Railway Miscellany – Bridges & Viaducts

Railway Miscellany

 Bridges & Viaducts

Ribblehead Viaduct

Ribblehead Viaduct

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Most early railway bridges were modest structures, many being simple arches made from brick or local stone.  In some cases metal was used, but the only material available in any quantity was cast iron – which had severe limitations.MaidenheadbridgeMaidenhead Bridge

The early engineers were cautious and usually allowed large margins for safety, making bridges stronger than they needed to be.  Many of these are still in use, carrying heavier loads at much higher speeds than planned.  Many people thought that the stone arches of Brunel’s viaduct across the Thames at Maidenhead were too shallow, and cracks soon appeared in them.  Later, the contractor admitted that the supports had been removed too soon, and, after the bridge had been repaired, Brunel ordered the scaffolding to be left in position, but no longer supporting the bridge.  During a storm, it was the scaffolding that blew down.  The bridge still stands today, taking far heavier loads at speeds even its designer did not consider.

Some early bridges did need to be replaced quickly or strengthened, and a few even collapsed.  The best known example is the Tay Bridge, which was destroyed in a storm because the designer did not allow enough strength for the effect of gales, which was made worse by poor construction and maintenance.tay_bridge_001Tay Bridge

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Wood was also used in bridge construction, even for viaducts, and Brunel built outstanding examples in Devon and Cornwall.  The piers were built of stone, but wooden timbers fanned out to support the track.  Wood has a limited life as a building material, but Brunel’s structures were designed so that individual timbers could be restored and renewed.  In the last part of the nineteenth century, steel was often used for bridge construction, the Forth Bridge being a famous example.ForthBridge03LForth Bridge

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With the invention of concrete, new construction methods were introduced.  The first concrete viaducts used separate blocks made to size, but later the whole bridge structure was cast on site.  A notable example is the Glenfinnan viaduct on the line from Fort William to Mallaig.800px-Glenfinnan_ViaductGlenfinnan Viaduct

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Reinforced concrete was followed by prestressing, a technique also used for making sleepers.  Nowadays designs in concrete are particularly advanced.  One viaduct had a continuous span cast at one side of a gap and then pushed over the top of piers to form the bridge.

220px-BarmouthBridge2007

Barmouth Bridge

 en.wikipedia.org

Many bridges crossing waterways had to be built high enough so that tall ships could pass underneath.  Another solution is the swing bridge, where one span pivots about its centre point to provide an opening.

 Foot bridge – almost every type of bridge has been used with success on the railways, except for suspension bridges.  As early as 1830, the Stockton & Darlington built one across the Tees on the line to Middlesbrough.  But it did not inspire confidence, and one driver was so distrustful of it that he set the locomotive’s controls so that the train took itself across while he crossed on foot!

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