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The Tay Bridge Disaster
Between the cities of Edinburgh and Dundee there has always been a large natural barrier of the Firth estuary. This was bought to attention the most when the railways expansion exploded in the industrial revolution. Due to the fact that if a passenger that wanted to travel from Edinburgh to Dundee they had to divert round the Firth going via Sterling, which added 60 miles to their journey. The only other alternative would have been to cross the firth using a ferry. This was from Tayport using the Broughty boat ferry. The fastest possible journey time from Edinburgh to Dundee was to use the boat ferry which, if leaving Waverly Station from Edinburgh at 6.25am would arrive in Dundee at 9.37am which was a total journey time of 3 hours and 12 minutes for a journey which was only 46miles in length.However on many occasions the weather was so bad that it was impossible for the ferries to run.
This service was operated by the Edinburgh and Northern Railway who realised that the service had to be improved. They therefore hired the services of Thomas Bouche in 1849, at the time only 26 years of age, with his main task to reduce the time of the journey. Immediately he focused on the ferry service and by 1850 the worlds first roll-on-roll-off train ferry was in operation. Nevertheless he realised this solution would not satisfy a higher demand of trains and when Eastern and Northern railways were taken over by the rapidly expanding North British Railways in 1854 Bouche concluded that the best solution would be to build a bridge across the firth. This caused great controversy and experts at the time stated the idea of a bridge was “the most insane idea ever to be propounded” Yet the case for the bridge was overwhelming as a better solution to crossing the Firth. An Act of Parliament was passed on 15th July 1870 for the construction of a bridge to cross the Tay. By this time Thomas Bouche had become an independent engineer and was assigned as chief engineer for the new bridge.
Charles de Bergue won the construction contract for the bridge which they signed on 8th May 1871 and on the 22nd July 1871 the foundation stone was laid. The original contract was for the works to come to £217,000. The works commenced to plan but after the construction of the first few piers problems had been encountered. It was found that the foundations for the piers had to made a lot deeper in the deeper sections of the estuary, this is due to the fact that deeper into the estuary the hard bed rock changed to sand and gravel. Therefore Hopkins, Gilkes & Co. replaced the original contractors as they had more experience. Due to this change when the bridge was opened on the 31st May 1878 the cost of construction had risen to £300,000.
Due to the problems encountered by the first contractors Thomas Bouche changed his design of piers from solid brick to the relatively new design of cast iron with wrought iron in selected areas due to them needing to be of lighter construction. Bouche had earlier used this method of construction on a previous bridge at Belah in the Pennines.
The general design of the bridge was that there was a single track supported on 85 piers. The first 14 were of the brick and masonry construction and the rest of cast iron. The cast iron columns had to be strengthened with additional wrought iron struts and ties to support the loads going to be applied. From the south side the bridge left the bank and climbed up to pier number 29 at a rate of 1 in 490. From pier 29 to 36 the track levelled out and after this had a rapid decent at 1 in 74 till it met the north bank at Dundee. At the navigable part of the estuary the bridge had to be at its highest so that boats could pass underneath. This was between sections 29 to 36 thus these were the highest girders of 88 feet above the high water mark, and of a raised, trussed box structure.
Thomas Bouche was subsequently knighted after the completion of the bridge as at the time it had been the “largest single engineering project in Britain” and was the longest bridge in the world at over two miles in length when it opened on 1st June 1878. However disaster was to strike just over a year after the successful construction.
On the 28th December 1879 it was scheduled for the 4.15pm train from Edinburgh to Dundee to cross the Tay Bridge from the south side of the Tay to the north side at Dundee. At around 7.15pm the train began to cross the bridge as scheduled. It was then due to pass the signal box at Dundee at 7.19pm however it never reached the signal box. When the train did not arrive the local foreman at Dundee decided to walk over the bridge to investigate why the train had not arrived on time. The wind had been growing all day coming from the west which is at right angles to the bridge and at this time gusts of wind were estimated at force 10/11 on the Beaufort scale. Due to the extreme winds that were being experienced it was extremely difficult to walk and at some points the foreman was forced to crawl as the wind was too strong to walk against. His progress was then completely halted as he came across the reason for the train not arriving. The bridge girders had completely collapsed and the train had plunged into the Tay. There had been 75 people on board the train and there were no survivors. This was the worst loss of life due to structural failure and collapse in the UK (other than due to war actions).
Due to the high winds the masonry column bolts lifted up by small amounts each time a strong gust came by, this was due to the fact that the bolts, used to hold the columns in place, were only attached to the first two layers of masonry. Observers witnessed that some diagonal ties snapping and the bridge began to shake violently mainly attributed to the fact of the extra force coming from the lifting of the column bolts. If however the column bolts had been attached to all layers of masonry, they would have withheld the extra force of the wind and therefore the diagonal ties would not have snapped. The snapping of the diagonal ties meant that the structure did not act structurally as it was designed. It caused the lateral stiffness of the structure to be a third of that of the original structure. This meant the bridge had become very flexible and would sway in the gusts of winds. Unfortunately at the same time as an extra strong gust of wind, the train was on the bridge. The extra weight of the train acting on the already reduced strength of the bridge caused disaster. It caused the leeward columns to fail in compression whilst at the same time the downwind ones failed in tension. This caused sections of the bridge to completely collapse and fall in to the estuary.
The Board of trade immediately called for an investigation of the failure and collapse of the bridge as soon as the news reached London. They appointed three commissioners who would consider the evidence gathered. There was Mr Rothery, a maths graduate and had also trained as a barrister, who was commissioner of wrecks, Colonel Yolland, a royal engineer who became chief railways inspector in 1877 and Mr W H Barlow the president of the institute of civil engineers and also a well respected practising civil engineer. There was an initial session held in Dundee court house which started on 3rd January 1880 and lasted 3 days.
It was concluded that an innovate method of investigating the disaster would be taken and it would be comprised of detailed mechanical tensile testing undertaken by David Kirkaldy. Also failed samples were used in the inquiry as exhibits. In addition a scale model of the bridge was constructed so hopefully the failure could be understood. Another critical part to the trail was that over 50 photographs were taken of the failed bridge less than a week after collapse. This was vital as all the failed members were more or less in the place after failure, thus it enabled experts to prove how the piers had failed.
Eye witness accounts were also vital as they included information that could not be determined by structural analysis or from analysis of the failed piers in situ. In total one hundred and twenty one eye witness' were called upon in the inquiry. The inquiry was completed in only 6months which was quite a feet as the scale of the disaster was so large and much evidence had to be collated. However the fast conclusions were necessary as lessons need to be learnt for future bridge design.
The enquiry found that “The Fall of the bridge was occasioned by the insufficiency of the cross bracing and its fastening to sustain the force of the gale.” Also stated was if the wind bracing and piers had been maintained and also constructed properly then The Tay Bridge would have endured the storm and not collapsed. After the inquiry in to the collapse of the tay bridge all previous bridges built by Thomas Bouche were examined and reinforced or rebuilt. Also The Board Of Trade banned the construction of bridges using cast iron columns and they started the inspection of bridges whilst under construction. In 1881 a Royal Commission on “Wind Pressure on Railway Structures” was initiated. This was undertaken formerly by W H Barlow, G G Stokes and W Yolland. They collated values of wind pressures applied on bridges using anemometers and calculations were completed. From these a maximum wind pressure of 56 pounds per square foot on the design of bridges was implemented. This was adapted for the construction of bridges of different designs so that hopefully another disaster would not occur.