Britannia Bridge - Pushing the Limits: New Adventures in Engineering - page 40

Britannia Bridge
Among the masterpieces of bridge building in the first half of the nineteenth century is the sus-
pension bridge across the Menai Strait designed by Thomas Telford and completed in 1824. In this
580-foot span high above the water, Telford used all three contemporary bridge-building materials:
stone, wood, and iron. The towers and arched approaches were of stone, the suspended roadway was
of wood, and the suspension chains were of wrought iron, which was known to be much stronger
and more reliable in tension than the cast iron used in the compressed arches of iron bridges. Telford
made sure he understood the behavior and strength of the iron by testing it.
In the 1840s, when it came time to bridge the Menai Strait with a railroad, the use of Telford's
existing suspension bridge came immediately to mind. However, since its roadway would deflect
excessively under heavy steam locomotives, driving the trains of the Chester & Holyhead Railway
over the bridge was out of the question. George Stephenson, the great railway engineer and Robert's
father, suggested that the railroad cars be unhitched from their locomotive when they reached the
bridge, hauled over by horsepower, and then rehitched to another locomotive on the other side.
Such a procedure was not in keeping with the image of a railroad that was intended to speed the
mail between London and the ferry to Dublin, however, and alternatives to bridging the strait were
sought.
Since the tides are particularly tricky in the Menai Strait, making it difficult enough to navigate in
its natural state, the Admiralty did not want any obstacles to shipping placed in the water. This elim-
inated the possibility of erecting falsework or piers anywhere but possibly on the Britannia Rock,
about a mile south of Telford's suspension bridge. The rock is about five hundred feet from either
shore, so it was natural to design a bridge supported there from beneath by arches or from above
by a tower and cables. The restrictions imposed by the Admiralty had driven Telford to choose a
suspension design earlier in the century, but now that the suspension principle had been dismissed
as unsuitable for a railroad—not because of lack of strength but because of lack of stiffness and re-
liability in high winds—an alternative to conventional bridge designs or construction methods had
to be found. Robert Stephenson even developed a scheme whereby an arch would be entirely as-
sembled near the shore on wooden scaffolding (known as centering) supported on pontoons, then
the whole assembly floated into place at high tide so as to insert the arch between the abutments