Dundee - Tay Bridge - postcard by GD & DL - c.1905-10

• Postcard

   

• Picture / Image:  Tay Bridge [Dundee] 
• Publisher: GD & DL 
• Postally used: no
• Stamp:  n/a
• Postmark(s): n/a
• Sent to:  n/a
• Notes / condition: 

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Text from the free encyclopedia WIKIPEDIA may appear below to give a little background information (internal links may not  work) :

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The Tay Bridge carries the main-line railway across the Firth of Tay in Scotland, between the city of Dundee and the suburb of Wormit in Fife. Its span is 2.75 miles (3.5 kilometres).

The present structure is the second one on its site.

From about 1854, there had been plans for a Tay crossing, to replace an early train-ferry. The first bridge, opened in 1878, was a single-track lattice design, notable for lightness and low cost. Its sudden collapse in a high wind on 28 December 1879 was one of the great engineering disasters of history, and its causes are still debated today.

The second bridge was a double-track construction of iron and steel, opened in 1887 and still in service. In 2003, a strengthening and refurbishing project was recognised by a major award for the scale and difficulty of the work.

The original Tay Bridge was designed by noted railway engineer Thomas Bouch, who received a knighthood following the bridge's completion.^[1] It was a lattice-grid design, combining cast and wrought iron. The design was well known, having been used first by Kennard in the Crumlin Viaduct in South Wales in 1858, following the innovative use of cast iron in The Crystal Palace. However, the Crystal Palace was not as heavily loaded as a railway bridge. A previous cast iron design, the Dee bridge which collapsed in 1847, failed due to poor use of cast-iron girders. Later, Gustave Eiffel used a similar design to create several large viaducts in the Massif Central (1867).

Proposals for constructing a bridge across the Tay date back to at least 1854. The North British Railway (Tay Bridge) Act received the Royal Assent on 15 July 1870 and the foundation stone was laid on 22 July 1871.

The original design was for lattice girders supported by brick piers resting on bedrock shown by trial borings to lie at no great depth under the river. At either end of the bridge the single rail track ran on top of the bridge girder, most of which therefore lay below the pier tops. In the centre section of the bridge (the “high girders”) the railway ran inside the bridge girder, which could then run above the pier tops to give the required clearance to allow passage of sailing ships upriver (e.g. to Perth). To accommodate thermal expansion there were few rigid connections between girders and piers.

As the bridge extended out into the river, it became clear (December 1873^[2]) that the bedrock really lay much deeper; too deep to act as a foundation for the bridge piers. Bouch redesigned it quickly to reduce the number of piers and correspondingly increase the span of the girders. The pier foundations were no longer taken down to bedrock; instead they were constructed by sinking brick-lined wrought-iron caissons onto the riverbed, removing sand until the caissons rested upon the consolidated gravel layer which had been misreported as rock, and then filling the caissons with concrete.^[3] To reduce the weight the ground under the caissons would have to support, the brick piers were replaced by open lattice iron skeleton piers (each pier had multiple cast-iron columns taking the weight of the bridging girders, with wrought iron horizontal braces and diagonal tiebars linking the columns of the pier to give rigidity and stability). The basic concept was well known, having been used first by Kennard in the Crumlin Viaduct in South Wales in 1858;^[4] Bouch had used it for viaducts (notably the Belah Viaduct (1860)) on the South Durham & Lancashire Union Railway line over Stainmore, but for the Tay Bridge, even with the largest practicable caissons the pier dimensions were significantly constrained by the caisson. Bouch’s pier design set 6 columns in a hexagon; this maximised the pier width but not the amount of diagonal bracing directly resisting sideways forces.