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Structure of the spire

The Spire of Dublin is a true collaboration between architecture and engineering.Engineer Arup worked with Ian Ritchie Architects at the competition stage and undertook detailed research into the design of the structure and its geotechnic, mechanical, electrical and public health aspects.The company also provided wind engineering services and supervised on-site construction.

The project was full of engineering challenges, but the really satisfying side to it was the holistic approach to design and the use of processes from outside the mainstream construction industry, such as polishing and peening. The spire is supported on stainless steel reinforced concrete walls of an underground access chamber, which houses electrical and drainage systems. This in turn is supported on eight steel-reinforced 900mm diameter concrete piles anchored into the bedrock.Once the piles were in place, the pile cap was monolithically connected to the bottom of the concrete chamber.

To counter dangers of wind-induced vortex shedding, a tuned mass damping system was built into the fifth of the Spire's eight frustums (sections). Designed by Arup and Motioneering, of Guelph, Ontario, this is a passive system consisting of two stainless steel masses suspended on cables.Each mass is connected to four viscous damping units and tuned to a specific frequency by adjustment of the length of the suspension cables.This technology has been around for some time, but the spire marks the first use of tuned mass dampers in Ireland .The mass damping system had to be coordinated to fit into a very tight space without hitting the internal access ladders or the electrical cables supplying the lighting system.

Many other aspects of the design - the selection of materials, wind analysis and wind profiling, pile design, fatigue analysis - involved fundamental research into the physics of specific problems.The complexities were further compounded by a specified design life of more than 120 years.

But the most demanding issue was the height of the spire and the effects of the wind's many moods; these factors determined plate thickness, joint design criteria, weld fatigue criteria, damper design and tolerances.

Cormac Deavy, associate director, Arup

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