The Tay Bridge Disaster
The completion of the world’s longest bridge in February1878 was heralded with great applaud and it was indeed a marvelous feat of British Engineering in the Victorian era.
However, there were certain flaws in its design that would lead to its eventual structural failure, and a mere nineteen months later, the high center section collapsed, making it Britain’s worst bridge disaster. There have been various theories offered as to the reason for the collapse, with the poor seabed survey, train derailment, bad design, and the use of materials inadequate for withstanding high wind stresses being some of the more popular ones.
The images below show the missing high girder section, taken soon after its collapse and, on a happier note, the New Tay Bridge.
As the bridge spanned the River Tay Estuary, a survey of the estuary seabed was required to evaluate its suitability to support the structure. Once completed, the original supports were designed on the results of this report.
The survey was later proved to be inaccurate and was the first error; the support design had to be altered. This along with cost-cutting measures being demanded by the railway company because of financial overrun, led to what was later referred to as under-designing of support structures.
The seabed of the shallower waters each side of the estuary was firm bedrock- conventional brick upstands being built to support the structural girders. These masonry platforms were not in any way responsible for the eventual structural failure; they are still evident to this day, forming a breakwater for the new replacement rail bridge.
However, the seabed under the deeper waters near the center of the estuary that were navigated by the sailing boats of the time was another matter. Here the seabed consisted of a mix of sand, sediment, and gravel, not a suitable base at all for the conventional heavy masonry foundation.
Therefore, a revised significantly lighter foundation design was proposed consisting of steel caissons piled into the seabed to support the vertical columns. As Bouch was under pressure to reduce construction costs, and as this support design offered a much cheaper option, he agreed to the design change in this section of the bridge.
The bridge structure was also required to be higher in this section to allow tall-masted sail boats to pass under. To accomplish this Bouch incorporated a raised trussed structure, known as "the high girder sector," being positioned about at the bridge’s midpoint.
It is here in the high girder sector that we will concentrate our examination of the structure, since subsequent inquiries have cited failure of the components in this area being responsible for the bridge failure.
Please turn to page 2 for further examination of these components.