5.12 Pole and Stewart report
Apparently prepared using the same methodology as Law, Pole and Stewart produced a report that calculated the loads at various points in the bridge under live locomotive loads and wind loading at various pressures. Stewart was employed by Bouch to perform the original design calculations for the bridge, while Pole was brought in as an independent expert. He had extensive experience of use of different materials in bridges, and indeed, had written a standard text book for engineers on the subject.
The court of enquiry asked the two experts to supply all design calculations made for the bridge before and during construction in a letter from the three commissioners. Seventeen points of detail were mentioned in their formal letter of instruction, including:
the weights of the different parts of the structure;
estimates of the surface area exposed to the wind;
the maximum pressure the piers could withstand;
stress on the diagonal tie bars for various wind pressures.
Pole and Stewart attempted to respond to the enquiries by noting that the commissioners ‘… seem to refer to the calculations originally made for the structure …’. Although they stated that the bridge parts were designed
… with full regard to all the strains likely to come upon them, the calculations have not been preserved in such a form as to be available for our present purpose ….
This is a curious way of answering the request. Were the calculations made when the bridge was built or not? If any calculations were made, surely they would be important evidence for the enquiry. Perhaps no calculations were made at all, and the bridge was built simply on previous experience, by make-and-do. Whatever the case, the two experts said they would perform new calculations from the original design drawings. In a later section of their detailed report, they stated that the calculations are difficult
… seeing that the strains are borne by the concurrent resistances of many bars in different positions.
How true is this remark in the light of current knowledge? The structure would now be referred to as being statically indeterminate, and it is impossible to calculate the loads in the bracing bars without considerable computing power. Law realised this and did not attempt the calculation.
However, Pole and Stewart – no doubt having to respond to the question posed by the court – did estimate the loads, which was more than Law felt able to do. Using a wind pressure of 30 pounds per square foot with a train on the bridge, they produced a figure of about 10 tonf inch−2. Given the cross-sectional area of a tie bar is about 2.25 square inches, it equates to a force in the bar of about 22.5 tonf, remarkably close to the breaking load of the bottom lugs.
Their calculations were duly performed for various wind pressures, although it is important to say that the approach of both Law and the two experts acting for Bouch and the NBR was highly theoretical and did not attempt to relate their results to the practical measurements made by Kirkaldy. Both analyses were static in nature, and made no allowances for dynamic effects, despite the evidence for oscillation of the bridge prior to failure, and the crucial evidence of Mr Noble about the looseness of the joints on the bracing bars. In their defence however, it is also true to say that neither engineers nor analysts of the day had much appreciation, theoretical or practical, of the dynamic effects of winds on large structures.
Their results were subject to severe criticism in the subsequent oral examination by the court. In vigorous cross-examination, neither expert would admit the piers were faulty, and Mr Stewart was caught in a trap, as the following part of the transcript shows:
19,143. Take the hypothesis that is put to you, that these tie bars were giving to the extent of a quarter of an inch, do you say that would add to the stability of the structure, or that it would detract from the stability of the structure? – Of course you mean this and the one opposite (pointing to the model)? It would add to the stability of the structure.
(Mr Bidder) Do you mean giving by extension, or by bolt bending?
(Mr Trayner) I mean giving by extension.
(Mr Stewart) I still hold to the view I have expressed. If Sir Thomas Bouch could have put in some kind of spring that would have allowed a yielding of a quarter of an inch it would have added to the strength of the structure. It would have been very difficult to do.
19,144. It would have been something like building a castle in the air? – Perhaps.
19,156. When these bars chattered in the way we have heard described, it was again a piece of unnecessary work on Mr Noble's part to fill them up with packing pieces? – You have already asked me that question. I do not know that it is always wise to increase the stability of a bridge by looseness, I think it wiser to pack it up. It is difficult to answer that question.
Mr Stewart was clearly caught on the horns of a dilemma: he said loose tie bars would make the bridge stronger, but yet supported packing them out with shims to tighten them.
Pole and Stewart also admitted they had not, unlike Mr Law, made a detailed survey of the fallen piers and examined the fractured and failed component parts of the columns.
So what, in their opinion, was the cause of failure of the bridge? They suggested the train hit the high girders, and the shock was transmitted to the piers, which fractured and brought the bridge down. There was indeed some evidence because two girders were damaged – but that was probably caused after the bridge had started to fall, with the train toppling over as a result. Impact-by-train was the same argument advanced by Stephenson to explain the failure of the Dee bridge in 1847.
The natural question was then put to the experts: if the bridge could be brought down by the train hitting the high girders, was it not then a faulty design? Stewart had apparently approved all the design changes made to the bridge by Bouch – cast-iron piers instead of brick piers, pier tops not strongly connected, six columns instead of eight, and so on. He had done calculations at the time, but could not produce them to the enquiry. Indeed, their calculations in their joint report were done entirely from scratch and without reference to any calculations done before construction was underway.
Finally, Mr Cochrane refuted that the train had collided with the girders. He was examined after Dr Pole, and showed how the tie bar failure caused collapse of pier 5.
Having seen both the recovered train and the girders, Mr Cochrane went on:
15,033. (Mr Trayner) You heard the suggestion that the train had gone off the rails and knocked over the bridge; have you formed any opinion as to whether the train had gone off the rails, and by so doing had contributed in any way to the result? – I think it did not from all I saw. I do not believe the train left the rails.