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The original Tay Bridge was completed in 1878. Designed by Civil engineer Thomas
Bouch, it took six years to build using ten million bricks, two million rivets, eightyseven thousand cubic feet of timber and fifteen thousand casks of cement. Six hundred men were employed throughout the construction, twenty of whom lost their lives. The bridge cost over £300,000. The Tay Bridge was nearly two miles long, consisting of 85 spans and at the time was the longest bridge in the world. The spans carried a single rail track; 72 of these spans were supported on spanning girders below the level of the track; the remaining 13 navigation spans were spanning girders above the level of the track (i.e. the train runs through a tunnel of girders). These "high girders", as they were known, were 74m in length with a 26.8m clearance above the high water mark.
On the stormy night of 28 December 1879, the central navigation spans of the Tay
Bridge collapsed into the Firth of Tay at Dundee, taking with them a train, 6 carriages and 75 souls to their fate. At the time, a gale estimated at Beaufort force10/11 was blowing down the Tay estuary at right angles to the bridge.
It was high girder spans which fell. Most of the girders below track level, all of which remained standing, were transferred to the present Tay rail Bridge. At the time of the collapse Bouch was working on the design of the proposed Forth Bridge. In consequence, the design of the bridge was transferred to Benjamin Baker and Sir John
Fowler.
Train derailment
This theory suggests that the train crossing at the time was involved in the collapse of the bridge. It suggests that the train came off the track due to a kink in the rails with uplift of the train attributed to aerodynamic forces similar to uplift of an aeroplane’s

Group 7 wing. This uplift then blew one of the side carriages which hit the bridge. The shock experienced by the pier caused the cast iron lugs which connected the wind bracing members to the columns, to fracture which lead to the subsequent collapse of the pier structure . This theory is supported by the fact that a girder fell into the river during construction and was straightened out and put back into place, the likelihood is that the girder returned to the shape it was before it was straightened and hence this was the kink in the track.
Wind Theory
This suggests that the bridge was simply not strong enough to withstand the wind on that night. When the train reached the high girders on the bridge there was a particularly strong gust of wind. This increased the overturning force enough to cause the base of the windward column to lift which in turn causing the diagonal ties to begin to fail, starting at the second level and developing upwards this weakened the second level causing the failure of the bolted connections in the column at that level.
Simultaneously, the bracing failure extends upwards. The column support on the opposite side would then become ineffective. That side would start to drop and the whole pier would start to rotate about the second level. As it falls there would be a kickback on the first level causing it to be demolished but retaining most of the first level wreckage on top of the foundation. The wind speed is the critical factor as it determines the lateral loading on the bridge.
The gale, which was blowing along the Tay estuary that night, was at right angles to the bridge, which is the worst angle at which it could be blowing. Meteorological
Office records show that during the 22 month life of the bridge it suffered gale force winds on a total of only 9 days. The only time on which stronger than gale force was recorded was on the evening of the collapse. The wind force at the time of the collapse was exceptionally high. The structure was not designed to take wind force it was only designed to take 10 pounds per square foot and the norm was 40 to 50 pounds. The windward columns were not properly anchored to the foundation and the uplift of their bases caused an increase in the tie forces. Tests on tie assemblages
(including the lugs) taken from the wreckage gave average strength for the ties as 24.1
Tons whereas Bouch believed that the ties would take 32.5 tons. Bouch effectively underestimated the loading and overestimated the strength whereas he should have done it the other way round.

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Fatigue theory
This theory, attributed to Dr. Peter Lewis, claims that dynamic effects caused the fatigue failure of the cast iron lugs. The evidence for the dynamic effects is based on the eye witness reports from painters and fitters that the high girders piers oscillated from side to side whenever a train crossed the bridge. Inspection of photographs of the failed parts show that the failure of the cast iron lugs was due to fatigue rather than overstressing. A train crossing the bridge at six o clock that evening manage to get across despite the very strong winds. It was noted however that sparks flew from the wheels as if it were being pushed over by the wind. The bridge itself might have been oscillating from side to side to cause the same effect. It got to safety on the other side and it is thought that the combination of wind pressure plus oscillations on the bridge probably caused the failure of a large number of tie bars (which stabilise the bridge). Eye witnesses said that they seen the bridge swaying from side to side and up and down. Therefore there was fatigue in the ties and with the weakness of the members it is possible that this could have been the reason for failure.
As a result of the disaster lessons were learnt and precautionary measures were taken.
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All Bouch's bridges were examined and reinforced or rebuilt.
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Steel was approved by the Board of Trade for use in bridges.
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Designs using cast iron columns were barred.
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Regular and frequent inspections of bridges were made during and following construction by Board of Trade personnel.
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A maximum wind pressure of 56 per square foot for design of bridges and rules for applying this specification to bridges of different construction were recommended.

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If the building of the Tay Bridge had been a better financed, Sir Thomas Bouch may have not compromised the strength and stability of the redesigned piers. Due to the project being behind schedule and over budget, his professional judgment was probably clouded, for example he had intended to use sets of 8 columns instead of 6 for each pier. If one looks at his other bridges it is hard to believe it’s the same man that designed the Tay Bridge. One big lesson is that professional engineers need to operate at a high level of integrity
A Court of Inquiry was set up to try and ascertain the reason for the collapse of the bridge. The Court of Inquiry report concluded that, "The fall of the bridge occasioned by the insufficiency of the cross bracing and its fastenings to sustain the force of the gale." The Court of Inquiry indicated that if the piers, and in particular the wind bracing, had been properly constructed and maintained, the bridge could have withstood the storm that night, albeit with a low factor of safety - 4 to 5 was the norm at the time.
Sir Thomas Bouch was held chiefly to blame for the collapse in not making adequate allowance for wind loading. He used a wind pressure of 10 lbs/sq ft for the design of the Tay Bridge. It is interesting to note that when working on the design of a proposed
Forth Bridge (1866) he used 30lbs/sq ft. To this day, however, there is still speculation as to the fundamental cause and as to whether or not the designer, Thomas
Bouch, was to blame.
http://www.dundeecity.gov.uk/centlib/taybridge/taybridge.htm http://taybridgedisaster.co.uk/ http://www.open2.net/forensic_engineering
Group 7
Mark Claffey
Claire French
Frank Mohan