Tay Bridge Disaster - The Bridge

The Bridge

Construction began in 1871 of a bridge to be 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 bridge girders were deck trusses, the tops of which were level with the pier tops, with the single track railway running on top. However, in the centre section of the bridge (the "high girders") the bridge girders ran as through trusses above the pier tops (with the railway inside them) in order to give the required clearance to allow passage of sailing ships to Perth.

Bedrock actually lay much deeper and Bouch had to redesign the bridge, with fewer piers and correspondingly longer span girders. The pier foundations were now constructed by sinking brick-lined wrought-iron caissons onto the riverbed, and filling these with concrete. To reduce the weight these had to support, Bouch used 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, but for the Tay Bridge, the pier dimensions were constrained by the caisson. There were 13 high girders spans; to accommodate thermal expansion, at only 3 of their 14 piers was there a fixed connection to the girders; there were therefore 3 divisions of linked high girder spans, the spans in each division being structurally connected to each other, but not to neighbouring spans in other divisions. The southern and central divisions were nearly level but the northern division descended towards Dundee at gradients of up to 1 in 73.

The bridge was built by Hopkin Gilkes and Company, a Middlesbrough company which had worked previously with Bouch on iron viaducts. Gilkes, having first intended to produce all ironwork on Teesside, used a foundry at Wormit to produce the cast-iron components, and to carry out limited post-casting machining. Gilkes were in some financial difficulty ; they ceased trading in 1880, but had begun liquidation in May 1879, before the disaster. Bouch's brother had been a director of Gilkes, and on his death in January 1876 Bouch had inherited Gilkes shares valued at £35,000 but also a guarantee of £100,000 of Gilkes borrowings and been unable to extricate himself.

The change in design increased cost and necessitated delay, intensified after two of the high girders fell when being lifted into place in February 1877, but the first engine crossed the bridge in September 1877. A Board of Trade inspection was conducted over three days of good weather in February 1878; the bridge was passed for use by passenger traffic subject to a 25 mph speed limit, but the inspection report noted:

'... When again visiting the spot I should wish, if possible, to have an opportunity of observing the effects of high wind when a train of carriages is running over the bridge ...'.

The bridge was opened for passenger services on 1 June 1878. Bouch was knighted in June 1879 soon after Queen Victoria had used the bridge.