Cast iron in moulds
Cast iron components were manufactured by casting the molten metal in sand moulds that allowed fairly complex shapes to be produced.
Cast iron was formed into structural elements by pouring the molten and extremely fluid material into moulds formed in damp sand. These moulds were normally in two halves which were brought together initially against a timber 'pattern' that left an impression in the sand of the desired shape of the iron component. The two halves of the mould were separated, the timber pattern removed and the mould re-assembled ready to receive the molten iron. When the metal had cooled and solidified, the sand was simply brushed away and then used again to form another mould. This meant that each individual cast iron element was specially made and that the shape could be as complex as the two-part sand mould would allow.
The ends of structural elements were shaped during the casting process to facilitate jointing.
By the end of the 18th century, the craft of sand moulding was highly developed so that intricate and complicated shapes could be formed quickly and accurately. Much of this skill went into stylistic enterprises like mass producing Corinthian columns, but it did mean that the problem of connecting one element to another could be tackled in a diverse range of ways.
In the Iron Bridge at Coalbrookdale, which was the first major cast iron structure, the joint configurations were similar to those used in carpentry.
Not unnaturally, at Iron Bridge the designers of this unique structure borrowed ideas from established materials. They opted for arrangements that were based on carpentry joints. The moulds were shaped so that a tongue was cast into the end of one piece and a groove was cast into the end of another. The pieces could then be slotted into each other and a cast iron wedge locked them together. In this way, the lightweight framework of the bridge could be assembled with no special jointing pieces other than the wedges. Looking at the bridge today, many extra bolted clamps and fasteners are visible. These are later attempts to strengthen the bridge, but they must not be taken to mean that the original connections were basically unsound. The defects in the bridge structure were due to movement of the abutments and the interlocking joints were always a satisfactory if somewhat elaborate means of connection.
Structural frameworks with cast iron beams and columns and brick jack arch floors were developed in the late eighteenth century to provide fireproof construction for industrial buildings.
By the end of the eighteenth century cast iron mill structures were being erected and these had to be justified on a purely functional basis. A significant requirement was for a textile mill that would be resistant to fire. In the 1790s and the 1800s a number of mills were built which were all variations on the theme of masonry jack floors, timber beams and cast iron columns. Later, cast iron was used for the beams as well. These structures were much less combustible than their all-timber predecessors, and the way the different materials connected together to give mutual structural support was a triumph for the adaptability of the shaped cast iron end pieces. Not only did the columns lock together vertically over one another, but the thrust from the jack arches was absorbed by the angled seating and transferred into the horizontal beam. In some cases tie rods were threaded through the connectors to prevent outward thrust being applied to the external masonry walls.
This drawing shows the superb way in which the extremely complex junction between cast iron column, tie rod, timber beam and brick jack arch floor including sand filling was resolved in the earliest cast iron mills of the late eighteenth century. Cast iron enabled such complex junctions to be achieved because it allowed skilled craftsmen to make geometrically elaborate shapes.
The brittleness of cast iron and its low tensile strength made it unsuitable for major engineering structures.
As the 19th century progressed, designers became more and more confident with cast iron, and spurred on by rapid industrialisation and particularly the railway network, they produced more and more ambitious structures. Cast iron, however, has one major structural inadequacy: it is very weak in tension. This is no problem with compression arches for bridges and roofs, but any structure with bending or tension forces was vulnerable. By using large sections with big connections, tension stresses could be kept quite low. As confidence grew, however, sections became more highly stressed, and longer span railway bridges required complex and elaborate special junctions to keep the tension stresses within bounds. When the Dee Bridge near Chester collapsed with the loss of five lives in 1847 and the greatest engineer of his day, Robert Stephenson, only narrowly avoided major censure, it was realised that a material that was both brittle and of low tensile strength could have only limited use for structures. Subsequently, wrought iron and steel became the predominant structural alloys of iron and carbon.
This complicated device was used by Robert Stephenson to make a bridge beam that utilised the tensile strength of expensive wrought iron with cheap cast iron which was only reliably strong in compression. This jointing system was used for the railway bridge over the River Dee but failed in 1847 soon after it had been opened. Such designs where disparate parts are struggling to compensate for the inherent weakness of each other are an example of basically poor engineering practice. Curiously Stephenson's contemporary design for the High Level Bridge over the River Tyne demonstrated how good design could overcome the same problem.
