The architect Thomas Pritchard (1723-1777) is credited with being the first person to design a bridge that would be built entirely of iron. In 1773 he discussed this novel idea with two of his friends in the iron business, John Wilkinson (1728-1808) and Abraham Darby III (1723-1777). The iron masters realized immediately that this project would provide the perfect opportunity to demonstrate the superior qualities of their new cast iron and its potential as a structural material of great strength, easy assembly, and modest cost. The site, across the Severn Gorge, was close to their foundries in Coalbrookdale.
Prichard's design was for a single arch bridge spanning some 120 feet that would be just as strong and durable as one built of stone. Equally important on this very narrow and busy stretch of the River Severn, the bridge would require no midstream piers. It would be built at a height that allowed tall masted ships to pass under it even at flood stage. It was to be cast in many pieces that would fit together like a puzzle with joints that locked in place for easy assembly by adopting many of the joinery principles of wood construction.
The site for the proposed bridge was a gorge so steep that ferry service was not possible for wheeled vehicles. The lack of a bridge in this developing area was a great inconvenience to the increasing road traffic which had to make long detours to cross the Severn elsewhere. The Commissioners were from the local gentry and clergy of Bosley and of Madeley Wood on the opposing banks. The Subscribers guaranteed the estimated cost of upwards of 3000 pounds sterling. They chose Thomas Pritchard's design, and they commissioned Abraham Darby III to build the bridge. The Commissioners' petition received the Royal Assent on the 25th of March 1776. After some initial disagreements over whether the bridge should indeed be built of iron and the accuracy of the estimated cost of construction as well as the legal interpretation of the wording of the Act, the Commissioners overcame their anxieties and began construction in November of 1777.
The approach roads were built immediately after the acquisition and demolition of some structures on the bankside. The stone abutments which were to receive the base of each arch were built next. Meanwhile the ribs were being cast in open sand molds, most likely at Darby's Upper Furnace complex at Coalbrookdale. The bridge consisted of more than 800 separate castings. Each rib of the arch was 70 feet long and weighed over five tons. A total of 378 tons of iron were used over all. This was an extraordinary amount given the capacity of the iron foundries of that time.
During the summer of 1779 the parts were brought to the site on barges. A wood scaffolding was erected as shown in a contemporary water color sketch by Elias Martin ".. which shows an elegant wooden framework, rather like a set of goal posts with the cross-bar supported by diagonal struts, which was used to raise the half-ribs from the deck of a vessel on the Severn River below.
"It appears that the base plates were fixed onto the masonry platforms (abutments) on either side of the river and the uprights inserted into the base plates. One of the main half-ribs was then inserted into the base plate, and then the other; the two halves being fixed with vertical and horizontal pins in a crown piece, a complex component that could only have been cast with great skill ...
"Once the main ribs were erected, a stable platform would have been completed. ... (This) may have been a series of (wooden) decks, erected on the main ribs from which other components, the middle and back ribs, the cross stays and braces, the deck bearers and the ornamental circles and ogees, could have been inserted. Most pieces were joined by dovetails, wedges, or shouldered joints according to wood-working practice" (Attributed to Shelley White of the Ironbridge Archaeological Unit: "The Iron Bridge" by Cossons and Trinder 2002 p. 21-28)
The bridge was essentially erected in three months without any serious accidents and without interrupting barge traffic on the Severn. The scaffolding was removed in November 1779. During 1780 stone abutments were built on each end beyond the bridge iron-work to support the approach roads to the bridge. The Iron Bridge was officially opened to traffic on New Year's Day, 1781.
The architect Thomas Prichard had died in 1777 and Abraham Darby III was almost ruined by the cost over-runs that he had guaranteed to cover. Yet from the very beginning the bridge caused a sensation and became a major tourist attraction. It more than fulfilled the hopes and the expectations of its proponents. Its success demonstrated definitively that iron was a practical, economical, and beautiful structural material.
Now that we have successfully circumnavigated such subjects as the way iron is made and how to build bridges that don't get washed away, it is time to tuck away all this information where you can easily retrieve it because we are going to take our final loop, this time around the subject of the building of new roads and bridges to meet the transportation needs of the still very youthful but vigorous Industrial Revolution.