgian city. Originally known as the 'Monument of Light', now officially the Dublin Spire, the new monument replaces an earlier structure, which attracted even more controversy in its time.

Dublin City Council's dramatic commission is a brave one given the backdrop of the staid, mainly Georgian architecture in O'Connell Street, Ireland's largest thoroughfare. Over the years the once grand street has become a long-time home to cheap burger joints and neon signs, but now it is to undergo a major renaissance.

At the centre of the street, near the General Post Office where the Easter Rising leaders surrendered in 1916, the 50m high Nelson's Pillar stood until 1966 when it was blown up by the IRA. The departure of Lord Nelson - dubbed Ireland's 'First Man in Space' - left a vacancy to claim the mantle of defining monument of the city.

The Millennium presented the opportunity to fill it, and the international competition was won by London-based architect Ian Ritchie's dramatic 'Monument of Light'.

'It's a new point of reference for the city and a clear statement of modernity for the 21st century, ' Ritchie says. 'The city wanted to mark the new millennium, but celebrating a moment in time is quite different.

Controversy soon followed when a losing entrant took a court action alleging Ritchie's design did not fulfil the brief.

This was rejected but delayed the project beyond its millennium deadline, with work only beginning in 2001.

Ritchie's long collaboration with Arup, from his work with the late great Irish engineer Peter Rice, continues on the Spire. Irish joint venture contractor is SIAC/Radley Engineering:

SIAC is doing the civils works while Radley has the arduous responsibility of the high-specification steelwork.

'It is unusual - the job is like a civils project, but has the precision required of an art piece, ' says Dublin City Council project engineer Michael O'Neill.

The Spire tapers from 3m diameter at its base to 150mm diameter at its 'sharp end'. Six hollow sections made from stainless steel plate ranging from 35mm thickness for the base section to 10mm at the top are bolted together to form the hollow cone. The 126t of steel ultimately rest on eight concrete piles - drilled and augured into granite rock up to 20m down through boulder clay.

Compressive working load in the piles is 1400kN, while socketing into the rock provides 1000kN resistance against tensile loads from sway in the Spire.

The piles support a reinforced concrete chamber housing a concrete plinth on which the Spire sits directly. This chamber also holds lighting and electrical equipment, and provides maintenance access. A flat patterned bronze annulus covers the chamber and surrounds the outside of the Spire's base.

Concrete is of the highest quality. 'It needs to last 150 years, so the grade is C65, while the reinforcement is also stainless steel, ' says O'Neill.

A template matching the holes in the flange at the base of the Spire is used to locate the 48 meaty 60mm diameter holding down bolts cast into the plinth.

Minute tolerances, a hallmark of the job, mean the highest precision is needed so everything fits perfectly on site. No site welding or insitu adjustments are allowed.

The high strength and corrosion resistant Grade 1.4404 austenitic ferritic stainless steel originates in France, where molten steel is rolled into plates which are polished and cut into plates before being 'folded' into curved sections by Barnshaw's in Scotland. Connecting and base flanges are made in Germany. All are finally welded to form the individual sections by Radley's in Dungarvan, 'a complex operation given the shrinkage and warping you can get from welding, ' says O'Neill.

Five individual frusta or truncated cone sections and a point section are fabricated: a bottom 20m section, then four 17m or 18m long pieces, with the last point section 37m long. All are connected with bolted flanges.

A huge 1000t crane to lift each section in place with surgicallike precision required to ensure the flange bolt-holes meet. Three sacrificial jacks in the plinth will plumb and level the bottom section. Maintenance ladders and platforms fabricated within the Spire will enable operatives to climb inside and tighten bolts varying from 60mm to 24mm.

Indicators will show when the required torque is reached or if further tightening is required should the weight of additional sections reduce tension in the bolts lower down. The last section - 37m long because its diameter is too small for anyone to get inside - is jointed with welded threaded collar joints.

An ingenious tuned mass damper system has been designed by Arup and Canadian specialist Motioneering. 'We needed damping to limit the movement and stresses from wind loading and vortex shedding which occurs around conical shapes, ' says Arup associate director and project engineer Cormac Deavy. 'The dampers are tuned to the lowest two modes of natural frequency of the Spire and then fine tuned.'

The system comes into play when wind speeds reach 10m/s, around a Force 5 fresh or stiff breeze. Deflection will be limited to around 1.5m at the tip for a 1 in 100 year design wind speed. Fine tuning can only take place immediately after erection, and the system is computerised to allow remote monitoring of movement in Dublin or Canada.

Distinctive lighting and surface finishing of the Spire is an essential aspect of Ritchie's creation. A shot-peened finish was chosen, where the surface is bombarded with minute steel balls to give an indented minutely dimpled surface.

Ritchie says he wanted a special pattern for the bottom 10m, 'a sort of Georgian height' like much of Dublin's old architecture, something defining the space of the site, as well as the life and light on the street.

A remarkable pattern of shotpeened and mirror-polished surface was chosen. The indentations of the face of a cylindrical rock sample taken beneath the Spire was projected onto a 2dimensional image. 'To symbolise the Irish diaspora, the ebb and flow of Irish people who emigrated and have returned, we then overlaid a double-helix of DNA over the scanned image of the rock pattern, and where they overlap is where we mirrorpolish, ' says Ritchie.

Lighting from the foundation chamber through a tiny gap between the bronze plate and spire will shine onto the bottom section. But above, 11,884 tiny perforations in the upper sections of the spire will enable light to shine through from an LED source in a tube within the Spire, providing a dramatic effect.

Provision is made for abseilers to climb the structure with harness eyes which can only be fixed from inside of the structure to prevent impromptu ascents.

Final fabrication checks are under way before erection takes place, hoped to be before Christmas.

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