Various theories have been expounded about the way in which the bridge was built, but the recent discovery of a painting thought to show it under construction prompted the BBC's Timewatch programme to attempt to recreate the event.
Ironbridge Gorge Museum Trust deputy director David de Haan had seen a watercolour sketch by Elias Martin which was believed to be painted when the bridge was under construction in 1779. The aim of the Timewatch programme was to build a half-size model of the bridge using the method shown in the painting, to test whether it was actually a credible engineering solution.
First step was to produce a set of half-size iron structural members, cast using the techniques that would have been available at the time. Six half-span ribs, six uprights and six baseplates were required to build the three arches, and dimensional information was provided both from David de Haan's own sketches and from the English Heritage dimensional survey of the existing bridge (Bd&e issue no 26).
One major problem that resulted from this collaborative effort was that it created confusion when an attempt was made to match the two versions - one was in metric, the other in imperial units. What's more, the technical diagrams that had been provided took no account of the fact that the bridge was erected on stone abutments - hence the iron arch was not a complete semi-circle. Interpretation of the diagrams was required by H Downs & Sons, the foundry which was contracted to produce the iron elements for the half-scale structure, and the sketchy information meant that this was not a straightforward process. All the dimensions had to be sorted out - they were not tallying - it was as if the bridge had been built in one language and recorded in another. When the designs had originally been done in imperial measurements, the designers would have chosen whole numbers wherever possible; the fact that the as-built structure had been recorded in 'accurate' metric measurements meant it was almost impossible to see the original logic behind the design. Changing the measurements back to imperial allowed the construction to be translated and understood.
The programme makers wanted the replica not only to follow the same erection process as the original construction would have done, but they also wanted to use the same material and casting methods as would have been applicable at the time. This meant open casting, which is essentially the same process, but as suggested by the name, the moulds are left open to the air rather than closed in. The other main difference was in the type of iron to be used - the original structure was made from 'grey' iron, which contains flaked graphite, rather than the SG iron which is used today and in which the addition of magnesium transforms the graphite to spheroids. This manifests itself in the ductility of the iron; modern castings are much more ductile, with tensile strengths from 350N/mm2 up to 900N/mm2, whereas the material used for the iron bridge would have had a tensile strength of just 250N/mm2 and would have been liable to fracture easily.
The main effect of using grey iron was that care had to be taken in the lifting and handling of the elements, but the open casting process had quite a significant effect on the quality of the finished elements. As the molten iron was poured out of the ladle into the first mould, which was more than 11m long, it flowed along at walking pace - so fast that a wave formed and the pouring had to be stopped to let it subside. By the time pouring started again, the metal at the end of the mould was solidifying and consequently the casting showed defects of overlapping and frozen waves. Pouring was carried out more slowly on the remaining castings, with somewhat better results; ironically the elements that were produced by this method showed very similar casting defects to those on the original bridge.
Once all the elements had been cast, the erection process could begin. This took place at Blists Hill Victorian village in Telford, where the half-size replica will be left in position for a year. The erection was directed by Buro Happold structural engineer Jamie Hillier, Lieutenant Ben Day of the Royal Engineers and David de Haan, with the brute force being provided by a squad of Royal Engineers and the whole process observed by two enthusiasts who had both built their own small-scale models at home. For the television programme, the army constructed a derrick at one end of the canal to be used to lift the castings from the bank onto the canal boat which would bring them to the site, and one on each side of the canal at the bridge site. Two uprights, as depicted in the painting, were connected with a crossbar which together formed a swinging derrick and built-in scaffolding to lift the castings off the boat and into their final position. Although the elements used in the reconstruction were only half size, the crew first carried out an experiment to see if these same derricks using the hemp ropes and wooden pulleys of the time would have been capable of lifting the actual bridge ribs - almost 6t in weight, which they proved conclusively.
Two wooden uprights were erected, one on each side of the canal, and these were connected with a crossbar; this formed the basic 'scaffolding' which would support the iron elements during erection of the bridge - the design of this set-up having been derived from the painting.
With the base plates in position on the abutments, the first upright was fitted into the base plate, and was wedged in. The first rib was then lifted into place; once the footing was correctly located in the base plate - using the traditional lubricant of lard for easy positioning - it was hoisted into the upright position and lashed to the crossbar. A similar process was used to raise the second half of the arch, but the most difficult part of the operation was connecting the two half-ribs at the complex dovetail joint at the crown. This required some adjustment and fine tuning on the pulleys until the two elements were aligned sufficiently accurately to allow the full connection to be made. Reaching this stage of the erection took four days including the erection of the timbers; the remaining elements were lifted into position by a crane, once the manual method had been proven but the use of ropes and pulleys had allowed more flexibility in the fine adjustment and neither method proved faster than the other.
