Science & Technology in Action

5th Edition

The Samuel Beckett Bridge

Dublin City Council

The iconic Samuel Beckett Bridge in Dublin illustrates many aspects of science and engineering. This lesson describes different bridge designs and analyses the power required to rotate such a structure.
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Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

The Samuel Beckett Bridge, which was designed by Santiago Calatrava, is a cable-stayed cantilever bridge with a span of 120 metres crossing the river Liffey in Dublin city. Its single support is 28 m from the South quay and the bridge can be turned through 90° to facilitate marine traffic. The shape resembles a harp on its side.

Beams and Cantilevers
A beam bridge is essentially a rigid structure supported at both ends. Various techniques are used to maximise strength while minimising weight. A girder bridge can be thought of as a variation of the beam bridge. I-beam and box steel girders are commonly used in construction; these shapes are much less prone to bending. Girder bridges can span more than 200 m.

If a beam overhangs its support – like a diving board – it is known as a cantilever. Simple cantilevers can be used for short spans. For longer spans various methods are used to strengthen the structure and prevent it from bending – for example, by using cables or networks of girders.

Truss Bridges
More complex shapes allow the load on the bridge to be distributed more evenly and extended over greater distances. In the case of truss bridges the strength is greatly enhanced by the fact that the individual lengths of the members are relatively short and much use is made of triangulation in the structure. There are many different variations of the basic truss bridge design.

True or False?

  1. An I-beam girder has a square cross-section. false
  2. The Vikings constructed steel truss bridges. false
  3. A cantilever is a horizontal beam that protrudes beyond the support point. true
  4. The deck of a cable-stayed cantilever bridge hangs from suspension cables. false
  5. A truss structure is much stronger than a simple beam of the same mass and length and of the same material. true
  6. The pylon of a cable-stayed cantilever bridge is in compression and the cables are in tension. true
  7. When a beam is balanced then the weight on each side is always the same. false
  8. The counterweight used in the Samuel Beckett Bridge is 250 tonnes. false
  9. The intensity of a stress has the same units as pressure, N/m2. true
  10. The moment of inertia of a simple beam is proportional to the square of the length. true