The Spire, due to be lit up tomorrow, was engineered by an Irishman notto snap in wind, writes Dick Ahlstrom.
Love it or hate it, the Spire of Dublin is with us for keeps - or at the very least for the next 120 years. Tomorrow night, more than six months after the Spire was erected, it is due to be lit: the lamp inside the stainless-steel spike will be turned on, to shine through 12,000 perforations. An inauguration ceremony is set to follow on Monday.
The orange warning beacon attached to its tip when the final section of the Spire was lifted into place, in January, blew off only days later. The Spire itself should stand the test of time, however. Having been designed by Ian Ritchie, an English architect whose early career included working on the glass pyramids at the Louvre, in Paris, the Spire was then engineered for survival in our persistent winds by an Irishman, Cormac Deavy.
The monument, which Ritchie likened to an underground cathedral whose steeple soars into the sky, won the 1998 competition to replace the ill-fated Nelson's Pillar, which the IRA blew up in 1966. The confidence of the boom days may have inspired Dublin City Council to select the dramatic design, a choice that went through legal battles and suffered public criticism as the "stiletto in the ghetto".
Deavy, who says the council was courageous to do something so different, had to find a way to keep the unusual structure standing no matter what the weather threw at it. An associate director at Arup Consulting Engineers, he led the design team responsible for delivering an engineering solution. His first response mirrored that of many people struck by the Spire's apparent simplicity. "The first five seconds you think this is straightforward," he says, "but you then begin to realise how complex it is."
For a structure that stands just over 120 metres high, the big issue is the wind: how do you keep it standing in difficult Irish conditions for its designed life of well over a century? Without the wind it would just be a metal spike supporting its own weight. Add the wind, however, and you need dampers to stop it rocking back and forth and, eventually, snapping in two."That is what the dampers are there to control," says Deavy, "vortex-induced oscillations."
Surprisingly it is not "big wind" but far more frequent slower, gusting wind that causes the most problems for this type of structure. Gusting winds shove the Spire, then slacken before giving another push. Without a damping system the frequency of the gusts could match the natural frequency of the Spire - the rate at which it resonates, almost as if it were a giant tuning fork - causing it to wobble violently and shear. Deavy and his team had to design for both strong and gusting wind - "and everything in between".
The original design called for a damper "like a spoon stuck in a jar of honey". The drag of a paddle through a viscous liquid would be enough to keep the frequencies of the Spire and the wind from matching. Then they switched to liquid-filled shock absorbers, much like those on a car. These connect the Spire to two "masses" - weights of 1,250 kilograms and 800 kilograms - hung inside the fifth segment of the Spire. "What we have done is put shock absorbers between the masses and the Spire wall," says Deavy.
If the wind starts to rock the Spire, the weights' inertia dampens its movement, preventing the Spire from establishing a coherent rocking motion. This should stop it from shaking apart, although it doesn't prevent it from swaying gently in the breeze.
For Deavy, a graduate of NUI Galway who has worked Germany, France, England and Malaysia, working on the Spire was a welcome opportunity to work on an Irish project. "It is nice to do something in your own country and something as striking as the Spire," he says. "It is very special. It is something to be proud of."