## The Spire of Dublin

### Dublin City Council

The design and construction of the Spire involved many aspects of engineering, science and technology. This lesson describes how potentially destructive resonance is prevented by the installation of two tuned mass dampers within the Spire.

Contains the full lesson along with a supporting toolkit, including teachers’ notes.

### Lesson excerpt

For 150 years the 40 metre high Nelson Pillar stood in the middle of Dublin’s O’Connell Street until it was demolished in 1966 after being damaged by an explosion. In 1998 Dublin City Council launched an international architectural competition for a replacement monument. There were 205 entries and the winning design was an elegant spire by Ian Ritchie Architects. It is a 120 m high cone that is three metres wide at the base and tapers to 150 mm at the top. Although it is essentially art, architecture and sculpture, it is also, as we will look at in this lesson, engineering, science and technology.

How was the Spire designed?
The spire was manufactured in eight sections of up to 20 m in length each of which is a frustum – a truncated cone. The sections were made from rolled sheets of stainless steel. A thickflange with holes in it was welded at the ends of adjoining sections to enable them to be bolted together. The top two frusta are joined by a threaded connector.

What effect has wind?
The force of the wind must be taken into account in the design of tall structures. The moment of a force, i.e. its turning effect, is the magnitude of the force multiplied by the perpendicular distance from its line of action to the point about which it can turn (the fulcrum). The force is a function of the wind speed and the surface area on which it acts.

For example, if a force Fdue to wind acts at a heighth then the moment of the force around the point A isF x h.

### True or False?

1. A frustum is the fixed point on which a lever can rotate. false
2. The moment of a force is also the turning effect of the force. true
3. The force of wind on a structure depends solely on wind speed. false
4. The suppression of resonant vibrations is called amplification. false
5. Resonant vibrations can be induced in metal rods and strings but not in columns of air. false
6. Harmonics are integral multiples of a fundamental frequency. true
7. Nodes are points at which the amplitude of vibration is a maximum. false
8. The total mass of the two tuned mass dampers in the Spire is about two tonnes. true
9. For a column that is fixed at one end the first two harmonics are in the ratio of 1:3. true
10. A steady wind can induce resonance. true
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