Tubular bridge

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Tubular bridge
Conwy Railway Bridge
Ancestor:	Plate girder bridge
Related:	Box girder bridge, Skyway, Jetway
Descendant:	None
Carries:	heavy rail
Span range:	Medium
Material:	Wrought iron, steel
Movable:	No
Design effort:	Very high
Falsework required:	Depends upon length and degree of prefabrication

The three bridges across the river Conwy

A tubular bridge is a bridge built as a rigid box girder section within which the traffic is carried. Famous examples include the original Britannia Bridge over the Menai Strait and the Conwy railway bridge over the River Conwy, designed and tested by William Fairbairn and built by Robert Stephenson between 1846 and 1850. The Conwy railway bridge carries the North Wales coast railway line across the River Conwy between Llandudno Junction and the town of Conwy. The wrought iron tubular bridge was built by Robert Stephenson to a design by William Fairbairn, and is similar in construction to Stephenson's other famous tubular bridge, the Britannia Bridge across the Menai Strait. It was officially opened in 1849, but had been completed in 1848. Being the first tubular bridge to be built, the design needed much testing on prototypes to confirm that it would be capable of carrying heavy locomotives, the testing being performed by Fairbairn. The successful result enabled the much larger Britannia bridge to be built. The current Conwy bridge has been reinforced by extra columns under the bridge into the river, but is otherwise virtually unchanged since it was built.

Since the destruction by fire of Britannia Bridge in 1970, Conwy railway bridge remains the only surviving example of this means of construction undertaken by Stephenson. In the case of the Britannia Bridge this technology allowed a bridge with spans up to 460-feet (140-m) long to be constructed, when until then the longest wrought iron span had been 31 ft 6 inches.

Section of the original wrought-iron tubular Britannia Bridge standing in front of the modern bridge

Bibliography

• John Rapley, The Britannia and Other Tubular Bridges: And the Men Who Built Them, Tempus (2003).

See also

• Box girder bridge - a similar bridge that carries the traffic atop the box.
• Covered bridge - a type which may employ a variety of structures but which also encloses the traffic (for the protection of the bridge)
• Jetway - a movable bridge that carries pedestrians from a terminal to an aircraft.
• Skyway - a bridge connecting buildings at an elevation above the ground

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