5.13 Conclusion of the BoT enquiry
The BoT enquiry issued two reports at the end of the enquiry, one authored by the chair, Mr Rothery, the other by the two other assessors. The Rothery report is Paper 3, linked below. They agreed about most of the issues in contention, as follows (Paper 3, page 47 of report).
There is no evidence to show that there has been any movement or settlement in the foundations of the piers;
The wrought iron was of fair quality;
The cast iron was also fairly good, though sluggish on melting;
The girders were fairly proportioned for the work they had to do;
The iron columns, though sufficient to support the vertical weight of the girders and trains, were owing to the weakness of the cross-bracing and its fastenings, unfit to resist the lateral pressure of the wind;
The imperfections in the work turned out at the Wormit foundry were due in great part to want of proper supervision;
The supervision of the bridge after its completion was unsatisfactory;
If by loosening of the tie bars the columns got out of shape, the mere introduction of packing pieces between the gibs and the cotters would not bring them back to their positions;
Trains were frequently run through the high girders at much higher speeds than 25 mph;
The fall of the bridge was probably due to the giving way of the cross-bracing and its fastenings;
The imperfections in the columns might also have contributed to the same result.
Click 'View document' below to open Paper 3 (35 pages, 39 MB).
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Rothery went on to look at specific design problems in the bridge, especially the narrow base, the slight inclination (batter) of the outer columns, and to the omission of spigots at their bases, and to the casting of holes in the lugs and flanges of the 18 inch columns. He laid the blame for faulty design, construction and maintenance at Bouch's door. The other two members of the enquiry did not go so far, but felt strongly enough to produce a second joint report on the disaster.
Finally, the second report stated that,
… we have to state that there is no requirement issued by the Board of Trade respecting wind pressure, and there does not appear to be any understood rule in the engineering profession regarding wind pressure in railway structures; and we therefore recommend that the Board of Trade should take some steps as may be necessary for the establishment of rules for that purpose.
Question 15
Review the general findings of the Rothery report in the light of the evidence shown in this unit. Concentrate on:
the possible cause or causes of the failure;
the material evidence provided by Kirkaldy.
Answer
The evidence reviewed in this unit would have been available to the original investigation, and the results of our examination broadly agree with what the enquiry team found – bearing in mind we have only seen a small part of the total amount of evidence before the enquiry. When examined in detail, many of the defects alleged by Henry Law were not supported by the photographs. Many of the casting defects alleged were not found in the castings visible in the extant pictures – blow holes, beaumont's egg filler, cold shuts. The most serious defects in the bridge were:
the system of cross bracing with a poorly designed tension joint;
cast-in lugs that were well below strength owing to stress concentration at the boltholes.
The wind braces loosened with time and applied load, so that they could not adequately support the normal loads from passing trains and wind loading. When higher than expected loads occurred as a result of a gale, the lugs fractured and caused the collapse of the entire high girders section of the bridge. Even those piers over which the train had not passed on the night of the disaster failed, showing they were clearly under-designed for their job. The bridge collapsed like a house of cards when strained by the train and moderate wind loading.
The role of ‘racking’ was not explored in greater detail by the enquiry, but it now seems clear it was oscillations in the structure that loosened the joints on the wind braces. Lateral vibrations would have been within the elastic limits of both the wrought and cast-iron components, but the effects led directly to deterioration of the bracing and ultimately failure.
Kirkaldy's results were not discussed in as great detail as they demanded, although it was clear the lower cast-iron lugs were well below strength. The lowered strength was attributed by the enquiry team to casting defects, due to engineers not apparently being aware of the effects of stress concentrations on the strength of shaped parts. Further information could have been explored in greater detail by the questioning Kirkaldy, but he was not called to present his evidence in person. Important lessons could have been learned about the importance of component design, the importance of testing materials prior to their use in critical structures and including such tests in the contract and bridge specification.
Question 16
Suggest a possible sequence of events leading to the final collapse. Use the evidence provided in the previous sections, as well as your own interpretation of that evidence from previous questions. Concentrate on the evidence provided by the remaining debris on the piers and the position of the train and piers found during the diving operations, to provide you with key evidence as to the cause of the disaster.
Answer
One possible sequence of events is as follows.
Cross bracing between the two sets of three columns on many piers of the high girders strained by repeated oscillations of bridge during passage of trains and high winds, leading to loosening of the gib and cotter joints.
Critical damage occurs when the last train before the disaster passes over the bridge, with braces broken at the lugs on many piers, and caused directly by the oscillations. An unknown number were broken, so making the bridge liable to oscillations of even greater amplitude.
The very last train passed into the high girders, having successfully negotiated the low girders, and reached pier 4, where the extra sideways load from the wind together with its own weight, caused pier 4 to shear and collapse to the east.
The position of the fallen girders and train found in the water (about 40 feet from pier 4, 21 feet from pier 5) suggests the 88 foot high columns broke up during the collapse. The position of the remaining debris on the piers is a consequence of the unpredictable disintegration of the pier following the initiation of collapse by the fracture of a critical number of lugs.
The girder span was strongly connected to the four spans to the south and the toppling action pulled the rest over, perhaps rather slowly, initiating toppling of those piers, and so exacerbating the problem. The train had in the meantime travelled further forward, but when it reached a critical angle, itself started to topple over.
The girders of the first falling span were not strongly connected to the next set of spans because there was an expansion joint here on pier 5. The rails to the north may have been sufficient to initiate pulling away of the next set of connected spans, which may also have started by collapse of the piers – already probably severely weakened by lug breakage.
The failure travelled the final length of the high girders section in a chain reaction.
The suggested sequence of events is of course somewhat speculative. However, it is a scenario based on the evidence available to the enquiry at the time. There are alternative scenarios that could be constructed, and you may well have arrived at a different sequence.
Arriving at a plausible sequence of events does not of course affect the conclusions of the enquiry, but does allow more detailed investigation of the causes of the disaster, stimulates further questions to be posed and raises issues concerned with the survival of the original evidence considered by the enquiry, especially the broken samples exhibited by Mr Law.