TAY BRIDGE DISASTER28th December

1879

The Tay bridge, completed February 1878 designed by noted railway engineer Sir Thomas Bouch, was a monument of strength for the railways, and for Scotland, nearly two miles long, consisting of 85 spans and at the time it was the longest bridge in the world. The 85 spans carried a single railway track, 72 of which were supported on deck trusses, the remaining 13 spans were longer through trusses above the railway track (“High Girders”) to allow ships with tall masts to pass under.

The Disaster 28th of December 1879, A violent storm

 estimated at Beaufort force 10/11 was blowing down the Tay estuary at right angles to the bridge. At approximately 7:15 p.m. , an express train consisting of a heavy locomotive, six passenger cars and 75 people, traversed the Tay bridge. As the train reached the centre of the bridge the 13 centre spans(“high girders”) of the bridge gave way and the train plunged into the icy water below, killing all 75 souls aboard.

BEFOREThrough Trusses (“High Girders”)

All 13 Through trusses(“High Girders”) gave way as the train passed over.

AFTER

The Investigation Investigators quickly determined many faults in the design,

material and process that had contributed to the failure of the bridge . The investigation found faults in 5 main areas:-Poor quality materials-Weak Lugs (projections from the columns where cross bracings were attached)-Piers were not constructed properly-Poor design-Maintenance

Poor Quality MaterialsThe columns supporting the 13 longest spans of the bridge were made out of cast iron, which is a very brittle under tension, whilst the wrought iron used in the cross bracing is very durable. It was also found that the cast iron columns, were cast (when material is introduced into a mould while liquid, and allowed to solidify into a specific shape) horizontally, with the result that the walls of the columns were not of even thickness. This design flaw in the use of materials with different properties led to the collapse of the Tay Bridge.

Piers not Constructed ProperlyInvestigations found that the Piers on which the columns were constructed were inadequate to support the bridge during high winds. On the Tay bridge, the columns supporting the spans were bolted into, only two layers of masonry on the pier. During wind the masonry into which the column bolts were anchored would lift up by a small amount. This movement may have led to the failure of cross bracing.T he diagram on the next page , shows how this type of movement was amplified in the final collapse. Thomas Bouch the bridge designer, used10 pounds per for the design of the bridge, he calculated that a wind load of this strength would not cause uplift on the bolts securing the bases of the columns. Bouch used a lower wind loading in an attempt to save money(the higher the wind loading the more bracing and structure strengthening must be used), the value he should have used should have been at least 40 pounds per square foot.

Here you can clearly see that the columns were only bolted into the first two layers of masonry

Weak LugsAnother element of the design faulted by investigators was how weak the lugs were, Lugs are projections from the columns where the cross bracing is attached to. It was found that The lugs, due to poor smelting and casting holes instead of drilling them, were unnecessarily weak. Thomas Bouch the designer believed that the individual lugs could hold 60 tons, but when tested in the investigation the lugs proved to break at only 20 tons, The lugs In the disaster play a major role, in all the theories presented later in this presentation.

A lug is a type of joint: a projection from a column is attached to cross bracing

Casting is the process of pouring molten metal into a mould. In this case errors in casting lead to the lugs being weak

Maintenance A clear Issue put forward by investigators was; maintenance or a lack of, the bridge bore clear evidence that the central structure had been deteriorating for months before the final accident . The Maintenance inspector, Henry Noble, had heard the joints of the wrought iron-cross bracings, “chattering” a few months after the bridge opened , this sound was an indication that the joints were loose and needed to be tightened or replaced.

When the cross bars are loose they become useless in bracing the columns, therefore the structure is weaker. Instead of reporting the incident or at least tightening the joints, the maintenance inspector hammered shims (small thin pieces) of iron in-between the joints to stop them from rattling, this may have stoped the rattling but did nothing to fix the underlying problem. From this It was clear that bridge inspectors needed more knowledge of bridges and how they work, in order to properly fix problems instead of doing “botch” jobs

Poor DesignThe design of the bridge was generally overall flawed, here are the reasons-The Trough Trusses (“High Girders”) Were top heavy, and vulnerable to high winds, no extra strengthening in the columns and cross bracing were present to compensate for this.-There was never enough cross bracing to support the bridge -Design of the piers were, as discussed earlier was inadequate stop uplift on the bolts securing the bases of the columns. -The poor quality or wrong materials were used in the design of the bridge-Lugs were not adequate (even if built correctly) a stronger way of connecting the cross bracing to the columns should have been used.-Thomas Bouch's design generally underestimated the load and overestimated the strength of the bridge, In a safe structural design Common practise would be to Overestimate the load and underestimate the strength, the complete opposite of what Bouch did

THEORIES There are so many factors and possible

causes involved, and not enough surviving evidence, that no-one cause has been proven, instead there are a number of theory's, all of which are explained in this presentation.-Blow down by the wind Theory-Train derailment theory-Fatigue theory

Blown Down By the Wind Theory As mentioned previously the columns were not correctly anchored into the

pier, such as-that in a strong wind the masonry into which the column bolts were anchored would lift up by a small amount.

This small movement is enough to cause excessive tension and compression in the cross bracing, especially in the centre cross bracings which are longer and take more weight. These forces are to much for the weak cast iron lugs which fail (attaching the cross bracing to the columns ).The cross bracing with the most strain (middle cross bracing on second layer above the pier) would fail first, this strain would then be transferred onto the next layer and then the next, and so on.

The result of this is that the whole pier has now only 1/3 of the strength that it would normally have if it was fully braced, the bridge is now very flexible and sways violently in strong gusts. The combined force of the wind blowing against the bridge and the heavy train traversing is to much for the unbraced columns, which fail and send the train plunging into the water below.

The windward column and masonry lifts up from the pier causing extra stress on the inner bracing

The lifting up of the columns and masonry causes the inner bracing to fail, the bridge now only has 1/3 of its original strength.

The combined wind and the train is too much for the weakened bridge to handle, the column opposite to the direction of the wind fails and the bridge collapses.

Lugs holding Inner bracing to column fail

Train Derailment TheoryThis theory was put forward in defence of the designer Thomas Bouch. This Theory suggests that the train which was traversing the bridge, travels over a kink in the rails, causing the train to derail, and crash into the side of the bridge. This sends a sudden shock through the entire structure, causing the cast iron lugs connecting the cross bracing to the columns, to fracture leading to the subsequent collapse of the pier structure. This Theory, however has the least evidence and fails to explain why 13 piers collapsed instead of just the piers the train was traversing

Cast iron lug at bottom of pier fails, leading to the collapse of the bridge

Fatigue Theory This theory, like the last suggests that the wind was not the

cause of the collapse of the bridge, instead it claims that dynamic (moving) effects caused the failure of the cast iron lugs due to fatigue (Fatigue refers to constant varying stresses, which overtime cause cracks to form in a material, these cracks eventually reach a size where the structure fails).

The only evidence to support this theory, comes from eye witness reports from painters and fitters who say that the piers would shake from side to side whenever a train crossed the bridge, this movement could have caused fatigue in the lugs, consequently causing the bridge to fail. Limited evidence can be found in high quality photographs which show some evidence of fatigue in the lugs, this limited evidence however is not enough to prove that the bridge failed due to fatigue.

Disaster Within the Community The downing of the Tay Bridge effectively cut off

the north of Scotland from the railway network, back in 1879 the railway was the only effective means of travel, the cities of Aberdeen and Dundee were servery deprived, everything and everyone that travelled in and out of the cities was by train. As a result of the bridge failure business and industry suffered greatly in northern Scotland, any goods or passengers had to be transferred onto a boat to make any journey to northern Scotland.

Prevention In order to have prevented the Tay bridge from collapsing,

several design aspects must be changed. Using the materials available at the time and at a reasonable price, this is what would have needed to be changed in Thomas Bouch’s design for it to be safe. -The columns, in order to support the bridge, needed to be anchored more deeply into the pier. -Columns, cross bracing and lugs should have been constructed with wrought iron and or steel.-Extra cross bracing was required to support the columns-Another solution entirely would be to build the whole pier out of stone rather than of iron, this would cost more, but would be virtually indestructible and easier to maintain.

Lessons learned and things changed

The bridge building industry after 1879 changed significantly, It saw the end of cast iron as a use for bridge building, the disaster also resulted in many cast iron bridges all over the world being replaced. An important lesson learned by the bridge building industry at the time was that it was better to spend more money on a safe bridge than on a cheap bridge that could fail at any time. The outcome of Tay bridge disaster saw, for the first time, quality control inspectors who make sure what is being built is to a certain degree of quality