ONE IN THE EYE FOR NELSON'S COLUMN
February 2006

Visitors to Dublin cannot help but be awe-inspired by the city’s latest piece of public art, the Spire – the world’s tallest sculpture. Reaching almost infinitely skyward and delicately reflecting ambient light, the Spire is simultaneously an artistic achievement, a civic statement and an engineering triumph that brought together some of the finest technology companies.

People are immediately struck by its height and elegance, and the keen eye will notice the quality of the light reflected from it at any time night or day. This is due to the precision peening of its stainless steel surface, achievable only because a bespoke machine was built especially for the manufacture of the Spire and controlled by a high tech motion control system developed by industrial control specialist Silverteam in Great Yarmouth.

Its base symbolises the roots of Ireland’s past, while its soaring tip, designed to dance gently in the breeze, represents to nation’s confidence in the future. The main body of the all-stainless steel Spire has been precision surfaced to softly reflect daylight and to be gently illuminated at night.

Replacing the O’Connell Street Nelson’s Pillar (a twin to the Column in London’s Trafalgar Square until blown up by dissidents in 1966), the Spire is the flagship of an ambitious urban renewal plan that will regenerate a large section of the city over the next decade. Its slender elegance is based on its slowly tapering girth, the 3m base diameter slimming down to 150mm at the very top and making it appear even taller than its 120m.

Designed by Ian Ritchie Architects, the structural engineering was given to Arups and production of the nine sections that make up the Spire was undertaken at Avensis in Sheffield. These were shipped to Radley Engineering in Dungarvan, Co Waterford for surface engineering to create the desired reflective surface. Radley also took delivery of a special peening made by the Metal Improvement Company (MIC) in Newbury, whose Keith Austin explains:

“For the artistic integrity of the piece it is imperative that light is reflected consistently from its entire surface, apart from the first 10m which are engraved with a symbolic pattern reflecting the human DNA double helix interwoven with rock core samples taken to assess the foundations.”

The base section was first polished to a mirror finish, and then a masking material was applied in the DNA pattern. The section was shot peened and the rock pattern computer cut into the surface. Finally the masking was removed to reveal the finished section.

An absolutely even peening pattern was required over the remaining 110m length and because of the taper MIC’s machine has to be especially for the purpose.

The machine was based on a standard welding rotator rig fitted with an additional traversing axis that would move a peening head along the length of the section being treated. Both axes of movement had to have variable speed capability so that peening density could be adjusted with the taper of the section.

MIC has developed many bespoke machines over the years and has a close working relationship with Silverteam, Hitachi’s drives and automation company in the UK, for developing their control systems. The solution arrived at was to fit a separate inverter to each axis and co-ordinate them through a PLC. The speed of the rotary axis is determined by the declining circumferential distance around the tapering section at any point and the traverse axis is slowed so that the peening progresses up the surface evenly like a screw thread being cut.

“The theories of conical geometry that we used to define the speed profiles are relatively simple, but making it work in reality to the required precision proved to be quite a challenge,” recalls Austin. “To check our calculations we did pre-peening dummy runs using a tracking sensor that allowed the PLC to build and remember a motion profile.”

In fact sacrificial plates were fitted to both ends of each plate and peening was started and finished on these, so that the main plates had a consistent pattern for their entire length.

“To ensure an even finish we used several tricks of the trade. We did the work in four passes, each time starting a quarter turn on from the previous pass, with an induction tag being used to define a datum. We used a seven nozzle peening head, the prime number meaning that there would be no tracking of one nozzle over the path of another. The head had a peening length of 525mm, which exactly matched the traversing speed of 525mm per full rotation, so that we progressed in a perfect helix up the length of the section.”

From conception of the idea, the Spire was about five years in the making. But most of this time was consumed in planning what was in fact a major engineering project. Production of the finished plates had to be completed in a matter of weeks, so MIC engineers joined Radley’s team in Dungarvan to make sure all went smoothly. Once on site the simple geometric design allowed swift and efficient erection so that O’Connell Street’s famous traffic was hardly disrupted at all.